rxrpc: Fix read-after-free in rxrpc_queue_local()

rxrpc_queue_local() attempts to queue the local endpoint it is given and
then, if successful, prints a trace line.  The trace line includes the
current usage count - but we're not allowed to look at the local endpoint
at this point as we passed our ref on it to the workqueue.

Fix this by reading the usage count before queuing the work item.

Also fix the reading of local->debug_id for trace lines, which must be done
with the same consideration as reading the usage count.

Fixes: 09d2bf59 ("rxrpc: Add a tracepoint to track rxrpc_local refcounting")
Reported-by: syzbot+78e71c5bab4f76a6a719@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>

include/trace/events/rxrpc.h

@@ -498,10 +498,10 @@ rxrpc_tx_points;
 #define E_(a, b)	{ a, b }
 
 TRACE_EVENT(rxrpc_local,
-	    TP_PROTO(struct rxrpc_local *local, enum rxrpc_local_trace op,
+	    TP_PROTO(unsigned int local_debug_id, enum rxrpc_local_trace op,
 		     int usage, const void *where),
 
-	    TP_ARGS(local, op, usage, where),
+	    TP_ARGS(local_debug_id, op, usage, where),
 
 	    TP_STRUCT__entry(
 		    __field(unsigned int,	local		)
@@ -511,7 +511,7 @@ TRACE_EVENT(rxrpc_local,
 			     ),
 
 	    TP_fast_assign(
-		    __entry->local = local->debug_id;
+		    __entry->local = local_debug_id;
 		    __entry->op = op;
 		    __entry->usage = usage;
 		    __entry->where = where;

net/rxrpc/local_object.c

@@ -93,7 +93,7 @@ static struct rxrpc_local *rxrpc_alloc_local(struct rxrpc_net *rxnet,
 		local->debug_id = atomic_inc_return(&rxrpc_debug_id);
 		memcpy(&local->srx, srx, sizeof(*srx));
 		local->srx.srx_service = 0;
-		trace_rxrpc_local(local, rxrpc_local_new, 1, NULL);
+		trace_rxrpc_local(local->debug_id, rxrpc_local_new, 1, NULL);
 	}
 
 	_leave(" = %p", local);
@@ -321,7 +321,7 @@ struct rxrpc_local *rxrpc_get_local(struct rxrpc_local *local)
 	int n;
 
 	n = atomic_inc_return(&local->usage);
-	trace_rxrpc_local(local, rxrpc_local_got, n, here);
+	trace_rxrpc_local(local->debug_id, rxrpc_local_got, n, here);
 	return local;
 }
 
@@ -335,7 +335,8 @@ struct rxrpc_local *rxrpc_get_local_maybe(struct rxrpc_local *local)
 	if (local) {
 		int n = atomic_fetch_add_unless(&local->usage, 1, 0);
 		if (n > 0)
-			trace_rxrpc_local(local, rxrpc_local_got, n + 1, here);
+			trace_rxrpc_local(local->debug_id, rxrpc_local_got,
+					  n + 1, here);
 		else
 			local = NULL;
 	}
@@ -343,16 +344,16 @@ struct rxrpc_local *rxrpc_get_local_maybe(struct rxrpc_local *local)
 }
 
 /*
- * Queue a local endpoint unless it has become unreferenced and pass the
- * caller's reference to the work item.
+ * Queue a local endpoint and pass the caller's reference to the work item.
  */
 void rxrpc_queue_local(struct rxrpc_local *local)
 {
 	const void *here = __builtin_return_address(0);
+	unsigned int debug_id = local->debug_id;
+	int n = atomic_read(&local->usage);
 
 	if (rxrpc_queue_work(&local->processor))
-		trace_rxrpc_local(local, rxrpc_local_queued,
-				  atomic_read(&local->usage), here);
+		trace_rxrpc_local(debug_id, rxrpc_local_queued, n, here);
 	else
 		rxrpc_put_local(local);
 }
@@ -367,7 +368,7 @@ void rxrpc_put_local(struct rxrpc_local *local)
 
 	if (local) {
 		n = atomic_dec_return(&local->usage);
-		trace_rxrpc_local(local, rxrpc_local_put, n, here);
+		trace_rxrpc_local(local->debug_id, rxrpc_local_put, n, here);
 
 		if (n == 0)
 			call_rcu(&local->rcu, rxrpc_local_rcu);
@@ -456,7 +457,7 @@ static void rxrpc_local_processor(struct work_struct *work)
 		container_of(work, struct rxrpc_local, processor);
 	bool again;
 
-	trace_rxrpc_local(local, rxrpc_local_processing,
+	trace_rxrpc_local(local->debug_id, rxrpc_local_processing,
 			  atomic_read(&local->usage), NULL);
 
 	do {