Re: [PATCH v2 2/2] ovl: get_acl: Fix null pointer dereference at realinode in rcu-walk mode

[Zhihao Cheng]

在 2023/5/16 19:40, Amir Goldstein 写道:
> On Mon, May 15, 2023 at 4:39 PM Zhihao Cheng <chengzhihao1@xxxxxxxxxx> wrote:
> > Following process:
> >           P1                     P2
> >   path_openat
> >    link_path_walk
> >     may_lookup
> >      inode_permission(rcu)
> >       ovl_permission
> >        acl_permission_check
> >         check_acl
> >          get_cached_acl_rcu
> >           ovl_get_inode_acl
> >            realinode = ovl_inode_real(ovl_inode)
> >                                drop_cache
> >                                 __dentry_kill(ovl_dentry)
> >                                  iput(ovl_inode)
> >                                   ovl_destroy_inode(ovl_inode)
> >                                    dput(oi->__upperdentry)
> >                                     dentry_kill(upperdentry)
> >                                      dentry_unlink_inode
> >                                       upperdentry->d_inode = NULL
> >              ovl_inode_upper
> >               upperdentry = ovl_i_dentry_upper(ovl_inode)
> >               d_inode(upperdentry) // returns NULL
> >            IS_POSIXACL(realinode) // NULL pointer dereference
> > , will trigger an null pointer dereference at realinode:
> >    [  205.472797] BUG: kernel NULL pointer dereference, address:
> >                   0000000000000028
> >    [  205.476701] CPU: 2 PID: 2713 Comm: ls Not tainted
> >                   6.3.0-12064-g2edfa098e750-dirty #1216
> >    [  205.478754] RIP: 0010:do_ovl_get_acl+0x5d/0x300
> >    [  205.489584] Call Trace:
> >    [  205.489812]  <TASK>
> >    [  205.490014]  ovl_get_inode_acl+0x26/0x30
> >    [  205.490466]  get_cached_acl_rcu+0x61/0xa0
> >    [  205.490908]  generic_permission+0x1bf/0x4e0
> >    [  205.491447]  ovl_permission+0x79/0x1b0
> >    [  205.491917]  inode_permission+0x15e/0x2c0
> >    [  205.492425]  link_path_walk+0x115/0x550
> >    [  205.493311]  path_lookupat.isra.0+0xb2/0x200
> >    [  205.493803]  filename_lookup+0xda/0x240
> >    [  205.495747]  vfs_fstatat+0x7b/0xb0
> >
> > Fetch a reproducer in [Link].
> >
> > Fix it by using helper ovl_i_path_realinode() to get realpath and real
> > inode after non-nullptr checking.
> >
> > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217404
> > Fixes: 332f606b32b6 ("ovl: enable RCU'd ->get_acl()")
>
> Note that this bug is also in 5.15.y, in method ovl_get_acl().
> I hope you will be able to follow up with a simple backport for 5.15 -
> i.e. only need to add a check for NULL realinode at the beginning.
> There was no realpath back then.

Sure. I notice that there is a '[ Upstream commit xxx ]' field in 5.15 
patch, so may I backport it after the fix applied into mainline(6.4)?

> AFAICT, both your patches should apply cleanly to 6.1.y, so should
> be picked up automatically by stable kernel bots.
>
> Thanks,
> Amir.
>
> > Signed-off-by: Zhihao Cheng <chengzhihao1@xxxxxxxxxx>
> > Suggested-by: Christian Brauner <brauner@xxxxxxxxxx>
> > ---
> >   fs/overlayfs/inode.c | 14 ++++++--------
> >   1 file changed, 6 insertions(+), 8 deletions(-)
> >
> > diff --git a/fs/overlayfs/inode.c b/fs/overlayfs/inode.c
> > index cc3ef5a6666a..b2021eada8be 100644
> > --- a/fs/overlayfs/inode.c
> > +++ b/fs/overlayfs/inode.c
> > @@ -576,20 +576,18 @@ struct posix_acl *do_ovl_get_acl(struct mnt_idmap *idmap,
> >                                   struct inode *inode, int type,
> >                                   bool rcu, bool noperm)
> >   {
> > -       struct inode *realinode = ovl_inode_real(inode);
> > +       struct inode *realinode;
> >          struct posix_acl *acl;
> >          struct path realpath;
> > +       int err;
> > +
> > +       err = ovl_i_path_realinode(inode, &realpath, &realinode, rcu);
> > +       if (err)
> > +               return ERR_PTR(err);
> >
> >          if (!IS_POSIXACL(realinode))
> >                  return NULL;
> >
> > -       /* Careful in RCU walk mode */
> > -       ovl_i_path_real(inode, &realpath);
> > -       if (!realpath.dentry) {
> > -               WARN_ON(!rcu);
> > -               return ERR_PTR(-ECHILD);
> > -       }
> > -
> >          if (rcu) {
> >                  /*
> >                   * If the layer is idmapped drop out of RCU path walk
> > --
> > 2.39.2
> .