[PATCH] exportfs: do not read dentry after free

[PATCH] exportfs: do not read dentry after free

[Pan Bian]

The function dentry_connected calls dput(dentry) to drop the previously
acquired reference to dentry. In this case, dentry can be released.
After that, IS_ROOT(dentry) checks the condition
(dentry == dentry->d_parent), which may result in a use-after-free bug.
This patch directly compares dentry with its parent obtained before
dropping the reference.

Fixes: a056cc8934c("exportfs: stop retrying once we race with
rename/remove")

Signed-off-by: Pan Bian <bianpan2016@163.com>
---
 fs/exportfs/expfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
index 645158d..a69aaf5 100644
--- a/fs/exportfs/expfs.c
+++ b/fs/exportfs/expfs.c
@@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
 		struct dentry *parent = dget_parent(dentry);
 
 		dput(dentry);
-		if (IS_ROOT(dentry)) {
+		if (dentry == parent) { /* is root entry */
 			dput(parent);
 			return false;
 		}
-- 
2.7.4

Re: [PATCH] exportfs: do not read dentry after free

[Amir Goldstein]

On Fri, Nov 23, 2018 at 5:16 AM Pan Bian <bianpan2016@163.com> wrote:
>
> The function dentry_connected calls dput(dentry) to drop the previously
> acquired reference to dentry. In this case, dentry can be released.
> After that, IS_ROOT(dentry) checks the condition
> (dentry == dentry->d_parent), which may result in a use-after-free bug.
> This patch directly compares dentry with its parent obtained before
> dropping the reference.
>
> Fixes: a056cc8934c("exportfs: stop retrying once we race with
> rename/remove")
>

CC Fixes patch author/reviewers

How did you find this? by code review or did this actually happen?

Normally a IS_ROOT dentry would be either DCACHE_DISCONNECTED or
pinned to some super block, but I guess there may be corner cases?

> Signed-off-by: Pan Bian <bianpan2016@163.com>
> ---
>  fs/exportfs/expfs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
> index 645158d..a69aaf5 100644
> --- a/fs/exportfs/expfs.c
> +++ b/fs/exportfs/expfs.c
> @@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
>                 struct dentry *parent = dget_parent(dentry);
>
>                 dput(dentry);
> -               if (IS_ROOT(dentry)) {
> +               if (dentry == parent) { /* is root entry */
>                         dput(parent);
>                         return false;
>                 }

The change itself looks right, but the name IS_ROOT is confusing
enough as it is. The explicit comment is just plain wrong.
If it was really a root dentry, it wouldn't have been DCACHE_DISCONNECTED
(unless it is a filesystem bug).

Thanks,
Amir.

Re: [PATCH] exportfs: do not read dentry after free

[PanBian]

On Fri, Nov 23, 2018 at 07:58:15AM +0200, Amir Goldstein wrote:
> On Fri, Nov 23, 2018 at 5:16 AM Pan Bian <bianpan2016@163.com> wrote:
> >
> > The function dentry_connected calls dput(dentry) to drop the previously
> > acquired reference to dentry. In this case, dentry can be released.
> > After that, IS_ROOT(dentry) checks the condition
> > (dentry == dentry->d_parent), which may result in a use-after-free bug.
> > This patch directly compares dentry with its parent obtained before
> > dropping the reference.
> >
> > Fixes: a056cc8934c("exportfs: stop retrying once we race with
> > rename/remove")
> >
> 
> CC Fixes patch author/reviewers
> 
> How did you find this? by code review or did this actually happen?
> 
> Normally a IS_ROOT dentry would be either DCACHE_DISCONNECTED or
> pinned to some super block, but I guess there may be corner cases?

I found this by code review, and I have not yet observed crash.

> 
> > Signed-off-by: Pan Bian <bianpan2016@163.com>
> > ---
> >  fs/exportfs/expfs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/exportfs/expfs.c b/fs/exportfs/expfs.c
> > index 645158d..a69aaf5 100644
> > --- a/fs/exportfs/expfs.c
> > +++ b/fs/exportfs/expfs.c
> > @@ -77,7 +77,7 @@ static bool dentry_connected(struct dentry *dentry)
> >                 struct dentry *parent = dget_parent(dentry);
> >
> >                 dput(dentry);
> > -               if (IS_ROOT(dentry)) {
> > +               if (dentry == parent) { /* is root entry */
> >                         dput(parent);
> >                         return false;
> >                 }
> 
> The change itself looks right, but the name IS_ROOT is confusing
> enough as it is. The explicit comment is just plain wrong.
> If it was really a root dentry, it wouldn't have been DCACHE_DISCONNECTED
> (unless it is a filesystem bug).

I will remove the comment and resubmit the patch.

Thanks a lot,
Pan

> 
> Thanks,
> Amir.