Re: [PATCH] ksmbd: fix lookup on idmapped mounts

[Christian Brauner <christian.brauner@ubuntu.com>]

On Sat, Aug 21, 2021 at 09:09:28PM +0900, Namjae Jeon wrote:
> 2021-08-21 20:39 GMT+09:00, Christian Brauner <christian.brauner@ubuntu.com>:
> > On Sat, Aug 21, 2021 at 01:11:17PM +0200, Christian Brauner wrote:
> >> On Sat, Aug 21, 2021 at 02:59:21PM +0900, Namjae Jeon wrote:
> >> > 2021-08-19 22:01 GMT+09:00, Christian Brauner
> >> > <christian.brauner@ubuntu.com>:
> >> > > On Thu, Aug 19, 2021 at 11:19:04AM +0900, Namjae Jeon wrote:
> >> > >> > On Tue, Aug 17, 2021 at 08:30:55AM +0900, Namjae Jeon wrote:
> >> > >> > > > From: Christian Brauner <christian.brauner@ubuntu.com>
> >> > >> > > >
> >> > >> > > > It's great that the new in-kernel ksmbd server will support
> >> > >> > > > idmapped
> >> > >> > > > mounts out of the box! However, lookup is currently broken.
> >> > >> > > > Lookup
> >> > >> > > > helpers such as lookup_one_len() call inode_permission()
> >> > >> > > > internally
> >> > >> > > > to ensure that the caller is privileged over the inode of the
> >> > >> > > > base
> >> > >> > > > dentry they are trying to
> >> > >> > lookup under. So the permission checking here is currently wrong.
> >> > >> > > >
> >> > >> > > > Linux v5.15 will gain a new lookup helper lookup_one() that
> >> > >> > > > does
> >> > >> > > > take idmappings into account. I've added it as part of my
> >> > >> > > > patch
> >> > >> > > > series to make btrfs support idmapped mounts. The new helper is
> >> > >> > > > in
> >> > >> > > > linux- next as part of David's (Sterba) btrfs for-next branch
> >> > >> > > > as
> >> > >> > > > commit c972214c133b ("namei: add
> >> > >> > mapping aware lookup helper").
> >> > >> > > >
> >> > >> > > > I've said it before during one of my first reviews: I would
> >> > >> > > > very
> >> > >> > > > much recommend adding fstests to
> >> > >> > [1].
> >> > >> > > > It already seems to have very rudimentary cifs support. There
> >> > >> > > > is a
> >> > >> > > > completely generic idmapped mount testsuite that supports
> >> > >> > > > idmapped
> >> > >> > > > mounts.
> >> > >> > > >
> >> > >> > > > [1]: https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git/
> >> > >> > > > Cc: Steve French <stfrench@microsoft.com>
> >> > >> > > > Cc: Christoph Hellwig <hch@infradead.org>
> >> > >> > > > Cc: Namjae Jeon <namjae.jeon@samsung.com>
> >> > >> > > > Cc: Hyunchul Lee <hyc.lee@gmail.com>
> >> > >> > > > Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
> >> > >> > > > Cc: David Sterba <dsterba@suse.com>
> >> > >> > > > Cc: linux-cifs@vger.kernel.org
> >> > >> > > > Signed-off-by: Christian Brauner
> >> > >> > > > <christian.brauner@ubuntu.com>
> >> > >> > > > ---
> >> > >> > > Hi Christian,
> >> > >> > >
> >> > >> > > > I merged David's for-next tree into cifsd-next to test this. I
> >> > >> > > > did
> >> > >> > > > only compile test this. If someone gives me a clear set of
> >> > >> > > > instructions how to test ksmbd on my local machine I can at
> >> > >> > > > least
> >> > >> > > > try to cut some time out of my week to do more reviews. (I'd
> >> > >> > > > especially like to see acl behavior with ksmbd.)
> >> > >> > >
> >> > >> > > There is "How to run ksmbd" section in patch cover letter.
> >> > >> > >
> >> > >> > > https://protect2.fireeye.com/v1/url?k=65ecaaf0-3a779239-65ed21bf-0cc47
> >> > >> > > a336fae-53bc47005a1a97a9&q=1&e=e44c9f9f-d7ae-4768-8cc2-8f02d748fc6e&u=
> >> > >> > > https%3A%2F%2Flkml.org%2Flkml%2F2021%2F8%2F5%2F54
> >> > >> > >
> >> > >> > > Let me know if it doesn't work well even if you try to run it
> >> > >> > > with
> >> > >> > > this step.
> >> > >> > > And We will also check whether your patch work fine.
> >> > >> > >
> >> > >> > > >
> >> > >> > > > One more thing, the tree for ksmbd was very hard to find. I had
> >> > >> > > > to
> >> > >> > > > do a lot archeology to end up
> >> > >> > at:
> >> > >> > > >
> >> > >> > > > git://git.samba.org/ksmbd.git
> >> > >> > > This is also in the patch cover letter. See "Mailing list and
> >> > >> > > repositories" section.
> >> > >> > > I think that you can use :
> >> > >> > >
> >> > >> > > https://protect2.fireeye.com/v1/url?k=8af83a5d-d5630294-8af9b112-0cc47
> >> > >> > > a336fae-e471ffbdb93d05b7&q=1&e=e44c9f9f-d7ae-4768-8cc2-8f02d748fc6e&u=
> >> > >> > > https%3A%2F%2Fgithub.com%2Fnamjaejeon%2Fsmb3-kernel%2Ftree%2Fksmbd-v7-
> >> > >> > > series
> >> > >> > >
> >> > >> > > >
> >> > >> > > > Would be appreciated if this tree could be reflected in
> >> > >> > > > MAINTAINERS
> >> > >> > > > or somewhere else. The github repos with the broken out
> >> > >> > > > patches/module aren't really that helpful.
> >> > >> > > Okay, I will add git address of ksmbd in MAINTAINERS on next
> >> > >> > > spin.
> >> > >> > >
> >> > >> > > >
> >> > >> > > > Thanks!
> >> > >> > > > Christian
> >> > >> > > Really thanks for your review and I will apply this patch after
> >> > >> > > checking it.
> >> > >> >
> >> > >> > Thank your for the pointers.
> >> > >> >
> >> > >> > Ok, so I've been taking the time to look into cifs and ksmbd today.
> >> > >> > My
> >> > >> > mental model was wrong. There
> >> > >> > are two things to consider here:
> >> > >> >
> >> > >> > 1. server: idmapped mounts with ksmbd
> >> > >> > 2. client: idmapped mounts with cifs
> >> > >> >
> >> > >> > Your patchset adds support for 1.
> >> > >> Right.
> >> > >>
> >> > >> > Let's say I have the following ksmbd config:
> >> > >> >
> >> > >> > root@f2-vm:~# cat /etc/ksmbd/smb.conf
> >> > >> > [global]
> >> > >> >         netbios name = SMBD
> >> > >> >         server max protocol = SMB3
> >> > >> > [test]
> >> > >> >         path = /opt
> >> > >> >         writeable = yes
> >> > >> >         comment = TEST
> >> > >> >         read only = no
> >> > >> >
> >> > >> > So /opt can be an idmapped mount and ksmb would know how to deal
> >> > >> > with
> >> > >> > that correctly, i.e. you could
> >> > >> > do:
> >> > >> >
> >> > >> > mount-idmapped --map-mount=b:1000:0:1 /opt /opt
> >> > >> >
> >> > >> > ksmbd.mountd
> >> > >> >
> >> > >> > and ksmbd would take the idmapping of /opt into account.
> >> > >> Right.
> >> > >>
> >> > >> >
> >> > >> > That however is different from 2. which is cifs itself being
> >> > >> > idmappable.
> >> > >> Right.
> >> > >>
> >> > >> > Whether or not that makes sense or is needed will need some
> >> > >> > thinking.
> >> > >> >
> >> > >> > In any case, this has consequences for xfstests and now I
> >> > >> > understand
> >> > >> > your earlier confusion. In
> >> > >> > another mail you pointed out that cifs reports that idmapped mounts
> >> > >> > are
> >> > >> > not supported. That is correct
> >> > >> > insofar as it means 2. is not supported, i.e. you can't do:
> >> > >> Right.
> >> > >>
> >> > >> >
> >> > >> > mount -t cifs -o username=foo,password=bar //server/files /mnt
> >> > >> >
> >> > >> > and then
> >> > >> >
> >> > >> > mount-idmapped --map-mount=b:1000:0:1 /mnt /mnt
> >> > >> >
> >> > >> > but that's also not what you want in order to test for ksmbd. What
> >> > >> > you
> >> > >> > want is to test 1.
> >> > >> Right. So we have manually tested it, not xfstests.
> >> > >>
> >> > >> >
> >> > >> > So your test setup would require you to setup an idmapped mount and
> >> > >> > have
> >> > >> > ksmbd use that which can then
> >> > >> > be mounted by a client.
> >> > >> >
> >> > >> > With your instructions I was at least able to get a ksmb instance
> >> > >> > running and be able to mount a
> >> > >> > client with -t cifs. All on the same machine, i.e. my server is
> >> > >> > localhost.
> >> > >> Okay.
> >> > >>
> >> > >> >
> >> > >> > However, I need to dig a bit into the semantics to make better
> >> > >> > assertions about what's going on.
> >> > >> Okay. And I have applied your patch to ksmbd.
> >> > >>
> >> > >> >
> >> > >> > Are unix extension supported with ksmb? Everytime I try to use
> >> > >> > "posix"
> >> > >> > as a mount option with mount -t cifs -o //127.0.0.1/test /mnt I
> >> > >> > get
> >> > >> > "uid=0" and "gid=0" and "noposix".
> >> > >> > I do set "unix extensions = yes" in both the samba and ksmbd
> >> > >> > smb.conf.
> >> > >> With posix mount option, It should work. It worked well before but it
> >> > >> is
> >> > >> strange now.
> >> > >>
> >> > >> I'm not sure this is the correct fix, But could you please try to
> >> > >> mount
> >> > >> with the below change ?
> >> > >>
> >> > >> diff --git a/fs/cifs/fs_context.c b/fs/cifs/fs_context.c
> >> > >> index eed59bc1d913..5fd0b0ddcc57 100644
> >> > >> --- a/fs/cifs/fs_context.c
> >> > >> +++ b/fs/cifs/fs_context.c
> >> > >> @@ -1268,8 +1268,10 @@ static int smb3_fs_context_parse_param(struct
> >> > >> fs_context *fc,
> >> > >>         case Opt_unix:
> >> > >>                 if (result.negated)
> >> > >>                         ctx->linux_ext = 0;
> >> > >> -               else
> >> > >> +               else {
> >> > >> +                       ctx->linux_ext = 1;
> >> > >>                         ctx->no_linux_ext = 1;
> >> > >> +               }
> >> > >>                 break;
> >> > >>         case Opt_nocase:
> >> > >>                 ctx->nocase = 1;
> >> > >
> >> > > That stops the bleeding indeed. :)
> >> > Okay, sorry for late response.
> >> > > I think a slightly nicer fix might be:
> >> > >
> >> > > diff --git a/fs/cifs/fs_context.c b/fs/cifs/fs_context.c
> >> > > index eed59bc1d913..424b8dc2232e 100644
> >> > > --- a/fs/cifs/fs_context.c
> >> > > +++ b/fs/cifs/fs_context.c
> >> > > @@ -1269,7 +1269,8 @@ static int smb3_fs_context_parse_param(struct
> >> > > fs_context *fc,
> >> > >                 if (result.negated)
> >> > >                         ctx->linux_ext = 0;
> >> > >                 else
> >> > > -                       ctx->no_linux_ext = 1;
> >> > > +                       ctx->linux_ext = 1;
> >> > > +               ctx->no_linux_ext = !ctx->linux_ext;
> >> > >                 break;
> >> > >         case Opt_nocase:
> >> > >                 ctx->nocase = 1;
> >> > >
> >> > > So with this patch applied I got unix permissions working after all.
> >> > > So
> >> > > I was able to do some more testing.
> >> > Okay.
> >> > >
> >> > > Just a few questions (independent of idmapped mounts):
> >> > >
> >> > > 1. Are the "uid=" and "gid=" mount options ignored when "username="
> >> > > is
> >> > >    specified and "posix" is specified?
> >> > >
> >> > >    It seems that "uid=" and "gid=" have are silently ignored when
> >> > >    "posix' is set. They are neither used to report file ownership
> >> > > under
> >> > >    the cifs mountpoint nor are they relevant when determining
> >> > > ownership
> >> > >    on disk?
> >> > >
> >> > >    As an example, assume I have added a user "foo" with uid 1000 to
> >> > >    ksmbd via:
> >> > >
> >> > >            ksmbd.adduser -a foo
> >> > >
> >> > >    And I mounted a share via:
> >> > >
> >> > >            mount -t cifs -o
> >> > > username=foo,password=bar,posix,uid=1234,gid=1234,forceuid,forcegid
> >> > > //127.0.0.1/test /mnt
> >> > >
> >> > >    i) Ownership in /mnt appears posix-correct, i.e. "uid=" and "gid="
> >> > > have
> >> > >       no effect on the reported ownership.
> >> > >
> >> > >    ii) Assume I'm logged in as the root user with uid 0. If I create
> >> > >        file or directory in /mnt it will be owned by user foo, i.e.
> >> > > uid
> >> > >        1000, i.e., the "uid=1234" and "gid=1234" mount option have
> >> > > zero
> >> > >        effect on the ownership of the files?
> >> > >
> >> > > 2. Are the "uid=" and "gid=" options ignored for permission checking
> >> > >    when "posix" is specified?
> >> > >
> >> > > 3. Am I correct in assuming that there is no mount option that makes
> >> > >    chown() or chmod() have an actual effect.
> >> > That will be an answer for 1,2, 3 question. There are mount options for
> >> > this.
> >> >  1. modefromsid
> >> >  2. idsfromsid
> >> >
> >> > You can use this mount option and please check it.
> >>
> >> Thank you! This works and finally I can hit some codepaths I wasn't able
> >> to until now.
> >>
> >> > >
> >> > >    cifs seems to have support for it but the ksmbd server doesn't
> >> > > seem
> >> > >    to?
> >> > No, you need to use mount options for this as I said.
> >> > ksmbd have supported it but I found an issue related to chown and have
> >> > fixed.
> >> >
> >> > Could you please check the below git branch (pulled David'tree + ksmbd
> >> > fixes) ?
> >> >
> >> >   git clone --branch=for-christian
> >> > https://github.com/namjaejeon/smb3-kernel
> >>
> >> Thanks, I've pulled that branch.
> >
> > There's two problems in this patch. I'll post and point out here
> > quickly, if you don't mind:
> Thanks for your reiview!
> >
> > commit fd7d13c387798cbc3abd68ecc07b2c868c6d96cb
> > Author: Namjae Jeon <namjae.jeon@samsung.com>
> > Date:   Sat Aug 21 14:39:43 2021 +0900
> >
> >     ksmbd: fix permission check issue on chown and chmod
> >
> >     When commanding chmod and chown on cifs&ksmbd, ksmbd allows it without
> > file
> >     permissions check. There is code to check it in settattr_prepare.
> >     Instead of setting the inode directly, update the mode and uid/gid
> >     through notify_change.
> >
> >     Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
> >     Signed-off-by: Steve French <stfrench@microsoft.com>
> >
> > diff --git a/fs/ksmbd/smb2pdu.c b/fs/ksmbd/smb2pdu.c
> > index 0131997c2177..d329ea49fa14 100644
> > --- a/fs/ksmbd/smb2pdu.c
> > +++ b/fs/ksmbd/smb2pdu.c
> > @@ -5861,10 +5861,15 @@ int smb2_set_info(struct ksmbd_work *work)
> >                 break;
> >         case SMB2_O_INFO_SECURITY:
> >                 ksmbd_debug(SMB, "GOT SMB2_O_INFO_SECURITY\n");
> > +               if (ksmbd_override_fsids(work)) {
> > +                       rc = -ENOMEM;
> > +                       goto err_out;
> > +               }
> >                 rc = smb2_set_info_sec(fp,
> >
> > le32_to_cpu(req->AdditionalInformation),
> >                                        req->Buffer,
> >                                        le32_to_cpu(req->BufferLength));
> > +               ksmbd_revert_fsids(work);
> >                 break;
> >         default:
> >                 rc = -EOPNOTSUPP;
> > diff --git a/fs/ksmbd/smbacl.c b/fs/ksmbd/smbacl.c
> > index 20455d810523..f28af33c0bd5 100644
> > --- a/fs/ksmbd/smbacl.c
> > +++ b/fs/ksmbd/smbacl.c
> > @@ -1300,6 +1300,7 @@ int set_info_sec(struct ksmbd_conn *conn, struct
> > ksmbd_tree_connect *tcon,
> >         struct smb_fattr fattr = {{0}};
> >         struct inode *inode = d_inode(path->dentry);
> >         struct user_namespace *user_ns = mnt_user_ns(path->mnt);
> > +       struct iattr newattrs;
> >
> >         fattr.cf_uid = INVALID_UID;
> >         fattr.cf_gid = INVALID_GID;
> > @@ -1309,12 +1310,28 @@ int set_info_sec(struct ksmbd_conn *conn, struct
> > ksmbd_tree_connect *tcon,
> >         if (rc)
> >                 goto out;
> >
> > -       inode->i_mode = (inode->i_mode & ~0777) | (fattr.cf_mode & 0777);
> > -       if (!uid_eq(fattr.cf_uid, INVALID_UID))
> > -               inode->i_uid = fattr.cf_uid;
> > -       if (!gid_eq(fattr.cf_gid, INVALID_GID))
> > +       newattrs.ia_valid = ATTR_CTIME;
> > +       if (!uid_eq(fattr.cf_uid, INVALID_UID)) {
> > +               newattrs.ia_valid |= ATTR_UID;
> > +               newattrs.ia_uid = fattr.cf_uid;
> > +       }
> > +       if (!gid_eq(fattr.cf_gid, INVALID_GID)) {
> >                 inode->i_gid = fattr.cf_gid;
> >
> > This needs to be removed. If setattr_prepare() fails you will still end
> > up with inode->i_gid changed, i.e. you're changing group ownership even
> > if you fail the subsequent setattr_prepare().
> Oops, My mistake, I am missing remove this line.
> >
> > -       mark_inode_dirty(inode);
> > +               newattrs.ia_valid |= ATTR_GID;
> > +               newattrs.ia_gid = fattr.cf_gid;
> > +       }
> > +       newattrs.ia_valid |= ATTR_MODE;
> > +       newattrs.ia_mode = (inode->i_mode & ~0777) | (fattr.cf_mode &
> > 0777);
> > +       rc = setattr_prepare(user_ns, path->dentry, &newattrs);

Sorry, I also forgot to mention that you need to remove this too. All of
this is handled in the underlying filesystem's ->setattr. So this just
needs a notify_change().