Re: [PATCH net] vsock/bpf: release sock lock while waiting for data in recvmsg

[Michal Luczaj <mhal@rbox.co>]

On 10/1/26 11:01, Stefano Garzarella wrote:
> +cc Bobby and Michal who touched this code recently
> 
> On Tue, Sep 29, 2026 at 06:33:04PM +0200, Jerome Mohm via B4 Relay wrote:
>> From: Jerome Mohm <jrmmhm.kernel@eldare.de>
>>
>> vsock_bpf_recvmsg() takes lock_sock(sk) and holds it across the whole
>> receive loop, including vsock_msg_wait_data(), which sleeps in
>> wait_woken() without dropping the lock. When data arrives, the
>> transport's delivery context (the vsock-loopback worker, or the
>> virtio/vhost rx path) calls virtio_transport_recv_pkt() -> lock_sock()
>> on the same socket and blocks. That context is the only one that would
>> queue the skb and call sk_data_ready() to wake the reader, so neither
>> side makes progress. A blocking recv() hangs until a signal arrives (a
>> finite SO_RCVTIMEO also breaks it); while it lasts the shared delivery
>> worker is stalled, so all vsock rx on that transport stops, not only
>> the affected socket. The hung-task watchdog reports the worker blocked
>> in D state:
>>
>>  INFO: task kworker/1:3:107 blocked for more than 5 seconds.
>>  task:kworker/1:3 state:D Workqueue: vsock-loopback vsock_loopback_work
>>  Call Trace:
>>   __schedule
>>   schedule
>>   __lock_sock
>>   lock_sock_nested
>>   virtio_transport_recv_pkt
>>   vsock_loopback_work
>>
>> The reader holds the same sk_lock-AF_VSOCK it is waiting on, taken in
>> vsock_bpf_recvmsg().
>>
>> This code is based on net/unix/unix_bpf.c, whose unix_msg_wait_data()
>> drops u->iolock around wait_woken() and re-takes it afterwards. The
>> vsock port substituted lock_sock() for that serialisation lock but
>> omitted the unlock and relock. tcp_bpf and the native
>> vsock_connectible_wait_data() both drop the lock across the wait;
>> vsock_bpf is the only one that does not.
>>
>> Release the socket lock around the wait and re-acquire it before
>> re-checking for data, so the caller's locking is unchanged.
>>
>> Fixes: 634f1a7110b4 ("vsock: support sockmap")
>> Cc: stable@vger.kernel.org
>> Assisted-by: LLM
>> Signed-off-by: Jerome Mohm <jrmmhm.kernel@eldare.de>
>> ---
>> net/vmw_vsock/vsock_bpf.c | 2 ++
>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/net/vmw_vsock/vsock_bpf.c b/net/vmw_vsock/vsock_bpf.c
>> index 9049d2648646..bb7d81a95baa 100644
>> --- a/net/vmw_vsock/vsock_bpf.c
>> +++ b/net/vmw_vsock/vsock_bpf.c
>> @@ -50,7 +50,9 @@ static bool vsock_msg_wait_data(struct sock *sk, struct sk_psock *psock, long ti
>> 	sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>> 	ret = vsock_has_data(sk, psock);
>> 	if (!ret) {
>> +		release_sock(sk);
>> 		wait_woken(&wait, TASK_INTERRUPTIBLE, timeo);
>> +		lock_sock(sk);
>> 		ret = vsock_has_data(sk, psock);
>> 	}
>> 	sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>>
>> ---
>> base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
>> change-id: 20260929-kbh3-1-022-fix-340aa3f7f071
>>
>> Best regards,
>> --
>> Jerome Mohm <jrmmhm.kernel@eldare.de>
>>
> 
> LGTM, but I'd like also Bobby and Michal opinion:
> 
> Acked-by: Stefano Garzarella <sgarzare@redhat.com>

I took a look at v2 and I don't really have any functional comments. Well,
besides the use of READ_ONCE, but that's probably something we need to
tackle separately?

A single-patch v2 addresses not only the original bug, but also sashiko's
complains about the pre-existing issues (making the subject stale?), but I
didn't know if you'd rather have all of that stuffed together, so I
refrained from commenting to avoid confusing the author.

thanks,
Michal