Re: [PATCH net v2] tcp: fix data-race in tcp_recv_should_stop

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 9/9/26 10:05 AM, Quanye Yang via B4 Relay wrote:
> From: Quanye Yang <quanyeyang@proton.me>
>
> BUG: KCSAN: data-race in do_recvmmsg / mptcp_recvmsg
>
> read-write (marked) to 0xffff8880134d391c of 4 bytes by task 2619 on cpu 1:
>   instrument_atomic_read_write include/linux/instrumented.h:113 [inline]
>   sock_error include/net/sock.h:2565 [inline]
>   do_recvmmsg+0x50c/0x580 net/socket.c:3049
>   __sys_recvmmsg net/socket.c:3144 [inline]
>   __do_sys_recvmmsg net/socket.c:3167 [inline]
>   __se_sys_recvmmsg net/socket.c:3160 [inline]
>   __x64_sys_recvmmsg+0x161/0x180 net/socket.c:3160
>   x64_sys_call+0x19c7/0x1ca0 arch/x86/include/generated/asm/syscalls_64.h:300
>   do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
>   do_syscall_64+0xde/0x3d0 arch/x86/entry/syscall_64.c:84
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> read to 0xffff8880134d391c of 4 bytes by task 2620 on cpu 0:
>   tcp_recv_should_stop include/net/tcp.h:3086 [inline]
>   mptcp_recvmsg+0x54d/0xd50 net/mptcp/protocol.c:2466
>   inet_recvmsg+0x204/0x210 net/ipv4/af_inet.c:894
>   sock_recvmsg_nosec net/socket.c:1151 [inline]
>   sock_recvmsg+0x11a/0x140 net/socket.c:1173
>   ____sys_recvmsg+0x14b/0x3c0 net/socket.c:2933
>   ___sys_recvmsg+0x116/0x160 net/socket.c:2975
>   __sys_recvmsg net/socket.c:3008 [inline]
>   __do_sys_recvmsg net/socket.c:3014 [inline]
>   __se_sys_recvmsg net/socket.c:3011 [inline]
>   __x64_sys_recvmsg+0xeb/0x160 net/socket.c:3011
>   x64_sys_call+0x1319/0x1ca0 arch/x86/include/generated/asm/syscalls_64.h:48
>   do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
>   do_syscall_64+0xde/0x3d0 arch/x86/entry/syscall_64.c:84
>   entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> value changed: 0x0000006b -> 0x00000000
>
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 0 UID: 0 PID: 2620 Comm: syz.2.33 Not tainted 7.2.0-g39d4f32c5d53 #76 PREEMPT(full)
> Hardware name: QEMU Ubuntu 26.04 PC (i440FX + PIIX, 1996), BIOS 1.17.0-debian-1.17.0-1ubuntu1 04/01/2014
>
> do_recvmmsg() calls sock_error() without the socket lock. sock_error()
> clears sk_err with xchg(), which races with the unmarked load in
> tcp_recv_should_stop(). The same race exists for plain TCP with
> concurrent recvmmsg() readers; KCSAN reported it on an MPTCP socket
> because mptcp_recvmsg() uses this helper.
>
> Use READ_ONCE() for the lockless peek. No extra ordering is needed:
> the value is only used to decide whether receiving should stop.
> This does not consume sk_err; the check-then-sock_error() TOCTOU
> on the no-data paths is a separate issue.
>
> Fixes: eb477fdd6803 ("tcp: add recv_should_stop helper")


Is the Fixes tag right? eb477fdd6803 only moved the existing check
into a helper, the plain read was already there before.


> Reported-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/632
> Signed-off-by: Quanye Yang <quanyeyang@proton.me>
> ---
> Link: https://lore.kernel.org/all/14749060-d011-41e7-9a4b-754eb5bd9d5b@redhat.com/
> ---
> Changes in v2:
> - add Reported-by and Closes for the MPTCP syzkaller report
> - Link to v1: https://patch.msgid.link/20260908-mptcp-sk-err-net-v1-1-da71aaec9afd@proton.me
> ---
>   include/net/tcp.h | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/include/net/tcp.h b/include/net/tcp.h
> index 436495ff2271..c61d8678eafd 100644
> --- a/include/net/tcp.h
> +++ b/include/net/tcp.h
> @@ -3082,7 +3082,8 @@ enum skb_drop_reason tcp_inbound_hash(struct sock *sk,
>   
>   static inline int tcp_recv_should_stop(struct sock *sk)
>   {
> -	return sk->sk_err ||
> +	/* sk_err can be cleared locklessly by sock_error(). */
> +	return READ_ONCE(sk->sk_err) ||
>   	       sk->sk_state == TCP_CLOSE ||
>   	       (sk->sk_shutdown & RCV_SHUTDOWN) ||
>   	       signal_pending(current);


The lockless writers are sock_error() from SO_ERROR and recvmmsg(),
so every plain read of sk_err on a TCP socket has the same race, not 
only this helper:
     tcp_recvmsg_locked
     tcp_sendmsg_locked
     tcp_splice_read
     sk_stream_wait_memory
     tcp_bpf_sendmsg
     tcp_bpf_recvmsg_parser
     mptcp_splice_read
     mptcp_recvmsg

I'd drop the Fixes tag, target net-next and annotate all of them like 
e13ec3da05d1, instead of pointing at a MPTCP commit.

Eric, Matthieu, does that sound OK?