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

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

[phind.uet]

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>
---
V2: Add reproducer steps to commit message.
V3: Fix truncated title and add annotations to reproducer steps.
 net/vmw_vsock/af_vsock.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 622dbd046799..43eddc33ed12 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -1847,14 +1847,11 @@ 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:
 	finish_wait(sk_sleep(sk), &wait);
 out:
-- 
2.43.0

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

[Paolo Abeni]

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

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

[Stefano Garzarella]

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

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

[Nguyen Dinh Phi [SG]]

On 4/8/26 17:37, Stefano Garzarella wrote:
> 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
> 

Sorry, I made a mistake when sending email.
I've just sent a new version.

Thanks,
Phi.