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, 14 insertions, 10 deletions

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 2353985..5e35ef3 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -6801,26 +6801,30 @@ no_packet:
 static void __sctp_write_space(struct sctp_association *asoc)
 {
 	struct sock *sk = asoc->base.sk;
-	struct socket *sock = sk->sk_socket;
 
-	if ((sctp_wspace(asoc) > 0) && sock) {
-		if (waitqueue_active(&asoc->wait))
-			wake_up_interruptible(&asoc->wait);
+	if (sctp_wspace(asoc) <= 0)
+		return;
+
+	if (waitqueue_active(&asoc->wait))
+		wake_up_interruptible(&asoc->wait);
 
-		if (sctp_writeable(sk)) {
-			wait_queue_head_t *wq = sk_sleep(sk);
+	if (sctp_writeable(sk)) {
+		struct socket_wq *wq;
 
-			if (wq && waitqueue_active(wq))
-				wake_up_interruptible(wq);
+		rcu_read_lock();
+		wq = rcu_dereference(sk->sk_wq);
+		if (wq) {
+			if (waitqueue_active(&wq->wait))
+				wake_up_interruptible(&wq->wait);
 
 			/* Note that we try to include the Async I/O support
 			 * here by modeling from the current TCP/UDP code.
 			 * We have not tested with it yet.
 			 */
 			if (!(sk->sk_shutdown & SEND_SHUTDOWN))
-				sock_wake_async(sock,
-						SOCK_WAKE_SPACE, POLL_OUT);
+				sock_wake_async(wq, SOCK_WAKE_SPACE, POLL_OUT);
 		}
+		rcu_read_unlock();
 	}
 }