Re: [PATCH v2] ovl: Use current fsuid and fsgid in ovl_link()

[Miklos Szeredi]

On Wed, 31 Aug 2022 at 15:43, Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> On Wed, Aug 31, 2022 at 03:00:18PM +0200, Miklos Szeredi wrote:
> > On Thu, 25 Aug 2022 at 15:08, Zhang Tianci
> > <zhangtianci.1997@xxxxxxxxxxxxx> wrote:
> > >
> > > There is a wrong case of link() on overlay:
> > >   $ 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 we will get an error(EACCES) because fuse daemon checks the link()'s
> > > caller is "bin", it denied this request.
> > >
> > > In 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. This is acceptable
> > > to use the caller's fsuid.
> > >
> > > 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 this is incorrect.
> > >
> > > Fix this bug by using current fsuid/fsgid to do underlying fs's link().
> > >
> > > Link: https://lore.kernel.org/all/20220817102951.xnvesg3a7rbv576x@wittgenstein/T
> > >
> > > Signed-off-by: Zhang Tianci <zhangtianci.1997@xxxxxxxxxxxxx>
> > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@xxxxxxxxxxxxx>
> > > Signed-off-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
> > > ---
> > >  fs/overlayfs/dir.c       | 16 +++++++++++-----
> > >  fs/overlayfs/overlayfs.h |  2 ++
> > >  2 files changed, 13 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > > index 6b03457f72bb..dd84e6fc5f6e 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(),
> > > +       };
> >
> > Why do we need to override fsuid/fsgid for the hardlink case?
> >
> > Wouldn't it be simpler to just use the mounter's creds unmodified in
> > this case?   The inode is not created in this case, so overriding with
> > current uid/gid is not necessary, I think.
> >
> > Another way to look at it is: rename(A, B) is equivalent to an
> > imaginary atomic [link(A, B) + unlink(A)] pair.  But we don't override
> > uid/gid for rename() or unlink() so why should we for link().
>
> So my assumption has been that we want to override for any creation
> request and so for the sake of consistency I would've expected to also
> use that for ->link().

But link() is *not* a creation op.  It's a namespace manipulation op.

> Plus, this is also what has been done before the
> commit  51f7e52dc943 ("ovl: share inode for hard link") iiuc.

It wouldn't have mattered back then either, since the upper inode was
linked and not copied.

> Fwiw, I would've opted for consistency and even use the caller's
> fs{g,u}id during ->rename() and ->unlink().
>
> Right now the caller's fs{g,u}id - indirectly through inode_init_owner()
> - is used to ensure that the ownership of newly created files in the
> upper layer are based on the caller's not on the mounter's fs{g,u}id
> afaict. If we continue to only override for those cases it would really
> help that we document this in a good comment in ovl_create_or_link().

Yep.

Thanks,
Miklos