[PATCH v5 1/3] vsock: don't check the listener's sk_err in vsock_accept()

[Nguyen Dinh Phi <phind.uet@gmail.com>]

Syzbot reported an issue which can be reproduced with these steps:
	r0 = socket(AF_VSOCK, SOCK_STREAM, 0)
	bind(r0, {VMADDR_CID_ANY, PORT})
	connect(r0, {VMADDR_CID_LOCAL, PORT}) -> -1, EPROTO (self-connect)
	listen(r0, backlog)                   -> 0
	r1 = socket(AF_VSOCK, SOCK_STREAM, 0)
	connect(r1, {VMADDR_CID_LOCAL, PORT}) -> 0
	accept(r0)                            -> -1, EPROTO (stale sk_err)

Basically, it creates a socket (r0) and triggers a self-connect after
binding it. This self-connect fails with EPROTO because it loops back
to r0 while the socket is still in the TCP_SYN_SENT state, causing it
to be incorrectly dispatched to the connecting-client path. The
unexpected packet type encountered there sets sk_err to EPROTO.

After that, it invokes a listen() call on the same socket. This
listen() call succeeds because the kernel's listening path never
inspects or clears sk_err. Then, a new socket (r1) is created as a
normal client and connects to r0. However, vsock_accept() rejects this
incoming connection because the listener's sk_err still holds the
EPROTO error from the earlier failed self-connect.

This rejection causes the child socket created for r1's connection to
never be freed on virtio or hyperv transports; only the VMCI transport
implements pending_work to revisit and clean up a rejected socket.

For a non-blocking connect(), vsock_connect() may return -EINPROGRESS
immediately, and vsock_connect_timeout() can later set sk->sk_err
asynchronously.

Since no vsock transport ever sets sk_err on a socket while it is in
TCP_LISTEN state, checking it in vsock_accept() serves no purpose and
only carries forward errors left behind by earlier, unrelated
connection attempts on the same socket. Remove the checks so accept()
no longer rejects valid incoming connections because of a stale
error, which also avoids the resource leak described above.

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Reported-by: syzbot+1b2c9c4a0f8708082678@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1b2c9c4a0f8708082678
Suggested-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
---
 net/vmw_vsock/af_vsock.c | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 622dbd046799..ff507761f472 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -1893,7 +1893,7 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 	timeout = sock_rcvtimeo(listener, arg->flags & O_NONBLOCK);
 
 	while ((connected = vsock_dequeue_accept(listener)) == NULL &&
-	       listener->sk_err == 0 && timeout != 0) {
+		timeout != 0) {
 		prepare_to_wait(sk_sleep(listener), &wait, TASK_INTERRUPTIBLE);
 		release_sock(listener);
 		timeout = schedule_timeout(timeout);
@@ -1906,12 +1906,6 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 		}
 	}
 
-	if (listener->sk_err) {
-		err = -listener->sk_err;
-	} else if (!connected) {
-		err = -EAGAIN;
-	}
-
 	if (connected) {
 		sk_acceptq_removed(listener);
 
@@ -1941,6 +1935,8 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 
 		release_sock(connected);
 		sock_put(connected);
+	} else {
+		err = -EAGAIN;
 	}
 
 out:
-- 
2.53.0