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

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, Aug 04, 2026 at 11:26:57AM +0200, Paolo Abeni wrote:
>
>
>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

Yeah, it seems the same conclusion we reached with Michal on v1 and Phi 
agreed on: https://lore.kernel.org/netdev/148e56ec-dc26-4be2-a7af-eb547b517a68@gmail.com/

Not sure why sk_err check was not removed in vsock_accept.

Phi can you check?

Thanks,
Stefano