[PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Zhang Tianci]

ovl_link() did not create a new inode after commit
51f7e52dc943 ("ovl: share inode for hard link"), so
in ovl_create_or_link() we should not override cred's
fsuid and fsgid when called by ovl_link().

Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
---
 fs/overlayfs/dir.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..568d338032db 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
 	err = -ENOMEM;
 	override_cred = prepare_creds();
 	if (override_cred) {
-		override_cred->fsuid = inode->i_uid;
-		override_cred->fsgid = inode->i_gid;
 		if (!attr->hardlink) {
+			override_cred->fsuid = inode->i_uid;
+			override_cred->fsgid = inode->i_gid;
 			err = security_dentry_create_files_as(dentry,
 					attr->mode, &dentry->d_name, old_cred,
 					override_cred);
-- 
2.32.1 (Apple Git-133)

Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Amir Goldstein]

On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
<zhangtianci.1997@bytedance.com> wrote:
>
> ovl_link() did not create a new inode after commit
> 51f7e52dc943 ("ovl: share inode for hard link"), so
> in ovl_create_or_link() we should not override cred's
> fsuid and fsgid when called by ovl_link().
>
> Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> ---
>  fs/overlayfs/dir.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..568d338032db 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>         err = -ENOMEM;
>         override_cred = prepare_creds();
>         if (override_cred) {
> -               override_cred->fsuid = inode->i_uid;
> -               override_cred->fsgid = inode->i_gid;
>                 if (!attr->hardlink) {
> +                       override_cred->fsuid = inode->i_uid;
> +                       override_cred->fsgid = inode->i_gid;
>                         err = security_dentry_create_files_as(dentry,
>                                         attr->mode, &dentry->d_name, old_cred,
>                                         override_cred);
> --

This change looks incorrect.
Unless I am missing something, fsuid/fsgid still need to
be overridden for calling link() on underlying fs.
What made you do this change?

Thanks,
Amir.

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[天赐张]

On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> <zhangtianci.1997@bytedance.com> wrote:
> >
> > ovl_link() did not create a new inode after commit
> > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > in ovl_create_or_link() we should not override cred's
> > fsuid and fsgid when called by ovl_link().
> >
> > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > ---
> >  fs/overlayfs/dir.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..568d338032db 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> >         err = -ENOMEM;
> >         override_cred = prepare_creds();
> >         if (override_cred) {
> > -               override_cred->fsuid = inode->i_uid;
> > -               override_cred->fsgid = inode->i_gid;
> >                 if (!attr->hardlink) {
> > +                       override_cred->fsuid = inode->i_uid;
> > +                       override_cred->fsgid = inode->i_gid;
> >                         err = security_dentry_create_files_as(dentry,
> >                                         attr->mode, &dentry->d_name, old_cred,
> >                                         override_cred);
> > --
>
> This change looks incorrect.
> Unless I am missing something, fsuid/fsgid still need to
> be overridden for calling link() on underlying fs.
> What made you do this change?
>
> Thanks,
> Amir.

Hi Amir,

I ran into an error when I tested overlay on fuse:
  $ mkdir /lower /fuse /merge
  $ mount -t fuse /fuse
  $ mkdir /fuse/upper /fuse/work
  $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
  $ touch /merge/file
  $ chown bin.bin /merge/file // the file's caller becomes "bin"
  $ ln /merge/file /merge/lnkfile

Then I got an error(EACCES) because fuse daemon checks the link()'s
caller is "bin", it denied this request.
I browsed the changing history of ovl_link(). There are two key commits:
The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
overrides the cred's fsuid/fsgid using the new inode. The new inode's
owner is initialized by inode_init_owner(), and inode->fsuid is
assigned to the current user. So the override fsuid becomes the
current user. We know link() is actually modifying the directory, so
the caller must have the MAY_WRITE permission on the directory. The
current caller may should have this permission. I think this is
acceptable to use the caller's fsuid(But I still feel a little
conflicted with the overlay's design).
The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
which removed the inode creation in ovl_link(). This commit move
inode_init_owner() into ovl_create_object(), so the ovl_link() just
give the old inode to ovl_create_or_link(). Then the override fsuid
becomes the old inode's fsuid, neither the caller nor the overlay's
creator! So I think this is incorrect.
I think the link() should be like unlink(), overlay fs should just use
the creator cred to do underlying fs's operations.

Thanks,
Tianci.

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Amir Goldstein]

On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
>
> On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > <zhangtianci.1997@bytedance.com> wrote:
> > >
> > > ovl_link() did not create a new inode after commit
> > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > in ovl_create_or_link() we should not override cred's
> > > fsuid and fsgid when called by ovl_link().
> > >
> > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > ---
> > >  fs/overlayfs/dir.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > index 6b03457f72bb..568d338032db 100644
> > > --- a/fs/overlayfs/dir.c
> > > +++ b/fs/overlayfs/dir.c
> > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > >         err = -ENOMEM;
> > >         override_cred = prepare_creds();
> > >         if (override_cred) {
> > > -               override_cred->fsuid = inode->i_uid;
> > > -               override_cred->fsgid = inode->i_gid;
> > >                 if (!attr->hardlink) {
> > > +                       override_cred->fsuid = inode->i_uid;
> > > +                       override_cred->fsgid = inode->i_gid;
> > >                         err = security_dentry_create_files_as(dentry,
> > >                                         attr->mode, &dentry->d_name, old_cred,
> > >                                         override_cred);
> > > --
> >
> > This change looks incorrect.
> > Unless I am missing something, fsuid/fsgid still need to
> > be overridden for calling link() on underlying fs.
> > What made you do this change?
> >
> > Thanks,
> > Amir.
>
> Hi Amir,
>
> I ran into an error when I tested overlay on fuse:
>   $ mkdir /lower /fuse /merge
>   $ mount -t fuse /fuse
>   $ mkdir /fuse/upper /fuse/work
>   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
>   $ touch /merge/file
>   $ chown bin.bin /merge/file // the file's caller becomes "bin"
>   $ ln /merge/file /merge/lnkfile
>
> Then I got an error(EACCES) because fuse daemon checks the link()'s
> caller is "bin", it denied this request.
> I browsed the changing history of ovl_link(). There are two key commits:
> The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> overrides the cred's fsuid/fsgid using the new inode. The new inode's
> owner is initialized by inode_init_owner(), and inode->fsuid is
> assigned to the current user. So the override fsuid becomes the
> current user. We know link() is actually modifying the directory, so
> the caller must have the MAY_WRITE permission on the directory. The
> current caller may should have this permission. I think this is
> acceptable to use the caller's fsuid(But I still feel a little
> conflicted with the overlay's design).
> The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> which removed the inode creation in ovl_link(). This commit move
> inode_init_owner() into ovl_create_object(), so the ovl_link() just
> give the old inode to ovl_create_or_link(). Then the override fsuid
> becomes the old inode's fsuid, neither the caller nor the overlay's
> creator! So I think this is incorrect.
> I think the link() should be like unlink(), overlay fs should just use
> the creator cred to do underlying fs's operations.
>

I see. The reproducer and explanation belong in the commit message.

Your argument makes sense to me, but CC Christian to make
sure I am not missing anything related to ACLs and what not.

Thanks,
Amir.

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Amir Goldstein]

On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> >
> > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > <zhangtianci.1997@bytedance.com> wrote:
> > > >
> > > > ovl_link() did not create a new inode after commit
> > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > in ovl_create_or_link() we should not override cred's
> > > > fsuid and fsgid when called by ovl_link().
> > > >
> > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > ---
> > > >  fs/overlayfs/dir.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > index 6b03457f72bb..568d338032db 100644
> > > > --- a/fs/overlayfs/dir.c
> > > > +++ b/fs/overlayfs/dir.c
> > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > >         err = -ENOMEM;
> > > >         override_cred = prepare_creds();
> > > >         if (override_cred) {
> > > > -               override_cred->fsuid = inode->i_uid;
> > > > -               override_cred->fsgid = inode->i_gid;
> > > >                 if (!attr->hardlink) {
> > > > +                       override_cred->fsuid = inode->i_uid;
> > > > +                       override_cred->fsgid = inode->i_gid;
> > > >                         err = security_dentry_create_files_as(dentry,
> > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > >                                         override_cred);
> > > > --
> > >
> > > This change looks incorrect.
> > > Unless I am missing something, fsuid/fsgid still need to
> > > be overridden for calling link() on underlying fs.
> > > What made you do this change?
> > >
> > > Thanks,
> > > Amir.
> >
> > Hi Amir,
> >
> > I ran into an error when I tested overlay on fuse:
> >   $ mkdir /lower /fuse /merge
> >   $ mount -t fuse /fuse
> >   $ mkdir /fuse/upper /fuse/work
> >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> >   $ touch /merge/file
> >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> >   $ ln /merge/file /merge/lnkfile
> >
> > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > caller is "bin", it denied this request.
> > I browsed the changing history of ovl_link(). There are two key commits:
> > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > owner is initialized by inode_init_owner(), and inode->fsuid is
> > assigned to the current user. So the override fsuid becomes the
> > current user. We know link() is actually modifying the directory, so
> > the caller must have the MAY_WRITE permission on the directory. The
> > current caller may should have this permission. I think this is
> > acceptable to use the caller's fsuid(But I still feel a little
> > conflicted with the overlay's design).
> > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > which removed the inode creation in ovl_link(). This commit move
> > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > give the old inode to ovl_create_or_link(). Then the override fsuid
> > becomes the old inode's fsuid, neither the caller nor the overlay's
> > creator! So I think this is incorrect.
> > I think the link() should be like unlink(), overlay fs should just use
> > the creator cred to do underlying fs's operations.
> >
>
> I see. The reproducer and explanation belong in the commit message.
>
> Your argument makes sense to me, but CC Christian to make
> sure I am not missing anything related to ACLs and what not.

Once again with correct email address...

>
> Thanks,
> Amir.

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Christian Brauner]

On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> >
> > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > >
> > > > > ovl_link() did not create a new inode after commit
> > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > in ovl_create_or_link() we should not override cred's
> > > > > fsuid and fsgid when called by ovl_link().
> > > > >
> > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > ---
> > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > >
> > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > index 6b03457f72bb..568d338032db 100644
> > > > > --- a/fs/overlayfs/dir.c
> > > > > +++ b/fs/overlayfs/dir.c
> > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > >         err = -ENOMEM;
> > > > >         override_cred = prepare_creds();
> > > > >         if (override_cred) {
> > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > -               override_cred->fsgid = inode->i_gid;
> > > > >                 if (!attr->hardlink) {
> > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > >                         err = security_dentry_create_files_as(dentry,
> > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > >                                         override_cred);
> > > > > --
> > > >
> > > > This change looks incorrect.
> > > > Unless I am missing something, fsuid/fsgid still need to
> > > > be overridden for calling link() on underlying fs.
> > > > What made you do this change?
> > > >
> > > > Thanks,
> > > > Amir.
> > >
> > > Hi Amir,
> > >
> > > I ran into an error when I tested overlay on fuse:
> > >   $ mkdir /lower /fuse /merge
> > >   $ mount -t fuse /fuse
> > >   $ mkdir /fuse/upper /fuse/work
> > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > >   $ touch /merge/file
> > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > >   $ ln /merge/file /merge/lnkfile
> > >
> > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > caller is "bin", it denied this request.
> > > I browsed the changing history of ovl_link(). There are two key commits:
> > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > assigned to the current user. So the override fsuid becomes the
> > > current user. We know link() is actually modifying the directory, so
> > > the caller must have the MAY_WRITE permission on the directory. The
> > > current caller may should have this permission. I think this is
> > > acceptable to use the caller's fsuid(But I still feel a little
> > > conflicted with the overlay's design).
> > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > which removed the inode creation in ovl_link(). This commit move
> > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > creator! So I think this is incorrect.
> > > I think the link() should be like unlink(), overlay fs should just use
> > > the creator cred to do underlying fs's operations.
> > >
> >
> > I see. The reproducer and explanation belong in the commit message.
> >
> > Your argument makes sense to me, but CC Christian to make
> > sure I am not missing anything related to ACLs and what not.
> 
> Once again with correct email address...

So we have:

ovl_create_object()
-> ovl_override_creds(ovl_sb)
-> ovl_new_inode()
   -> inode_init_owner()
      {
              inode->i_uid = current_fsuid();
              inode->i_gid = current_fsgid();
      }
-> ovl_create_or_link(inode, ...)
-> prepare_creds() // Copy of caller's creds
{
        override_creds->fsuid = inode->i_uid;
        override_creds->fsgid = inode->i_gid;
}
-> revert_creds()

which afaict means that the mounter's credentials are used apart from
the fs{g,u}id which is taken from inode->i_{g,u}id which should
correspond to current_fs{g,u}id().

The commit that is pointed out in the patch
51f7e52dc943 ("ovl: share inode for hard link")
seems to have broken that assumption.

Given that the intention was to use the creator's creds _with the
caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
the caller's fs{g,u}id are always used instead of using the full
creator's creds just for the link operation? So something like this
(untested):

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..4a3ee16a6d70 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
        const struct cred *old_cred;
        struct cred *override_cred;
        struct dentry *parent = dentry->d_parent;
+       /* Retrieve caller's fs{g,u}id before we override creds below. */
+       kuid_t caller_fsuid = current_fsuid();
+       kgid_t caller_fsgid = current_fsgid();

        err = ovl_copy_up(parent);
        if (err)
@@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
        err = -ENOMEM;
        override_cred = prepare_creds();
        if (override_cred) {
-               override_cred->fsuid = inode->i_uid;
-               override_cred->fsgid = inode->i_gid;
+               override_cred->fsuid = caller_fsuid;
+               override_cred->fsgid = caller_fsgid;
                if (!attr->hardlink) {
                        err = security_dentry_create_files_as(dentry,
                                        attr->mode, &dentry->d_name, old_cred,

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Christian Brauner]

On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > >
> > > > > > ovl_link() did not create a new inode after commit
> > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > fsuid and fsgid when called by ovl_link().
> > > > > >
> > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > ---
> > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > >
> > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > --- a/fs/overlayfs/dir.c
> > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > >         err = -ENOMEM;
> > > > > >         override_cred = prepare_creds();
> > > > > >         if (override_cred) {
> > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > >                 if (!attr->hardlink) {
> > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > >                                         override_cred);
> > > > > > --
> > > > >
> > > > > This change looks incorrect.
> > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > be overridden for calling link() on underlying fs.
> > > > > What made you do this change?
> > > > >
> > > > > Thanks,
> > > > > Amir.
> > > >
> > > > Hi Amir,
> > > >
> > > > I ran into an error when I tested overlay on fuse:
> > > >   $ mkdir /lower /fuse /merge
> > > >   $ mount -t fuse /fuse
> > > >   $ mkdir /fuse/upper /fuse/work
> > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > >   $ touch /merge/file
> > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > >   $ ln /merge/file /merge/lnkfile
> > > >
> > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > caller is "bin", it denied this request.
> > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > assigned to the current user. So the override fsuid becomes the
> > > > current user. We know link() is actually modifying the directory, so
> > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > current caller may should have this permission. I think this is
> > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > conflicted with the overlay's design).
> > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > which removed the inode creation in ovl_link(). This commit move
> > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > creator! So I think this is incorrect.
> > > > I think the link() should be like unlink(), overlay fs should just use
> > > > the creator cred to do underlying fs's operations.
> > > >
> > >
> > > I see. The reproducer and explanation belong in the commit message.
> > >
> > > Your argument makes sense to me, but CC Christian to make
> > > sure I am not missing anything related to ACLs and what not.
> > 
> > Once again with correct email address...
> 
> So we have:
> 
> ovl_create_object()
> -> ovl_override_creds(ovl_sb)
> -> ovl_new_inode()
>    -> inode_init_owner()
>       {
>               inode->i_uid = current_fsuid();
>               inode->i_gid = current_fsgid();
>       }
> -> ovl_create_or_link(inode, ...)
> -> prepare_creds() // Copy of caller's creds

s/caller's/creator's/

> {
>         override_creds->fsuid = inode->i_uid;
>         override_creds->fsgid = inode->i_gid;
> }
> -> revert_creds()
> 
> which afaict means that the mounter's credentials are used apart from
> the fs{g,u}id which is taken from inode->i_{g,u}id which should
> correspond to current_fs{g,u}id().
> 
> The commit that is pointed out in the patch
> 51f7e52dc943 ("ovl: share inode for hard link")
> seems to have broken that assumption.
> 
> Given that the intention was to use the creator's creds _with the
> caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> the caller's fs{g,u}id are always used instead of using the full
> creator's creds just for the link operation? So something like this
> (untested):
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..4a3ee16a6d70 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>         const struct cred *old_cred;
>         struct cred *override_cred;
>         struct dentry *parent = dentry->d_parent;
> +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> +       kuid_t caller_fsuid = current_fsuid();
> +       kgid_t caller_fsgid = current_fsgid();
> 
>         err = ovl_copy_up(parent);
>         if (err)
> @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>         err = -ENOMEM;
>         override_cred = prepare_creds();
>         if (override_cred) {
> -               override_cred->fsuid = inode->i_uid;
> -               override_cred->fsgid = inode->i_gid;
> +               override_cred->fsuid = caller_fsuid;
> +               override_cred->fsgid = caller_fsgid;
>                 if (!attr->hardlink) {
>                         err = security_dentry_create_files_as(dentry,
>                                         attr->mode, &dentry->d_name, old_cred,

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[天赐张]

On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <brauner@kernel.org> wrote:
>
> On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > >
> > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > > >
> > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > >
> > > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > > ---
> > > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > >
> > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > >         err = -ENOMEM;
> > > > > > >         override_cred = prepare_creds();
> > > > > > >         if (override_cred) {
> > > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > >                 if (!attr->hardlink) {
> > > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > > >                                         override_cred);
> > > > > > > --
> > > > > >
> > > > > > This change looks incorrect.
> > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > be overridden for calling link() on underlying fs.
> > > > > > What made you do this change?
> > > > > >
> > > > > > Thanks,
> > > > > > Amir.
> > > > >
> > > > > Hi Amir,
> > > > >
> > > > > I ran into an error when I tested overlay on fuse:
> > > > >   $ mkdir /lower /fuse /merge
> > > > >   $ mount -t fuse /fuse
> > > > >   $ mkdir /fuse/upper /fuse/work
> > > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > >   $ touch /merge/file
> > > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > >   $ ln /merge/file /merge/lnkfile
> > > > >
> > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > caller is "bin", it denied this request.
> > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > assigned to the current user. So the override fsuid becomes the
> > > > > current user. We know link() is actually modifying the directory, so
> > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > current caller may should have this permission. I think this is
> > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > conflicted with the overlay's design).
> > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > creator! So I think this is incorrect.
> > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > the creator cred to do underlying fs's operations.
> > > > >
> > > >
> > > > I see. The reproducer and explanation belong in the commit message.
> > > >
> > > > Your argument makes sense to me, but CC Christian to make
> > > > sure I am not missing anything related to ACLs and what not.
> > >
> > > Once again with correct email address...
> >
> > So we have:
> >
> > ovl_create_object()
> > -> ovl_override_creds(ovl_sb)
> > -> ovl_new_inode()
> >    -> inode_init_owner()
> >       {
> >               inode->i_uid = current_fsuid();
> >               inode->i_gid = current_fsgid();

In inode_init_owner(), the inode->i_gid may inherit from parent dir.
And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
sgid on directory").

> >       }
> > -> ovl_create_or_link(inode, ...)
> > -> prepare_creds() // Copy of caller's creds
>
> s/caller's/creator's/
>
> > {
> >         override_creds->fsuid = inode->i_uid;
> >         override_creds->fsgid = inode->i_gid;
> > }
> > -> revert_creds()
> >
> > which afaict means that the mounter's credentials are used apart from
> > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > correspond to current_fs{g,u}id().
> >
> > The commit that is pointed out in the patch
> > 51f7e52dc943 ("ovl: share inode for hard link")
> > seems to have broken that assumption.
> >
> > Given that the intention was to use the creator's creds _with the
> > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > the caller's fs{g,u}id are always used instead of using the full
> > creator's creds just for the link operation? So something like this
> > (untested):
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..4a3ee16a6d70 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> >         const struct cred *old_cred;
> >         struct cred *override_cred;
> >         struct dentry *parent = dentry->d_parent;
> > +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> > +       kuid_t caller_fsuid = current_fsuid();
> > +       kgid_t caller_fsgid = current_fsgid();
> >
> >         err = ovl_copy_up(parent);
> >         if (err)
> > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> >         err = -ENOMEM;
> >         override_cred = prepare_creds();
> >         if (override_cred) {
> > -               override_cred->fsuid = inode->i_uid;
> > -               override_cred->fsgid = inode->i_gid;
> > +               override_cred->fsuid = caller_fsuid;
> > +               override_cred->fsgid = caller_fsgid;

So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.

> >                 if (!attr->hardlink) {
> >                         err = security_dentry_create_files_as(dentry,
> >                                         attr->mode, &dentry->d_name, old_cred,

So your meaning should be like this:

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..9aead6ddc071 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
        const struct cred *old_cred;
        struct cred *override_cred;
        struct dentry *parent = dentry->d_parent;
+       kuid_t caller_fsuid = current_fsuid();
+       kgid_t caller_fsgid = current_fsgid();

        err = ovl_copy_up(parent);
        if (err)
@@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
        err = -ENOMEM;
        override_cred = prepare_creds();
        if (override_cred) {
-               override_cred->fsuid = inode->i_uid;
-               override_cred->fsgid = inode->i_gid;
                if (!attr->hardlink) {
+                       override_cred->fsuid = inode->i_uid;
+                       override_cred->fsgid = inode->i_gid;
                        err = security_dentry_create_files_as(dentry,
                                        attr->mode, &dentry->d_name, old_cred,
                                        override_cred);
@@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
*dentry, struct inode *inode,
                                put_cred(override_cred);
                                goto out_revert_creds;
                        }
+               } else {
+                       override_cred->fsuid = caller_fsuid;
+                       override_cred->fsgid = caller_fsgid;
                }
                put_cred(override_creds(override_cred));
                put_cred(override_cred);

As I said before, I think this is acceptable to use the caller's
fsuid. But I still feel a little conflicted with the overlay's design.
Because I am not sure if there should be some difference between
link() and unlink() after commit 51f7e52dc943 ("ovl: share inode for
hard link") remove the creation in ovl_link().

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Christian Brauner]

On Wed, Aug 17, 2022 at 07:36:44PM +0800, 天赐张 wrote:
> On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <brauner@kernel.org> wrote:
> >
> > On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > > > >
> > > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > >
> > > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > >
> > > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > > >
> > > > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > > > ---
> > > > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > >
> > > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > > >         err = -ENOMEM;
> > > > > > > >         override_cred = prepare_creds();
> > > > > > > >         if (override_cred) {
> > > > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > > >                 if (!attr->hardlink) {
> > > > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > > > >                                         override_cred);
> > > > > > > > --
> > > > > > >
> > > > > > > This change looks incorrect.
> > > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > > be overridden for calling link() on underlying fs.
> > > > > > > What made you do this change?
> > > > > > >
> > > > > > > Thanks,
> > > > > > > Amir.
> > > > > >
> > > > > > Hi Amir,
> > > > > >
> > > > > > I ran into an error when I tested overlay on fuse:
> > > > > >   $ mkdir /lower /fuse /merge
> > > > > >   $ mount -t fuse /fuse
> > > > > >   $ mkdir /fuse/upper /fuse/work
> > > > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > > >   $ touch /merge/file
> > > > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > > >   $ ln /merge/file /merge/lnkfile
> > > > > >
> > > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > > caller is "bin", it denied this request.
> > > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > > assigned to the current user. So the override fsuid becomes the
> > > > > > current user. We know link() is actually modifying the directory, so
> > > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > > current caller may should have this permission. I think this is
> > > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > > conflicted with the overlay's design).
> > > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > > creator! So I think this is incorrect.
> > > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > > the creator cred to do underlying fs's operations.
> > > > > >
> > > > >
> > > > > I see. The reproducer and explanation belong in the commit message.
> > > > >
> > > > > Your argument makes sense to me, but CC Christian to make
> > > > > sure I am not missing anything related to ACLs and what not.
> > > >
> > > > Once again with correct email address...
> > >
> > > So we have:
> > >
> > > ovl_create_object()
> > > -> ovl_override_creds(ovl_sb)
> > > -> ovl_new_inode()
> > >    -> inode_init_owner()
> > >       {
> > >               inode->i_uid = current_fsuid();
> > >               inode->i_gid = current_fsgid();
> 
> In inode_init_owner(), the inode->i_gid may inherit from parent dir.
> And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
> sgid on directory").
> 
> > >       }
> > > -> ovl_create_or_link(inode, ...)
> > > -> prepare_creds() // Copy of caller's creds
> >
> > s/caller's/creator's/
> >
> > > {
> > >         override_creds->fsuid = inode->i_uid;
> > >         override_creds->fsgid = inode->i_gid;
> > > }
> > > -> revert_creds()
> > >
> > > which afaict means that the mounter's credentials are used apart from
> > > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > > correspond to current_fs{g,u}id().
> > >
> > > The commit that is pointed out in the patch
> > > 51f7e52dc943 ("ovl: share inode for hard link")
> > > seems to have broken that assumption.
> > >
> > > Given that the intention was to use the creator's creds _with the
> > > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > > the caller's fs{g,u}id are always used instead of using the full
> > > creator's creds just for the link operation? So something like this
> > > (untested):
> > >
> > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > index 6b03457f72bb..4a3ee16a6d70 100644
> > > --- a/fs/overlayfs/dir.c
> > > +++ b/fs/overlayfs/dir.c
> > > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > >         const struct cred *old_cred;
> > >         struct cred *override_cred;
> > >         struct dentry *parent = dentry->d_parent;
> > > +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> > > +       kuid_t caller_fsuid = current_fsuid();
> > > +       kgid_t caller_fsgid = current_fsgid();
> > >
> > >         err = ovl_copy_up(parent);
> > >         if (err)
> > > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > >         err = -ENOMEM;
> > >         override_cred = prepare_creds();
> > >         if (override_cred) {
> > > -               override_cred->fsuid = inode->i_uid;
> > > -               override_cred->fsgid = inode->i_gid;
> > > +               override_cred->fsuid = caller_fsuid;
> > > +               override_cred->fsgid = caller_fsgid;
> 
> So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.
> 
> > >                 if (!attr->hardlink) {
> > >                         err = security_dentry_create_files_as(dentry,
> > >                                         attr->mode, &dentry->d_name, old_cred,
> 
> So your meaning should be like this:
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..9aead6ddc071 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
> *dentry, struct inode *inode,
>         const struct cred *old_cred;
>         struct cred *override_cred;
>         struct dentry *parent = dentry->d_parent;
> +       kuid_t caller_fsuid = current_fsuid();
> +       kgid_t caller_fsgid = current_fsgid();
> 
>         err = ovl_copy_up(parent);
>         if (err)
> @@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
> *dentry, struct inode *inode,
>         err = -ENOMEM;
>         override_cred = prepare_creds();
>         if (override_cred) {
> -               override_cred->fsuid = inode->i_uid;
> -               override_cred->fsgid = inode->i_gid;
>                 if (!attr->hardlink) {
> +                       override_cred->fsuid = inode->i_uid;
> +                       override_cred->fsgid = inode->i_gid;
>                         err = security_dentry_create_files_as(dentry,
>                                         attr->mode, &dentry->d_name, old_cred,
>                                         override_cred);
> @@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
> *dentry, struct inode *inode,
>                                 put_cred(override_cred);
>                                 goto out_revert_creds;
>                         }
> +               } else {
> +                       override_cred->fsuid = caller_fsuid;
> +                       override_cred->fsgid = caller_fsgid;
>                 }
>                 put_cred(override_creds(override_cred));
>                 put_cred(override_cred);

Hah, wait. I had a pretty obvious braino when I did that. I forgot to
account for setgid handling in inode_init_owner(). Let me take another
close look...

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Christian Brauner]

On Wed, Aug 17, 2022 at 01:56:43PM +0200, Christian Brauner wrote:
> On Wed, Aug 17, 2022 at 07:36:44PM +0800, 天赐张 wrote:
> > On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <brauner@kernel.org> wrote:
> > >
> > > On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > > > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > >
> > > > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > > > > >
> > > > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > > >
> > > > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > > > >
> > > > > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > > > > ---
> > > > > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > > >
> > > > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > > > >         err = -ENOMEM;
> > > > > > > > >         override_cred = prepare_creds();
> > > > > > > > >         if (override_cred) {
> > > > > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > > > >                 if (!attr->hardlink) {
> > > > > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > > > > >                                         override_cred);
> > > > > > > > > --
> > > > > > > >
> > > > > > > > This change looks incorrect.
> > > > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > > > be overridden for calling link() on underlying fs.
> > > > > > > > What made you do this change?
> > > > > > > >
> > > > > > > > Thanks,
> > > > > > > > Amir.
> > > > > > >
> > > > > > > Hi Amir,
> > > > > > >
> > > > > > > I ran into an error when I tested overlay on fuse:
> > > > > > >   $ mkdir /lower /fuse /merge
> > > > > > >   $ mount -t fuse /fuse
> > > > > > >   $ mkdir /fuse/upper /fuse/work
> > > > > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > > > >   $ touch /merge/file
> > > > > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > > > >   $ ln /merge/file /merge/lnkfile
> > > > > > >
> > > > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > > > caller is "bin", it denied this request.
> > > > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > > > assigned to the current user. So the override fsuid becomes the
> > > > > > > current user. We know link() is actually modifying the directory, so
> > > > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > > > current caller may should have this permission. I think this is
> > > > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > > > conflicted with the overlay's design).
> > > > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > > > creator! So I think this is incorrect.
> > > > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > > > the creator cred to do underlying fs's operations.
> > > > > > >
> > > > > >
> > > > > > I see. The reproducer and explanation belong in the commit message.
> > > > > >
> > > > > > Your argument makes sense to me, but CC Christian to make
> > > > > > sure I am not missing anything related to ACLs and what not.
> > > > >
> > > > > Once again with correct email address...
> > > >
> > > > So we have:
> > > >
> > > > ovl_create_object()
> > > > -> ovl_override_creds(ovl_sb)
> > > > -> ovl_new_inode()
> > > >    -> inode_init_owner()
> > > >       {
> > > >               inode->i_uid = current_fsuid();
> > > >               inode->i_gid = current_fsgid();
> > 
> > In inode_init_owner(), the inode->i_gid may inherit from parent dir.
> > And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
> > sgid on directory").
> > 
> > > >       }
> > > > -> ovl_create_or_link(inode, ...)
> > > > -> prepare_creds() // Copy of caller's creds
> > >
> > > s/caller's/creator's/
> > >
> > > > {
> > > >         override_creds->fsuid = inode->i_uid;
> > > >         override_creds->fsgid = inode->i_gid;
> > > > }
> > > > -> revert_creds()
> > > >
> > > > which afaict means that the mounter's credentials are used apart from
> > > > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > > > correspond to current_fs{g,u}id().
> > > >
> > > > The commit that is pointed out in the patch
> > > > 51f7e52dc943 ("ovl: share inode for hard link")
> > > > seems to have broken that assumption.
> > > >
> > > > Given that the intention was to use the creator's creds _with the
> > > > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > > > the caller's fs{g,u}id are always used instead of using the full
> > > > creator's creds just for the link operation? So something like this
> > > > (untested):
> > > >
> > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > index 6b03457f72bb..4a3ee16a6d70 100644
> > > > --- a/fs/overlayfs/dir.c
> > > > +++ b/fs/overlayfs/dir.c
> > > > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > >         const struct cred *old_cred;
> > > >         struct cred *override_cred;
> > > >         struct dentry *parent = dentry->d_parent;
> > > > +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> > > > +       kuid_t caller_fsuid = current_fsuid();
> > > > +       kgid_t caller_fsgid = current_fsgid();
> > > >
> > > >         err = ovl_copy_up(parent);
> > > >         if (err)
> > > > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > >         err = -ENOMEM;
> > > >         override_cred = prepare_creds();
> > > >         if (override_cred) {
> > > > -               override_cred->fsuid = inode->i_uid;
> > > > -               override_cred->fsgid = inode->i_gid;
> > > > +               override_cred->fsuid = caller_fsuid;
> > > > +               override_cred->fsgid = caller_fsgid;
> > 
> > So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.
> > 
> > > >                 if (!attr->hardlink) {
> > > >                         err = security_dentry_create_files_as(dentry,
> > > >                                         attr->mode, &dentry->d_name, old_cred,
> > 
> > So your meaning should be like this:
> > 
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..9aead6ddc071 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
> > *dentry, struct inode *inode,
> >         const struct cred *old_cred;
> >         struct cred *override_cred;
> >         struct dentry *parent = dentry->d_parent;
> > +       kuid_t caller_fsuid = current_fsuid();
> > +       kgid_t caller_fsgid = current_fsgid();
> > 
> >         err = ovl_copy_up(parent);
> >         if (err)
> > @@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
> > *dentry, struct inode *inode,
> >         err = -ENOMEM;
> >         override_cred = prepare_creds();
> >         if (override_cred) {
> > -               override_cred->fsuid = inode->i_uid;
> > -               override_cred->fsgid = inode->i_gid;
> >                 if (!attr->hardlink) {
> > +                       override_cred->fsuid = inode->i_uid;
> > +                       override_cred->fsgid = inode->i_gid;
> >                         err = security_dentry_create_files_as(dentry,
> >                                         attr->mode, &dentry->d_name, old_cred,
> >                                         override_cred);
> > @@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
> > *dentry, struct inode *inode,
> >                                 put_cred(override_cred);
> >                                 goto out_revert_creds;
> >                         }
> > +               } else {
> > +                       override_cred->fsuid = caller_fsuid;
> > +                       override_cred->fsgid = caller_fsgid;
> >                 }
> >                 put_cred(override_creds(override_cred));
> >                 put_cred(override_cred);
> 
> Hah, wait. I had a pretty obvious braino when I did that. I forgot to
> account for setgid handling in inode_init_owner(). Let me take another
> close look...

Ok, so if we look at all the callchains:

(1) .create = ovl_create()
    -> ovl_create_object()
       -> ovl_create_or_link()
   
(2) .mkdir = ovl_mkdir()
    -> ovl_create_object()
       -> ovl_create_or_link()
   
(3) .mknod = ovl_mknod()
    -> ovl_create_object()
       -> ovl_create_or_link()
   
(4) .symlink = ovl_symlink()
    -> ovl_create_object()
       -> ovl_create_or_link()

(5) .link = ovl_link()
     -> ovl_create_or_link()

we see that (1) to (4) set the caller's fs{g,u}id to the
inode->i_{g,u}id. That was instantiated by inode_init_owner(). This will
be current_fs{g,u}id() of the caller unless the directory the file is
created in was setgid in which case inode->i_gid can be different from
current_fsgid().

So iiuc, taking the inode->i_{g,u}id is done to get fsgid set to
inode->i_gid when the parent directory is setgid.

But for (5) the inode->i_{g,u}id should be irrelevant because it's a
hardlink and so we should just use the caller's fs{g,u}id. So with all
that in mind we should be doing:

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..74b3813eda47 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
        inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
        attr.mode = inode->i_mode;

+       attr.fsuid = inode->i_uid;
+       attr.fsgid = inode->i_gid;
        err = ovl_create_or_link(dentry, inode, &attr, false);
        /* Did we end up using the preallocated inode? */
        if (inode != d_inode(dentry))
@@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
 {
        int err;
        struct inode *inode;
+       struct ovl_cattr attr;

        err = ovl_want_write(old);
        if (err)
@@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
        inode = d_inode(old);
        ihold(inode);

-       err = ovl_create_or_link(new, inode,
-                       &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
-                       ovl_type_origin(old));
+       attr = (struct ovl_cattr){
+               .hardlink = ovl_dentry_upper(old),
+               .fsuid = current_fsuid(),
+               .fsgid = current_fsgid(),
+       };
+       err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
        if (err)
                iput(inode);

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 87759165d32b..85043123a103 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -655,6 +655,8 @@ struct ovl_cattr {
        umode_t mode;
        const char *link;
        struct dentry *hardlink;
+       kuid_t fsuid;
+       kgid_t fsgid;
 };

 #define OVL_CATTR(m) (&(struct ovl_cattr) { .mode = (m) })

Using the full creator's credentials for just hardlinks seems odd to me
when all creation requests use the caller's fs{g,u}id...

Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[Christian Brauner]

On Wed, Aug 17, 2022 at 02:29:20PM +0200, Christian Brauner wrote:
> On Wed, Aug 17, 2022 at 01:56:43PM +0200, Christian Brauner wrote:
> > On Wed, Aug 17, 2022 at 07:36:44PM +0800, 天赐张 wrote:
> > > On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <brauner@kernel.org> wrote:
> > > >
> > > > On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > > > > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > > > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > >
> > > > > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > >
> > > > > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > > > >
> > > > > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > > > > > ---
> > > > > > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > > > >
> > > > > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > > > > >         err = -ENOMEM;
> > > > > > > > > >         override_cred = prepare_creds();
> > > > > > > > > >         if (override_cred) {
> > > > > > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > > > > >                 if (!attr->hardlink) {
> > > > > > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > > > > > >                                         override_cred);
> > > > > > > > > > --
> > > > > > > > >
> > > > > > > > > This change looks incorrect.
> > > > > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > > > > be overridden for calling link() on underlying fs.
> > > > > > > > > What made you do this change?
> > > > > > > > >
> > > > > > > > > Thanks,
> > > > > > > > > Amir.
> > > > > > > >
> > > > > > > > Hi Amir,
> > > > > > > >
> > > > > > > > I ran into an error when I tested overlay on fuse:
> > > > > > > >   $ mkdir /lower /fuse /merge
> > > > > > > >   $ mount -t fuse /fuse
> > > > > > > >   $ mkdir /fuse/upper /fuse/work
> > > > > > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > > > > >   $ touch /merge/file
> > > > > > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > > > > >   $ ln /merge/file /merge/lnkfile
> > > > > > > >
> > > > > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > > > > caller is "bin", it denied this request.
> > > > > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > > > > assigned to the current user. So the override fsuid becomes the
> > > > > > > > current user. We know link() is actually modifying the directory, so
> > > > > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > > > > current caller may should have this permission. I think this is
> > > > > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > > > > conflicted with the overlay's design).
> > > > > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > > > > creator! So I think this is incorrect.
> > > > > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > > > > the creator cred to do underlying fs's operations.
> > > > > > > >
> > > > > > >
> > > > > > > I see. The reproducer and explanation belong in the commit message.
> > > > > > >
> > > > > > > Your argument makes sense to me, but CC Christian to make
> > > > > > > sure I am not missing anything related to ACLs and what not.
> > > > > >
> > > > > > Once again with correct email address...
> > > > >
> > > > > So we have:
> > > > >
> > > > > ovl_create_object()
> > > > > -> ovl_override_creds(ovl_sb)
> > > > > -> ovl_new_inode()
> > > > >    -> inode_init_owner()
> > > > >       {
> > > > >               inode->i_uid = current_fsuid();
> > > > >               inode->i_gid = current_fsgid();
> > > 
> > > In inode_init_owner(), the inode->i_gid may inherit from parent dir.
> > > And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
> > > sgid on directory").
> > > 
> > > > >       }
> > > > > -> ovl_create_or_link(inode, ...)
> > > > > -> prepare_creds() // Copy of caller's creds
> > > >
> > > > s/caller's/creator's/
> > > >
> > > > > {
> > > > >         override_creds->fsuid = inode->i_uid;
> > > > >         override_creds->fsgid = inode->i_gid;
> > > > > }
> > > > > -> revert_creds()
> > > > >
> > > > > which afaict means that the mounter's credentials are used apart from
> > > > > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > > > > correspond to current_fs{g,u}id().
> > > > >
> > > > > The commit that is pointed out in the patch
> > > > > 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > seems to have broken that assumption.
> > > > >
> > > > > Given that the intention was to use the creator's creds _with the
> > > > > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > > > > the caller's fs{g,u}id are always used instead of using the full
> > > > > creator's creds just for the link operation? So something like this
> > > > > (untested):
> > > > >
> > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > index 6b03457f72bb..4a3ee16a6d70 100644
> > > > > --- a/fs/overlayfs/dir.c
> > > > > +++ b/fs/overlayfs/dir.c
> > > > > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > >         const struct cred *old_cred;
> > > > >         struct cred *override_cred;
> > > > >         struct dentry *parent = dentry->d_parent;
> > > > > +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> > > > > +       kuid_t caller_fsuid = current_fsuid();
> > > > > +       kgid_t caller_fsgid = current_fsgid();
> > > > >
> > > > >         err = ovl_copy_up(parent);
> > > > >         if (err)
> > > > > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > >         err = -ENOMEM;
> > > > >         override_cred = prepare_creds();
> > > > >         if (override_cred) {
> > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > +               override_cred->fsuid = caller_fsuid;
> > > > > +               override_cred->fsgid = caller_fsgid;
> > > 
> > > So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.
> > > 
> > > > >                 if (!attr->hardlink) {
> > > > >                         err = security_dentry_create_files_as(dentry,
> > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > 
> > > So your meaning should be like this:
> > > 
> > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > index 6b03457f72bb..9aead6ddc071 100644
> > > --- a/fs/overlayfs/dir.c
> > > +++ b/fs/overlayfs/dir.c
> > > @@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
> > > *dentry, struct inode *inode,
> > >         const struct cred *old_cred;
> > >         struct cred *override_cred;
> > >         struct dentry *parent = dentry->d_parent;
> > > +       kuid_t caller_fsuid = current_fsuid();
> > > +       kgid_t caller_fsgid = current_fsgid();
> > > 
> > >         err = ovl_copy_up(parent);
> > >         if (err)
> > > @@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
> > > *dentry, struct inode *inode,
> > >         err = -ENOMEM;
> > >         override_cred = prepare_creds();
> > >         if (override_cred) {
> > > -               override_cred->fsuid = inode->i_uid;
> > > -               override_cred->fsgid = inode->i_gid;
> > >                 if (!attr->hardlink) {
> > > +                       override_cred->fsuid = inode->i_uid;
> > > +                       override_cred->fsgid = inode->i_gid;
> > >                         err = security_dentry_create_files_as(dentry,
> > >                                         attr->mode, &dentry->d_name, old_cred,
> > >                                         override_cred);
> > > @@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
> > > *dentry, struct inode *inode,
> > >                                 put_cred(override_cred);
> > >                                 goto out_revert_creds;
> > >                         }
> > > +               } else {
> > > +                       override_cred->fsuid = caller_fsuid;
> > > +                       override_cred->fsgid = caller_fsgid;
> > >                 }
> > >                 put_cred(override_creds(override_cred));
> > >                 put_cred(override_cred);
> > 
> > Hah, wait. I had a pretty obvious braino when I did that. I forgot to
> > account for setgid handling in inode_init_owner(). Let me take another
> > close look...
> 
> Ok, so if we look at all the callchains:
> 
> (1) .create = ovl_create()
>     -> ovl_create_object()
>        -> ovl_create_or_link()
>    
> (2) .mkdir = ovl_mkdir()
>     -> ovl_create_object()
>        -> ovl_create_or_link()
>    
> (3) .mknod = ovl_mknod()
>     -> ovl_create_object()
>        -> ovl_create_or_link()
>    
> (4) .symlink = ovl_symlink()
>     -> ovl_create_object()
>        -> ovl_create_or_link()
> 
> (5) .link = ovl_link()
>      -> ovl_create_or_link()
> 
> we see that (1) to (4) set the caller's fs{g,u}id to the
> inode->i_{g,u}id. That was instantiated by inode_init_owner(). This will
> be current_fs{g,u}id() of the caller unless the directory the file is
> created in was setgid in which case inode->i_gid can be different from
> current_fsgid().
> 
> So iiuc, taking the inode->i_{g,u}id is done to get fsgid set to
> inode->i_gid when the parent directory is setgid.
> 
> But for (5) the inode->i_{g,u}id should be irrelevant because it's a
> hardlink and so we should just use the caller's fs{g,u}id. So with all
> that in mind we should be doing:
> 
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..74b3813eda47 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
>         inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
>         attr.mode = inode->i_mode;
> 
> +       attr.fsuid = inode->i_uid;
> +       attr.fsgid = inode->i_gid;
>         err = ovl_create_or_link(dentry, inode, &attr, false);
>         /* Did we end up using the preallocated inode? */
>         if (inode != d_inode(dentry))
> @@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
>  {
>         int err;
>         struct inode *inode;
> +       struct ovl_cattr attr;
> 
>         err = ovl_want_write(old);
>         if (err)
> @@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
>         inode = d_inode(old);
>         ihold(inode);
> 
> -       err = ovl_create_or_link(new, inode,
> -                       &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
> -                       ovl_type_origin(old));
> +       attr = (struct ovl_cattr){
> +               .hardlink = ovl_dentry_upper(old),
> +               .fsuid = current_fsuid(),
> +               .fsgid = current_fsgid(),
> +       };
> +       err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
>         if (err)
>                 iput(inode);
> 
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 87759165d32b..85043123a103 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -655,6 +655,8 @@ struct ovl_cattr {
>         umode_t mode;
>         const char *link;
>         struct dentry *hardlink;
> +       kuid_t fsuid;
> +       kgid_t fsgid;
>  };
> 
>  #define OVL_CATTR(m) (&(struct ovl_cattr) { .mode = (m) })
> 
> Using the full creator's credentials for just hardlinks seems odd to me
> when all creation requests use the caller's fs{g,u}id...

Sigh, omitted something in the patch:

diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
index 6b03457f72bb..cf4dc303fa4d 100644
--- a/fs/overlayfs/dir.c
+++ b/fs/overlayfs/dir.c
@@ -595,8 +595,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
        err = -ENOMEM;
        override_cred = prepare_creds();
        if (override_cred) {
-               override_cred->fsuid = inode->i_uid;
-               override_cred->fsgid = inode->i_gid;
+               override_cred->fsuid = attr->fsuid;
+               override_cred->fsgid = attr->fsgid;
                if (!attr->hardlink) {
                        err = security_dentry_create_files_as(dentry,
                                        attr->mode, &dentry->d_name, old_cred,
@@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
        inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
        attr.mode = inode->i_mode;

+       attr.fsuid = inode->i_uid;
+       attr.fsgid = inode->i_gid;
        err = ovl_create_or_link(dentry, inode, &attr, false);
        /* Did we end up using the preallocated inode? */
        if (inode != d_inode(dentry))
@@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
 {
        int err;
        struct inode *inode;
+       struct ovl_cattr attr;

        err = ovl_want_write(old);
        if (err)
@@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
        inode = d_inode(old);
        ihold(inode);

-       err = ovl_create_or_link(new, inode,
-                       &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
-                       ovl_type_origin(old));
+       attr = (struct ovl_cattr){
+               .hardlink = ovl_dentry_upper(old),
+               .fsuid = current_fsuid(),
+               .fsgid = current_fsgid(),
+       };
+       err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
        if (err)
                iput(inode);

diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
index 87759165d32b..85043123a103 100644
--- a/fs/overlayfs/overlayfs.h
+++ b/fs/overlayfs/overlayfs.h
@@ -655,6 +655,8 @@ struct ovl_cattr {
        umode_t mode;
        const char *link;
        struct dentry *hardlink;
+       kuid_t fsuid;
+       kgid_t fsgid;
 };

Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()

[天赐张]

Christian Brauner <brauner@kernel.org> 于2022年8月17日周三 20:37写道：
>
> On Wed, Aug 17, 2022 at 02:29:20PM +0200, Christian Brauner wrote:
> > On Wed, Aug 17, 2022 at 01:56:43PM +0200, Christian Brauner wrote:
> > > On Wed, Aug 17, 2022 at 07:36:44PM +0800, 天赐张 wrote:
> > > > On Wed, Aug 17, 2022 at 6:29 PM Christian Brauner <brauner@kernel.org> wrote:
> > > > >
> > > > > On Wed, Aug 17, 2022 at 12:27:27PM +0200, Christian Brauner wrote:
> > > > > > On Wed, Aug 17, 2022 at 12:55:22PM +0300, Amir Goldstein wrote:
> > > > > > > On Wed, Aug 17, 2022 at 12:53 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > > >
> > > > > > > > On Wed, Aug 17, 2022 at 12:11 PM 天赐张 <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, Aug 17, 2022 at 3:36 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > > > > > > > >
> > > > > > > > > > On Wed, Aug 17, 2022 at 6:49 AM Zhang Tianci
> > > > > > > > > > <zhangtianci.1997@bytedance.com> wrote:
> > > > > > > > > > >
> > > > > > > > > > > ovl_link() did not create a new inode after commit
> > > > > > > > > > > 51f7e52dc943 ("ovl: share inode for hard link"), so
> > > > > > > > > > > in ovl_create_or_link() we should not override cred's
> > > > > > > > > > > fsuid and fsgid when called by ovl_link().
> > > > > > > > > > >
> > > > > > > > > > > Signed-off-by: Zhang Tianci <zhangtianci.1997@bytedance.com>
> > > > > > > > > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > > > > > > > > ---
> > > > > > > > > > >  fs/overlayfs/dir.c | 4 ++--
> > > > > > > > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > > > > > > >
> > > > > > > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > > > > > > index 6b03457f72bb..568d338032db 100644
> > > > > > > > > > > --- a/fs/overlayfs/dir.c
> > > > > > > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > > > > > > @@ -595,9 +595,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > > > > > > >         err = -ENOMEM;
> > > > > > > > > > >         override_cred = prepare_creds();
> > > > > > > > > > >         if (override_cred) {
> > > > > > > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > > > > > >                 if (!attr->hardlink) {
> > > > > > > > > > > +                       override_cred->fsuid = inode->i_uid;
> > > > > > > > > > > +                       override_cred->fsgid = inode->i_gid;
> > > > > > > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > > > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > > > > > > > > >                                         override_cred);
> > > > > > > > > > > --
> > > > > > > > > >
> > > > > > > > > > This change looks incorrect.
> > > > > > > > > > Unless I am missing something, fsuid/fsgid still need to
> > > > > > > > > > be overridden for calling link() on underlying fs.
> > > > > > > > > > What made you do this change?
> > > > > > > > > >
> > > > > > > > > > Thanks,
> > > > > > > > > > Amir.
> > > > > > > > >
> > > > > > > > > Hi Amir,
> > > > > > > > >
> > > > > > > > > I ran into an error when I tested overlay on fuse:
> > > > > > > > >   $ mkdir /lower /fuse /merge
> > > > > > > > >   $ mount -t fuse /fuse
> > > > > > > > >   $ mkdir /fuse/upper /fuse/work
> > > > > > > > >   $ mount -t overlay /merge -o lowerdir=/lower,upperdir=/fuse/upper,workdir=work
> > > > > > > > >   $ touch /merge/file
> > > > > > > > >   $ chown bin.bin /merge/file // the file's caller becomes "bin"
> > > > > > > > >   $ ln /merge/file /merge/lnkfile
> > > > > > > > >
> > > > > > > > > Then I got an error(EACCES) because fuse daemon checks the link()'s
> > > > > > > > > caller is "bin", it denied this request.
> > > > > > > > > I browsed the changing history of ovl_link(). There are two key commits:
> > > > > > > > > The first is commit bb0d2b8ad296 ("ovl: fix sgid on directory") which
> > > > > > > > > overrides the cred's fsuid/fsgid using the new inode. The new inode's
> > > > > > > > > owner is initialized by inode_init_owner(), and inode->fsuid is
> > > > > > > > > assigned to the current user. So the override fsuid becomes the
> > > > > > > > > current user. We know link() is actually modifying the directory, so
> > > > > > > > > the caller must have the MAY_WRITE permission on the directory. The
> > > > > > > > > current caller may should have this permission. I think this is
> > > > > > > > > acceptable to use the caller's fsuid(But I still feel a little
> > > > > > > > > conflicted with the overlay's design).
> > > > > > > > > The second is commit 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > > > > > which removed the inode creation in ovl_link(). This commit move
> > > > > > > > > inode_init_owner() into ovl_create_object(), so the ovl_link() just
> > > > > > > > > give the old inode to ovl_create_or_link(). Then the override fsuid
> > > > > > > > > becomes the old inode's fsuid, neither the caller nor the overlay's
> > > > > > > > > creator! So I think this is incorrect.
> > > > > > > > > I think the link() should be like unlink(), overlay fs should just use
> > > > > > > > > the creator cred to do underlying fs's operations.
> > > > > > > > >
> > > > > > > >
> > > > > > > > I see. The reproducer and explanation belong in the commit message.
> > > > > > > >
> > > > > > > > Your argument makes sense to me, but CC Christian to make
> > > > > > > > sure I am not missing anything related to ACLs and what not.
> > > > > > >
> > > > > > > Once again with correct email address...
> > > > > >
> > > > > > So we have:
> > > > > >
> > > > > > ovl_create_object()
> > > > > > -> ovl_override_creds(ovl_sb)
> > > > > > -> ovl_new_inode()
> > > > > >    -> inode_init_owner()
> > > > > >       {
> > > > > >               inode->i_uid = current_fsuid();
> > > > > >               inode->i_gid = current_fsgid();
> > > >
> > > > In inode_init_owner(), the inode->i_gid may inherit from parent dir.
> > > > And this is the main purpose of the commit bb0d2b8ad296 ("ovl: fix
> > > > sgid on directory").
> > > >
> > > > > >       }
> > > > > > -> ovl_create_or_link(inode, ...)
> > > > > > -> prepare_creds() // Copy of caller's creds
> > > > >
> > > > > s/caller's/creator's/
> > > > >
> > > > > > {
> > > > > >         override_creds->fsuid = inode->i_uid;
> > > > > >         override_creds->fsgid = inode->i_gid;
> > > > > > }
> > > > > > -> revert_creds()
> > > > > >
> > > > > > which afaict means that the mounter's credentials are used apart from
> > > > > > the fs{g,u}id which is taken from inode->i_{g,u}id which should
> > > > > > correspond to current_fs{g,u}id().
> > > > > >
> > > > > > The commit that is pointed out in the patch
> > > > > > 51f7e52dc943 ("ovl: share inode for hard link")
> > > > > > seems to have broken that assumption.
> > > > > >
> > > > > > Given that the intention was to use the creator's creds _with the
> > > > > > caller's fs{g,u}id_ wouldn't it make more sense to simply ensure that
> > > > > > the caller's fs{g,u}id are always used instead of using the full
> > > > > > creator's creds just for the link operation? So something like this
> > > > > > (untested):
> > > > > >
> > > > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > > > index 6b03457f72bb..4a3ee16a6d70 100644
> > > > > > --- a/fs/overlayfs/dir.c
> > > > > > +++ b/fs/overlayfs/dir.c
> > > > > > @@ -575,6 +575,9 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > >         const struct cred *old_cred;
> > > > > >         struct cred *override_cred;
> > > > > >         struct dentry *parent = dentry->d_parent;
> > > > > > +       /* Retrieve caller's fs{g,u}id before we override creds below. */
> > > > > > +       kuid_t caller_fsuid = current_fsuid();
> > > > > > +       kgid_t caller_fsgid = current_fsgid();
> > > > > >
> > > > > >         err = ovl_copy_up(parent);
> > > > > >         if (err)
> > > > > > @@ -595,8 +598,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
> > > > > >         err = -ENOMEM;
> > > > > >         override_cred = prepare_creds();
> > > > > >         if (override_cred) {
> > > > > > -               override_cred->fsuid = inode->i_uid;
> > > > > > -               override_cred->fsgid = inode->i_gid;
> > > > > > +               override_cred->fsuid = caller_fsuid;
> > > > > > +               override_cred->fsgid = caller_fsgid;
> > > >
> > > > So the override_cred->fsgid should be inode->i_gid if the inode is a new inode.
> > > >
> > > > > >                 if (!attr->hardlink) {
> > > > > >                         err = security_dentry_create_files_as(dentry,
> > > > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > >
> > > > So your meaning should be like this:
> > > >
> > > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > > index 6b03457f72bb..9aead6ddc071 100644
> > > > --- a/fs/overlayfs/dir.c
> > > > +++ b/fs/overlayfs/dir.c
> > > > @@ -575,6 +575,8 @@ static int ovl_create_or_link(struct dentry
> > > > *dentry, struct inode *inode,
> > > >         const struct cred *old_cred;
> > > >         struct cred *override_cred;
> > > >         struct dentry *parent = dentry->d_parent;
> > > > +       kuid_t caller_fsuid = current_fsuid();
> > > > +       kgid_t caller_fsgid = current_fsgid();
> > > >
> > > >         err = ovl_copy_up(parent);
> > > >         if (err)
> > > > @@ -595,9 +597,9 @@ static int ovl_create_or_link(struct dentry
> > > > *dentry, struct inode *inode,
> > > >         err = -ENOMEM;
> > > >         override_cred = prepare_creds();
> > > >         if (override_cred) {
> > > > -               override_cred->fsuid = inode->i_uid;
> > > > -               override_cred->fsgid = inode->i_gid;
> > > >                 if (!attr->hardlink) {
> > > > +                       override_cred->fsuid = inode->i_uid;
> > > > +                       override_cred->fsgid = inode->i_gid;
> > > >                         err = security_dentry_create_files_as(dentry,
> > > >                                         attr->mode, &dentry->d_name, old_cred,
> > > >                                         override_cred);
> > > > @@ -605,6 +607,9 @@ static int ovl_create_or_link(struct dentry
> > > > *dentry, struct inode *inode,
> > > >                                 put_cred(override_cred);
> > > >                                 goto out_revert_creds;
> > > >                         }
> > > > +               } else {
> > > > +                       override_cred->fsuid = caller_fsuid;
> > > > +                       override_cred->fsgid = caller_fsgid;
> > > >                 }
> > > >                 put_cred(override_creds(override_cred));
> > > >                 put_cred(override_cred);
> > >
> > > Hah, wait. I had a pretty obvious braino when I did that. I forgot to
> > > account for setgid handling in inode_init_owner(). Let me take another
> > > close look...
> >
> > Ok, so if we look at all the callchains:
> >
> > (1) .create = ovl_create()
> >     -> ovl_create_object()
> >        -> ovl_create_or_link()
> >
> > (2) .mkdir = ovl_mkdir()
> >     -> ovl_create_object()
> >        -> ovl_create_or_link()
> >
> > (3) .mknod = ovl_mknod()
> >     -> ovl_create_object()
> >        -> ovl_create_or_link()
> >
> > (4) .symlink = ovl_symlink()
> >     -> ovl_create_object()
> >        -> ovl_create_or_link()
> >
> > (5) .link = ovl_link()
> >      -> ovl_create_or_link()
> >
> > we see that (1) to (4) set the caller's fs{g,u}id to the
> > inode->i_{g,u}id. That was instantiated by inode_init_owner(). This will
> > be current_fs{g,u}id() of the caller unless the directory the file is
> > created in was setgid in which case inode->i_gid can be different from
> > current_fsgid().
> >
> > So iiuc, taking the inode->i_{g,u}id is done to get fsgid set to
> > inode->i_gid when the parent directory is setgid.
> >
> > But for (5) the inode->i_{g,u}id should be irrelevant because it's a
> > hardlink and so we should just use the caller's fs{g,u}id. So with all
> > that in mind we should be doing:
> >
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 6b03457f72bb..74b3813eda47 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
> >         inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
> >         attr.mode = inode->i_mode;
> >
> > +       attr.fsuid = inode->i_uid;
> > +       attr.fsgid = inode->i_gid;
> >         err = ovl_create_or_link(dentry, inode, &attr, false);
> >         /* Did we end up using the preallocated inode? */
> >         if (inode != d_inode(dentry))
> > @@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
> >  {
> >         int err;
> >         struct inode *inode;
> > +       struct ovl_cattr attr;
> >
> >         err = ovl_want_write(old);
> >         if (err)
> > @@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
> >         inode = d_inode(old);
> >         ihold(inode);
> >
> > -       err = ovl_create_or_link(new, inode,
> > -                       &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
> > -                       ovl_type_origin(old));
> > +       attr = (struct ovl_cattr){
> > +               .hardlink = ovl_dentry_upper(old),
> > +               .fsuid = current_fsuid(),
> > +               .fsgid = current_fsgid(),
> > +       };
> > +       err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
> >         if (err)
> >                 iput(inode);
> >
> > diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> > index 87759165d32b..85043123a103 100644
> > --- a/fs/overlayfs/overlayfs.h
> > +++ b/fs/overlayfs/overlayfs.h
> > @@ -655,6 +655,8 @@ struct ovl_cattr {
> >         umode_t mode;
> >         const char *link;
> >         struct dentry *hardlink;
> > +       kuid_t fsuid;
> > +       kgid_t fsgid;
> >  };
> >
> >  #define OVL_CATTR(m) (&(struct ovl_cattr) { .mode = (m) })
> >
> > Using the full creator's credentials for just hardlinks seems odd to me
> > when all creation requests use the caller's fs{g,u}id...
>
> Sigh, omitted something in the patch:
>
> diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> index 6b03457f72bb..cf4dc303fa4d 100644
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -595,8 +595,8 @@ static int ovl_create_or_link(struct dentry *dentry, struct inode *inode,
>         err = -ENOMEM;
>         override_cred = prepare_creds();
>         if (override_cred) {
> -               override_cred->fsuid = inode->i_uid;
> -               override_cred->fsgid = inode->i_gid;
> +               override_cred->fsuid = attr->fsuid;
> +               override_cred->fsgid = attr->fsgid;
>                 if (!attr->hardlink) {
>                         err = security_dentry_create_files_as(dentry,
>                                         attr->mode, &dentry->d_name, old_cred,
> @@ -646,6 +646,8 @@ static int ovl_create_object(struct dentry *dentry, int mode, dev_t rdev,
>         inode_init_owner(&init_user_ns, inode, dentry->d_parent->d_inode, mode);
>         attr.mode = inode->i_mode;
>
> +       attr.fsuid = inode->i_uid;
> +       attr.fsgid = inode->i_gid;
>         err = ovl_create_or_link(dentry, inode, &attr, false);
>         /* Did we end up using the preallocated inode? */
>         if (inode != d_inode(dentry))
> @@ -702,6 +704,7 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
>  {
>         int err;
>         struct inode *inode;
> +       struct ovl_cattr attr;
>
>         err = ovl_want_write(old);
>         if (err)
> @@ -728,9 +731,12 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
>         inode = d_inode(old);
>         ihold(inode);
>
> -       err = ovl_create_or_link(new, inode,
> -                       &(struct ovl_cattr) {.hardlink = ovl_dentry_upper(old)},
> -                       ovl_type_origin(old));
> +       attr = (struct ovl_cattr){
> +               .hardlink = ovl_dentry_upper(old),
> +               .fsuid = current_fsuid(),
> +               .fsgid = current_fsgid(),
> +       };
> +       err = ovl_create_or_link(new, inode, &attr, ovl_type_origin(old));
>         if (err)
>                 iput(inode);
>
> diff --git a/fs/overlayfs/overlayfs.h b/fs/overlayfs/overlayfs.h
> index 87759165d32b..85043123a103 100644
> --- a/fs/overlayfs/overlayfs.h
> +++ b/fs/overlayfs/overlayfs.h
> @@ -655,6 +655,8 @@ struct ovl_cattr {
>         umode_t mode;
>         const char *link;
>         struct dentry *hardlink;
> +       kuid_t fsuid;
> +       kgid_t fsgid;
>  };
>

There were no more comments some days ago so I will send the v2 patch
according to your modification later.

Thanks,
Tianci.