rxrpc: Fix a couple of potential use-after-frees

At the end of rxrpc_recvmsg(), if a call is found, the call is put and then
a trace line is emitted referencing that call in a couple of places - but
the call may have been deallocated by the time those traces happen.

Fix this by stashing the call debug_id in a variable and passing that to
the tracepoint rather than the call pointer.

Fixes: 8499790 ("rxrpc: Add a tracepoint to follow what recvmsg does")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: David S. Miller <davem@davemloft.net>

include/trace/events/rxrpc.h

@@ -1062,10 +1062,10 @@ TRACE_EVENT(rxrpc_receive,
 	    );
 
 TRACE_EVENT(rxrpc_recvmsg,
-	    TP_PROTO(struct rxrpc_call *call, enum rxrpc_recvmsg_trace why,
+	    TP_PROTO(unsigned int call_debug_id, enum rxrpc_recvmsg_trace why,
 		     int ret),
 
-	    TP_ARGS(call, why, ret),
+	    TP_ARGS(call_debug_id, why, ret),
 
 	    TP_STRUCT__entry(
 		    __field(unsigned int,		call		)
@@ -1074,7 +1074,7 @@ TRACE_EVENT(rxrpc_recvmsg,
 			     ),
 
 	    TP_fast_assign(
-		    __entry->call = call ? call->debug_id : 0;
+		    __entry->call = call_debug_id;
 		    __entry->why = why;
 		    __entry->ret = ret;
 			   ),

net/rxrpc/recvmsg.c

@@ -388,13 +388,14 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	struct rxrpc_call *call;
 	struct rxrpc_sock *rx = rxrpc_sk(sock->sk);
 	struct list_head *l;
+	unsigned int call_debug_id = 0;
 	size_t copied = 0;
 	long timeo;
 	int ret;
 
 	DEFINE_WAIT(wait);
 
-	trace_rxrpc_recvmsg(NULL, rxrpc_recvmsg_enter, 0);
+	trace_rxrpc_recvmsg(0, rxrpc_recvmsg_enter, 0);
 
 	if (flags & (MSG_OOB | MSG_TRUNC))
 		return -EOPNOTSUPP;
@@ -431,7 +432,7 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		if (list_empty(&rx->recvmsg_q)) {
 			if (signal_pending(current))
 				goto wait_interrupted;
-			trace_rxrpc_recvmsg(NULL, rxrpc_recvmsg_wait, 0);
+			trace_rxrpc_recvmsg(0, rxrpc_recvmsg_wait, 0);
 			timeo = schedule_timeout(timeo);
 		}
 		finish_wait(sk_sleep(&rx->sk), &wait);
@@ -450,7 +451,8 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 		rxrpc_get_call(call, rxrpc_call_get_recvmsg);
 	write_unlock(&rx->recvmsg_lock);
 
-	trace_rxrpc_recvmsg(call, rxrpc_recvmsg_dequeue, 0);
+	call_debug_id = call->debug_id;
+	trace_rxrpc_recvmsg(call_debug_id, rxrpc_recvmsg_dequeue, 0);
 
 	/* We're going to drop the socket lock, so we need to lock the call
 	 * against interference by sendmsg.
@@ -531,22 +533,22 @@ int rxrpc_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 error_unlock_call:
 	mutex_unlock(&call->user_mutex);
 	rxrpc_put_call(call, rxrpc_call_put_recvmsg);
-	trace_rxrpc_recvmsg(call, rxrpc_recvmsg_return, ret);
+	trace_rxrpc_recvmsg(call_debug_id, rxrpc_recvmsg_return, ret);
 	return ret;
 
 error_requeue_call:
 	if (!(flags & MSG_PEEK)) {
 		write_lock(&rx->recvmsg_lock);
 		list_add(&call->recvmsg_link, &rx->recvmsg_q);
 		write_unlock(&rx->recvmsg_lock);
-		trace_rxrpc_recvmsg(call, rxrpc_recvmsg_requeue, 0);
+		trace_rxrpc_recvmsg(call_debug_id, rxrpc_recvmsg_requeue, 0);
 	} else {
 		rxrpc_put_call(call, rxrpc_call_put_recvmsg);
 	}
 error_no_call:
 	release_sock(&rx->sk);
 error_trace:
-	trace_rxrpc_recvmsg(call, rxrpc_recvmsg_return, ret);
+	trace_rxrpc_recvmsg(call_debug_id, rxrpc_recvmsg_return, ret);
 	return ret;
 
 wait_interrupted: