rxrpc: Fix notification vs call-release vs recvmsg

When a call is released, rxrpc takes the spinlock and removes it from
->recvmsg_q in an effort to prevent racing recvmsg() invocations from
seeing the same call.  Now, rxrpc_recvmsg() only takes the spinlock when
actually removing a call from the queue; it doesn't, however, take it in
the lead up to that when it checks to see if the queue is empty.  It *does*
hold the socket lock, which prevents a recvmsg/recvmsg race - but this
doesn't prevent sendmsg from ending the call because sendmsg() drops the
socket lock and relies on the call->user_mutex.

Fix this by firstly removing the bit in rxrpc_release_call() that dequeues
the released call and, instead, rely on recvmsg() to simply discard
released calls (done in a preceding fix).

Secondly, rxrpc_notify_socket() is abandoned if the call is already marked
as released rather than trying to be clever by setting both pointers in
call->recvmsg_link to NULL to trick list_empty().  This isn't perfect and
can still race, resulting in a released call on the queue, but recvmsg()
will now clean that up.

Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Junvyyang, Tencent Zhuque Lab <zhuque@tencent.com>
cc: LePremierHomme <kwqcheii@proton.me>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250717074350.3767366-4-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 12 insertions, 16 deletions

diff --git a/net/rxrpc/call_object.c b/net/rxrpc/call_object.c
index 15067ff7b1f2..918f41d97a2f 100644
--- a/net/rxrpc/call_object.c
+++ b/net/rxrpc/call_object.c
@@ -561,7 +561,7 @@ static void rxrpc_cleanup_rx_buffers(struct rxrpc_call *call)
 void rxrpc_release_call(struct rxrpc_sock *rx, struct rxrpc_call *call)
 {
 	struct rxrpc_connection *conn = call->conn;
-	bool put = false, putu = false;
+	bool putu = false;
 
 	_enter("{%d,%d}", call->debug_id, refcount_read(&call->ref));
 
@@ -573,23 +573,13 @@ void rxrpc_release_call(struct rxrpc_sock *rx, struct rxrpc_call *call)
 
 	rxrpc_put_call_slot(call);
 
-	/* Make sure we don't get any more notifications */
+	/* Note that at this point, the call may still be on or may have been
+	 * added back on to the socket receive queue.  recvmsg() must discard
+	 * released calls.  The CALL_RELEASED flag should prevent further
+	 * notifications.
+	 */
 	spin_lock_irq(&rx->recvmsg_lock);
-
-	if (!list_empty(&call->recvmsg_link)) {
-		_debug("unlinking once-pending call %p { e=%lx f=%lx }",
-		       call, call->events, call->flags);
-		list_del(&call->recvmsg_link);
-		put = true;
-	}
-
-	/* list_empty() must return false in rxrpc_notify_socket() */
-	call->recvmsg_link.next = NULL;
-	call->recvmsg_link.prev = NULL;
-
 	spin_unlock_irq(&rx->recvmsg_lock);
-	if (put)
-		rxrpc_put_call(call, rxrpc_call_put_unnotify);
 
 	write_lock(&rx->call_lock);
 
@@ -638,6 +628,12 @@ void rxrpc_release_calls_on_socket(struct rxrpc_sock *rx)
 		rxrpc_put_call(call, rxrpc_call_put_release_sock);
 	}
 
+	while ((call = list_first_entry_or_null(&rx->recvmsg_q,
+						struct rxrpc_call, recvmsg_link))) {
+		list_del_init(&call->recvmsg_link);
+		rxrpc_put_call(call, rxrpc_call_put_release_recvmsg_q);
+	}
+
 	_leave("");
 }