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

[Luis Henriques <lhenriques@suse.de>]

Jeff Layton <jlayton@kernel.org> writes:

> 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.

Hmm, yeah.  I guess the changelog could mention that.  Thanks, Jeff.

Cheers,
-- 
Luis

>> > 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>
>