rxrpc: Fix potential deadlock

There is a potential deadlock in rxrpc_peer_keepalive_dispatch() whereby
rxrpc_put_peer() is called with the peer_hash_lock held, but if it reduces
the peer's refcount to 0, rxrpc_put_peer() calls __rxrpc_put_peer() - which
the tries to take the already held lock.

Fix this by providing a version of rxrpc_put_peer() that can be called in
situations where the lock is already held.

The bug may produce the following lockdep report:

============================================
WARNING: possible recursive locking detected
5.2.0-next-20190718 #41 Not tainted
--------------------------------------------
kworker/0:3/21678 is trying to acquire lock:
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at: spin_lock_bh
/./include/linux/spinlock.h:343 [inline]
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at:
__rxrpc_put_peer /net/rxrpc/peer_object.c:415 [inline]
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at:
rxrpc_put_peer+0x2d3/0x6a0 /net/rxrpc/peer_object.c:435

but task is already holding lock:
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at: spin_lock_bh
/./include/linux/spinlock.h:343 [inline]
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at:
rxrpc_peer_keepalive_dispatch /net/rxrpc/peer_event.c:378 [inline]
00000000aa5eecdf (&(&rxnet->peer_hash_lock)->rlock){+.-.}, at:
rxrpc_peer_keepalive_worker+0x6b3/0xd02 /net/rxrpc/peer_event.c:430

Fixes: 330bdcfa ("rxrpc: Fix the keepalive generator [ver #2]")
Reported-by: syzbot+72af434e4b3417318f84@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>

net/rxrpc/ar-internal.h

@@ -1061,6 +1061,7 @@ void rxrpc_destroy_all_peers(struct rxrpc_net *);
 struct rxrpc_peer *rxrpc_get_peer(struct rxrpc_peer *);
 struct rxrpc_peer *rxrpc_get_peer_maybe(struct rxrpc_peer *);
 void rxrpc_put_peer(struct rxrpc_peer *);
+void rxrpc_put_peer_locked(struct rxrpc_peer *);
 
 /*
  * proc.c

net/rxrpc/peer_event.c

@@ -378,7 +378,7 @@ static void rxrpc_peer_keepalive_dispatch(struct rxrpc_net *rxnet,
 		spin_lock_bh(&rxnet->peer_hash_lock);
 		list_add_tail(&peer->keepalive_link,
 			      &rxnet->peer_keepalive[slot & mask]);
-		rxrpc_put_peer(peer);
+		rxrpc_put_peer_locked(peer);
 	}
 
 	spin_unlock_bh(&rxnet->peer_hash_lock);

net/rxrpc/peer_object.c

@@ -436,6 +436,24 @@ void rxrpc_put_peer(struct rxrpc_peer *peer)
 	}
 }
 
+/*
+ * Drop a ref on a peer record where the caller already holds the
+ * peer_hash_lock.
+ */
+void rxrpc_put_peer_locked(struct rxrpc_peer *peer)
+{
+	const void *here = __builtin_return_address(0);
+	int n;
+
+	n = atomic_dec_return(&peer->usage);
+	trace_rxrpc_peer(peer, rxrpc_peer_put, n, here);
+	if (n == 0) {
+		hash_del_rcu(&peer->hash_link);
+		list_del_init(&peer->keepalive_link);
+		kfree_rcu(peer, rcu);
+	}
+}
+
 /*
  * Make sure all peer records have been discarded.
  */