Re: [PATCH v3] vsock: use sock_error() to consume sk_err after a failed connect

[Paolo Abeni <pabeni@redhat.com>]

On 7/30/26 10:18 AM, phind.uet@gmail.com wrote:
> From: Nguyen Dinh Phi <phind.uet@gmail.com>
> 
> Syzbot report 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 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()...
> 
> Reported-by: syzbot+1b2c9c4a0f8708082678@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=1b2c9c4a0f8708082678
> Fixes: d021c344051af ("VSOCK: Introduce VM Sockets")
> Signed-off-by: Nguyen Dinh Phi <phind.uet@gmail.com>
> Tested-by: Wupeng Ma <mawupeng1@huawei.com>
Sashiko nipa points out that the race still exits:

https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260730081843.287563-1-phind.uet%40gmail.com

/P