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

[Matthieu Baerts]

Hi Jiayuan,

On 09/09/2026 04:57, Jiayuan Chen wrote:
> 
> On 9/9/26 10:05 AM, Quanye Yang via B4 Relay wrote:
>> From: Quanye Yang <quanyeyang@xxxxxxxxx>
>>
>> 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@xxxxxxxxxx>
>> Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/632
>> Signed-off-by: Quanye Yang <quanyeyang@xxxxxxxxx>
>> ---
>> Link: https://lore.kernel.org/all/14749060-
>> d011-41e7-9a4b-754eb5bd9d5b@xxxxxxxxxx/
>> ---
>> 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@xxxxxxxxx
>> ---
>>   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?

Sounds good to me to target net-next without the Fixes tag.

Cheers,
Matt
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