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

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Message-ID: <Z9tCnq0rBw+nETfW@pop-os.localdomain>
Date: Wed, 19 Mar 2025 15:18:06 -0700
From: Cong Wang <xiyou.wangcong@...il.com>
To: Michal Luczaj <mhal@...x.co>
Cc: Stefano Garzarella <sgarzare@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Bobby Eshleman <bobby.eshleman@...edance.com>,
	Andrii Nakryiko <andrii@...nel.org>,
	Eduard Zingerman <eddyz87@...il.com>,
	Mykola Lysenko <mykolal@...com>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Martin KaFai Lau <martin.lau@...ux.dev>, Song Liu <song@...nel.org>,
	Yonghong Song <yonghong.song@...ux.dev>,
	John Fastabend <john.fastabend@...il.com>,
	KP Singh <kpsingh@...nel.org>, Stanislav Fomichev <sdf@...ichev.me>,
	Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
	Shuah Khan <shuah@...nel.org>, netdev@...r.kernel.org,
	bpf@...r.kernel.org, virtualization@...ts.linux.dev,
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH net v4 3/3] vsock/bpf: Fix bpf recvmsg() racing transport
 reassignment

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.
> 
> 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?

>     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?

Thanks.