Re: [PATCH net v4 3/3] vsock/bpf: Fix bpf recvmsg() racing transport reassignment

[Michal Luczaj <mhal@rbox.co>]

On 3/19/25 23:18, Cong Wang wrote:
> On Mon, Mar 17, 2025 at 10:52:25AM +0100, Michal Luczaj wrote:
>> Signal delivery during connect() may lead to a disconnect of an already
>> established socket. That involves removing socket from any sockmap and
>> resetting state to SS_UNCONNECTED. While it correctly restores socket's
>> proto, a call to vsock_bpf_recvmsg() might have been already under way in
>> another thread. If the connect()ing thread reassigns the vsock transport to
>> NULL, the recvmsg()ing thread may trigger a WARN_ON_ONCE.
>>

   *THREAD 1*                      *THREAD 2*

>> connect
>>   / state = SS_CONNECTED /
>>                                 sock_map_update_elem
>>                                 vsock_bpf_recvmsg
>>                                   psock = sk_psock_get()
>>   lock sk
>>   if signal_pending
>>     unhash
>>       sock_map_remove_links
> 
> So vsock's ->recvmsg() should be restored after this, right? Then how is
> vsock_bpf_recvmsg() called afterward?

I'm not sure I understand the question, so I've added a header above: those
are 2 parallel flows of execution. vsock_bpf_recvmsg() wasn't called
afterwards. It was called before sock_map_remove_links(). Note that at the
time of sock_map_remove_links() (in T1), vsock_bpf_recvmsg() is still
executing (in T2).

>>     state = SS_UNCONNECTED
>>   release sk
>>
>> connect
>>   transport = NULL
>>                                   lock sk
>>                                   WARN_ON_ONCE(!vsk->transport)
>>
> 
> And I am wondering why we need to WARN here since we can handle this error
> case correctly?

The WARN and transport check are here for defensive measures, and to state
a contract.

But I think I get your point. If we accept for a fact of life that BPF code
should be able to handle transport disappearing - then WARN can be removed
(while keeping the check) and this patch can be dropped.

My aim, instead, was to keep things consistent. By which I mean sticking to
the conditions expressed in vsock_bpf_update_proto() as invariants; so that
vsock with a psock is guaranteed to have transport assigned.