[PATCH v4] vsock: use sock_error() to consume sk_err after a failed connect

[phind.uet@gmail.com]

From: 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.

Fix the issue in blocking connect() by using sock_error() to read the
sk_err to prevent the rejection branch from occurring in this scenario.

sock_error() atomically reads and clears sk_err, ensuring the error is
consumed when vsock_connect() returns and cannot affect subsequent
operations on the same socket. This matches the established pattern
used by other protocol connect() implementations in the network
stack like __inet_stream_connect(), tipc_wait_for_connect()...

For non-blocking connection, vsock_connect_timeout() may set
sk->sk_err after vsock_connect() has returned. To handle it, we also
remove the sk_err checks from vsock_accept(). Nothing in vsock sets
sk_err on a listening socket, so accept() has no reason to inspect it
at all.

Reported-by: syzbot+1b2c9c4a0f8708082678@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1b2c9c4a0f8708082678
Fixes: d021c344051af ("VSOCK: Introduce VM Sockets")
Suggested-by: Michal Luczaj <mhal@rbox.co>
Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
Tested-by: Wupeng Ma <mawupeng1@huawei.com>
---
V2: Add reproducer steps to commit message.
V3: Fix truncated title and add annotations to reproducer steps.
V4: Remove sk_err checks from vsock_accept()

 net/vmw_vsock/af_vsock.c | 13 ++++---------
 1 file changed, 4 insertions(+), 9 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 622dbd046799..594fe27d2ebe 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -1847,12 +1847,10 @@ static int vsock_connect(struct socket *sock, struct sockaddr_unsized *addr,
 		prepare_to_wait(sk_sleep(sk), &wait, TASK_INTERRUPTIBLE);
 	}
 
-	if (sk->sk_err) {
-		err = -sk->sk_err;
+	err = sock_error(sk);
+	if (err) {
 		sk->sk_state = TCP_CLOSE;
 		sock->state = SS_UNCONNECTED;
-	} else {
-		err = 0;
 	}
 
 out_wait:
@@ -1893,7 +1891,7 @@ 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);
@@ -1906,11 +1904,8 @@ static int vsock_accept(struct socket *sock, struct socket *newsock,
 		}
 	}
 
-	if (listener->sk_err) {
-		err = -listener->sk_err;
-	} else if (!connected) {
+	if (!connected)
 		err = -EAGAIN;
-	}
 
 	if (connected) {
 		sk_acceptq_removed(listener);
-- 
2.43.0