nfsd: fix use-after-free due to delegation race

A delegation break could arrive as soon as we've called vfs_setlease.  A
delegation break runs a callback which immediately (in
nfsd4_cb_recall_prepare) adds the delegation to del_recall_lru.  If we
then exit nfs4_set_delegation without hashing the delegation, it will be
freed as soon as the callback is done with it, without ever being
removed from del_recall_lru.

Symptoms show up later as use-after-free or list corruption warnings,
usually in the laundromat thread.

I suspect aba2072f "nfsd: grant read delegations to clients holding
writes" made this bug easier to hit, but I looked as far back as v3.0
and it looks to me it already had the same problem.  So I'm not sure
where the bug was introduced; it may have been there from the beginning.

Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -1207,6 +1207,11 @@ hash_delegation_locked(struct nfs4_delegation *dp, struct nfs4_file *fp)
 	return 0;
 }
 
+static bool delegation_hashed(struct nfs4_delegation *dp)
+{
+	return !(list_empty(&dp->dl_perfile));
+}
+
 static bool
 unhash_delegation_locked(struct nfs4_delegation *dp)
 {
@@ -1214,7 +1219,7 @@ unhash_delegation_locked(struct nfs4_delegation *dp)
 
 	lockdep_assert_held(&state_lock);
 
-	if (list_empty(&dp->dl_perfile))
+	if (!delegation_hashed(dp))
 		return false;
 
 	dp->dl_stid.sc_type = NFS4_CLOSED_DELEG_STID;
@@ -4598,7 +4603,7 @@ static void nfsd4_cb_recall_prepare(struct nfsd4_callback *cb)
 	 * queued for a lease break. Don't queue it again.
 	 */
 	spin_lock(&state_lock);
-	if (dp->dl_time == 0) {
+	if (delegation_hashed(dp) && dp->dl_time == 0) {
 		dp->dl_time = ktime_get_boottime_seconds();
 		list_add_tail(&dp->dl_recall_lru, &nn->del_recall_lru);
 	}