rxrpc: Fix trace-after-put looking at the put peer record

rxrpc_put_peer() calls trace_rxrpc_peer() after it has done the decrement
of the refcount - which looks at the debug_id in the peer record.  But
unless the refcount was reduced to zero, we no longer have the right to
look in the record and, indeed, it may be deleted by some other thread.

Fix this by getting the debug_id out before decrementing the refcount and
then passing that into the tracepoint.

This can cause the following symptoms:

    BUG: KASAN: use-after-free in __rxrpc_put_peer net/rxrpc/peer_object.c:411
    [inline]
    BUG: KASAN: use-after-free in rxrpc_put_peer+0x685/0x6a0
    net/rxrpc/peer_object.c:435
    Read of size 8 at addr ffff888097ec0058 by task syz-executor823/24216

Fixes: 1159d4b4 ("rxrpc: Add a tracepoint to track rxrpc_peer refcounting")
Reported-by: syzbot+b9be979c55f2bea8ed30@syzkaller.appspotmail.com
Signed-off-by: David Howells <dhowells@redhat.com>

include/trace/events/rxrpc.h

@@ -519,10 +519,10 @@ TRACE_EVENT(rxrpc_local,
 	    );
 
 TRACE_EVENT(rxrpc_peer,
-	    TP_PROTO(struct rxrpc_peer *peer, enum rxrpc_peer_trace op,
+	    TP_PROTO(unsigned int peer_debug_id, enum rxrpc_peer_trace op,
 		     int usage, const void *where),
 
-	    TP_ARGS(peer, op, usage, where),
+	    TP_ARGS(peer_debug_id, op, usage, where),
 
 	    TP_STRUCT__entry(
 		    __field(unsigned int,	peer		)
@@ -532,7 +532,7 @@ TRACE_EVENT(rxrpc_peer,
 			     ),
 
 	    TP_fast_assign(
-		    __entry->peer = peer->debug_id;
+		    __entry->peer = peer_debug_id;
 		    __entry->op = op;
 		    __entry->usage = usage;
 		    __entry->where = where;

net/rxrpc/peer_object.c

@@ -382,7 +382,7 @@ struct rxrpc_peer *rxrpc_get_peer(struct rxrpc_peer *peer)
 	int n;
 
 	n = atomic_inc_return(&peer->usage);
-	trace_rxrpc_peer(peer, rxrpc_peer_got, n, here);
+	trace_rxrpc_peer(peer->debug_id, rxrpc_peer_got, n, here);
 	return peer;
 }
 
@@ -396,7 +396,7 @@ struct rxrpc_peer *rxrpc_get_peer_maybe(struct rxrpc_peer *peer)
 	if (peer) {
 		int n = atomic_fetch_add_unless(&peer->usage, 1, 0);
 		if (n > 0)
-			trace_rxrpc_peer(peer, rxrpc_peer_got, n + 1, here);
+			trace_rxrpc_peer(peer->debug_id, rxrpc_peer_got, n + 1, here);
 		else
 			peer = NULL;
 	}
@@ -426,11 +426,13 @@ static void __rxrpc_put_peer(struct rxrpc_peer *peer)
 void rxrpc_put_peer(struct rxrpc_peer *peer)
 {
 	const void *here = __builtin_return_address(0);
+	unsigned int debug_id;
 	int n;
 
 	if (peer) {
+		debug_id = peer->debug_id;
 		n = atomic_dec_return(&peer->usage);
-		trace_rxrpc_peer(peer, rxrpc_peer_put, n, here);
+		trace_rxrpc_peer(debug_id, rxrpc_peer_put, n, here);
 		if (n == 0)
 			__rxrpc_put_peer(peer);
 	}
@@ -443,10 +445,11 @@ void rxrpc_put_peer(struct rxrpc_peer *peer)
 void rxrpc_put_peer_locked(struct rxrpc_peer *peer)
 {
 	const void *here = __builtin_return_address(0);
+	unsigned int debug_id = peer->debug_id;
 	int n;
 
 	n = atomic_dec_return(&peer->usage);
-	trace_rxrpc_peer(peer, rxrpc_peer_put, n, here);
+	trace_rxrpc_peer(debug_id, rxrpc_peer_put, n, here);
 	if (n == 0) {
 		hash_del_rcu(&peer->hash_link);
 		list_del_init(&peer->keepalive_link);