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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Wed, 17 Aug 2022 12:53:44 +0300
Message-ID: <CAOQ4uxhD0an61z5wArn=uRsXF7+_3soNgPB7ikp2HYX8nQGkCg@mail.gmail.com>
Subject: Re: [External] Re: [PATCH] ovl: Do not override fsuid and fsgid in ovl_link()
To:     =?UTF-8?B?5aSp6LWQ5byg?= <zhangtianci.1997@bytedance.com>
Cc:     Miklos Szeredi <miklos@szeredi.hu>,
        overlayfs <linux-unionfs@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>,
        Christian Brauner <christian.brauner@ubuntu.com>
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On Wed, Aug 17, 2022 at 12:11 PM =E5=A4=A9=E8=B5=90=E5=BC=A0 <zhangtianci.1=
997@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 *dent=
ry, struct inode *inode,
> > >         err =3D -ENOMEM;
> > >         override_cred =3D prepare_creds();
> > >         if (override_cred) {
> > > -               override_cred->fsuid =3D inode->i_uid;
> > > -               override_cred->fsgid =3D inode->i_gid;
> > >                 if (!attr->hardlink) {
> > > +                       override_cred->fsuid =3D inode->i_uid;
> > > +                       override_cred->fsgid =3D inode->i_gid;
> > >                         err =3D security_dentry_create_files_as(dentr=
y,
> > >                                         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=3D/lower,upperdir=3D/fuse/upper,w=
orkdir=3Dwork
>   $ 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.