Re: [PATCH] ceph: fix race in concurrent __ceph_remove_cap invocations

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2020-11-12 at 20:43 +0800, Yan, Zheng wrote:
> On Thu, Nov 12, 2020 at 6:48 PM Luis Henriques <lhenriques@suse.de> wrote:
> > 
> > A NULL pointer dereference may occur in __ceph_remove_cap with some of the
> > callbacks used in ceph_iterate_session_caps, namely trim_caps_cb and
> > remove_session_caps_cb.  These aren't protected against the concurrent
> > execution of __ceph_remove_cap.
> > 
> 
> they are protected by session mutex, never get executed concurrently
> 

Maybe not concurrently with one another, but the s_mutex is _not_ held
when __ceph_remove_caps is called from ceph_evict_inode. We can't rely
on it to protect this.

> > Since the callers of this function hold the i_ceph_lock, the fix is simply
> > a matter of returning immediately if caps->ci is NULL.
> > 
> > Based on a patch from Jeff Layton.
> > 
> > Cc: stable@vger.kernel.org
> > URL: https://tracker.ceph.com/issues/43272
> > Link: https://www.spinics.net/lists/ceph-devel/msg47064.html
> > Signed-off-by: Luis Henriques <lhenriques@suse.de>
> > ---
> >  fs/ceph/caps.c | 11 +++++++++--
> >  1 file changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> > index ded4229c314a..443f164760d5 100644
> > --- a/fs/ceph/caps.c
> > +++ b/fs/ceph/caps.c
> > @@ -1140,12 +1140,19 @@ void __ceph_remove_cap(struct ceph_cap *cap, bool queue_release)
> >  {
> >         struct ceph_mds_session *session = cap->session;
> >         struct ceph_inode_info *ci = cap->ci;
> > -       struct ceph_mds_client *mdsc =
> > -               ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc;
> > +       struct ceph_mds_client *mdsc;
> >         int removed = 0;
> > 
> > +       /* 'ci' being NULL means he remove have already occurred */
> > +       if (!ci) {
> > +               dout("%s: cap inode is NULL\n", __func__);
> > +               return;
> > +       }
> > +
> >         dout("__ceph_remove_cap %p from %p\n", cap, &ci->vfs_inode);
> > 
> > +       mdsc = ceph_inode_to_client(&ci->vfs_inode)->mdsc;
> > +
> >         /* remove from inode's cap rbtree, and clear auth cap */
> >         rb_erase(&cap->ci_node, &ci->i_caps);
> >         if (ci->i_auth_cap == cap) {

-- 
Jeff Layton <jlayton@kernel.org>