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

[Jeff Layton <jlayton@kernel.org>]

On Thu, 2020-11-12 at 10:45 +0000, Luis Henriques 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.
> 
> 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) {

Merged into testing branch (with a minor fix to the comment).
-- 
Jeff Layton <jlayton@kernel.org>