net: fix sock_wake_async() rcu protection

Dmitry provided a syzkaller (http://github.com/google/syzkaller)
triggering a fault in sock_wake_async() when async IO is requested.

Said program stressed af_unix sockets, but the issue is generic
and should be addressed in core networking stack.

The problem is that by the time sock_wake_async() is called,
we should not access the @flags field of 'struct socket',
as the inode containing this socket might be freed without
further notice, and without RCU grace period.

We already maintain an RCU protected structure, "struct socket_wq"
so moving SOCKWQ_ASYNC_NOSPACE & SOCKWQ_ASYNC_WAITDATA into it
is the safe route.

It also reduces number of cache lines needing dirtying, so might
provide a performance improvement anyway.

In followup patches, we might move remaining flags (SOCK_NOSPACE,
SOCK_PASSCRED, SOCK_PASSSEC) to save 8 bytes and let 'struct socket'
being mostly read and let it being shared between cpus.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 14 deletions

diff --git a/net/socket.c b/net/socket.c
index 16be908205fc..456fadb3d819 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1056,27 +1056,20 @@ static int sock_fasync(int fd, struct file *filp, int on)
 	return 0;
 }
 
-/* This function may be called only under socket lock or callback_lock or rcu_lock */
+/* This function may be called only under rcu_lock */
 
-int sock_wake_async(struct socket *sock, int how, int band)
+int sock_wake_async(struct socket_wq *wq, int how, int band)
 {
-	struct socket_wq *wq;
-
-	if (!sock)
-		return -1;
-	rcu_read_lock();
-	wq = rcu_dereference(sock->wq);
-	if (!wq || !wq->fasync_list) {
-		rcu_read_unlock();
+	if (!wq || !wq->fasync_list)
 		return -1;
-	}
+
 	switch (how) {
 	case SOCK_WAKE_WAITD:
-		if (test_bit(SOCKWQ_ASYNC_WAITDATA, &sock->flags))
+		if (test_bit(SOCKWQ_ASYNC_WAITDATA, &wq->flags))
 			break;
 		goto call_kill;
 	case SOCK_WAKE_SPACE:
-		if (!test_and_clear_bit(SOCKWQ_ASYNC_NOSPACE, &sock->flags))
+		if (!test_and_clear_bit(SOCKWQ_ASYNC_NOSPACE, &wq->flags))
 			break;
 		/* fall through */
 	case SOCK_WAKE_IO:
@@ -1086,7 +1079,7 @@ call_kill:
 	case SOCK_WAKE_URG:
 		kill_fasync(&wq->fasync_list, SIGURG, band);
 	}
-	rcu_read_unlock();
+
 	return 0;
 }
 EXPORT_SYMBOL(sock_wake_async);