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Date:   Wed, 31 Aug 2022 17:29:04 +0200
From:   Christian Brauner <brauner@kernel.org>
To:     Miklos Szeredi <miklos@szeredi.hu>
Cc:     Zhang Tianci <zhangtianci.1997@bytedance.com>,
        linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        amir73il@gmail.com,
        Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
Subject: Re: [PATCH v2] ovl: Use current fsuid and fsgid in ovl_link()
Message-ID: <20220831152904.efqd56kayvrxsdj6@wittgenstein>
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On Wed, Aug 31, 2022 at 03:53:58PM +0200, Miklos Szeredi wrote:
> On Wed, 31 Aug 2022 at 15:43, Christian Brauner <brauner@kernel.org> 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@bytedance.com> 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@bytedance.com>
> > > > Signed-off-by: Jiachen Zhang <zhangjiachen.jaycee@bytedance.com>
> > > > Signed-off-by: Christian Brauner (Microsoft) <brauner@kernel.org>
> > > > ---
> > > >  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.

Yeah, I know what you mean but it's borderline in so far as the
underlying fs might still perform permission checking based on the
caller's fs{g,u}id which together with what I'm saying below in a bit
was what made me go oh well, we should use the caller's fs{g,u}id then
for consistency.

> 
> > 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.

What I meant was back then even for link the fs{g,u}id was based on a
newly allocated inode->i_{g,u}id that was initialized through
inode_init_owner() with the caller's fs{g,u}id:

	inode = ovl_new_inode(dentry->d_sb, mode);
	if (!inode)
		goto out;

	err = ovl_copy_up(dentry->d_parent);
	if (err)
		goto out_iput;

	inode_init_owner(inode, dentry->d_parent->d_inode, mode);
	stat.mode = inode->i_mode;

	old_cred = ovl_override_creds(dentry->d_sb);
	err = -ENOMEM;
	override_cred = prepare_creds();
	if (override_cred) {
		override_cred->fsuid = inode->i_uid;
		override_cred->fsgid = inode->i_gid;

and it changed after that commit to be based on old inode. So emulating
the old behavior seemed the better approach.

> 
> > 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.

Sounds good.