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

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

[Quanye Yang via B4 Relay]

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")
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);

---
base-commit: 38b6be101006d3e7af972999f45d4f1e8250587a
change-id: 20260908-mptcp-sk-err-net-7ff88ef05044

Best regards,
--  
Quanye Yang <quanyeyang@proton.me>

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

[Jiayuan Chen]

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?

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@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?

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

Cheers,
Matt
-- 
Sponsored by the NGI0 Core fund.

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

[netdev-bot+sashiko]

Thank you for your contribution! Sashiko AI review found 1 potential
issue(s) to consider:

Critical: 0 · High: 1 · Medium: 0 · Low: 0

- [High] Incomplete data-race fix: the patch converts only the sk_err
  load inside tcp_recv_should_stop() (include/net/tcp.h:3086) to…

--

tcp: fix data-race in tcp_recv_should_stop

This converts the sk_err load inside tcp_recv_should_stop() to READ_ONCE()
because do_recvmmsg() calls sock_error() without the socket lock and
sock_error() clears sk_err with xchg(). It carries a Fixes: tag for
eb477fdd6803 and a Closes: tag for the MPTCP syzkaller report.

Link: https://lore.kernel.org/all/14749060-d011-41e7-9a4b-754eb5bd9d5b@redhat.com/

> 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);

[Severity: High]
Is annotating only this read enough to close the reported race?

Every caller of tcp_recv_should_stop() keeps a plain read of the same
field in the other arm of the very same loop.  In mptcp_recvmsg(), the
function named in the KCSAN splat:

net/mptcp/protocol.c:mptcp_recvmsg() {
	...
		if (copied) {
			if (tcp_recv_should_stop(sk) ||
			    !timeo)
				break;
		} else {
			if (sk->sk_err) {
				copied = sock_error(sk);
				break;
			}
	...
}

The same shape exists in tcp_recvmsg_locked():

net/ipv4/tcp.c:tcp_recvmsg_locked() {
	...
		if (copied) {
			if (!timeo ||
			    tcp_recv_should_stop(sk))
				break;
		} else {
			if (sock_flag(sk, SOCK_DONE))
				break;

			if (sk->sk_err) {
				copied = sock_error(sk);
				break;
			}
	...
}

and in tcp_splice_read() and mptcp_splice_read(), whose loops both end
with the now-annotated helper:

net/ipv4/tcp.c:tcp_splice_read() {
	...
			if (sk->sk_err) {
				ret = sock_error(sk);
				break;
			}
	...
		if (tcp_recv_should_stop(sk))
			break;
	...
}

The writer described in the changelog, do_recvmmsg() calling
sock_error(sock->sk) with no lock_sock() and clearing the field with
xchg(&sk->sk_err, 0), races with those four plain reads exactly as it
raced with the one that was annotated, since lock_sock() in the readers
gives no exclusion against it.

Would a reproducer that lands in the blocking, copied == 0 arm still
produce the identical "data-race in do_recvmmsg / mptcp_recvmsg" report,
just attributed to mptcp_recvmsg() rather than to
tcp_recv_should_stop()?  If so, should the Closes: tag be dropped, or
should the four remaining reads be converted in the same patch?

The Fixes: tag points at eb477fdd6803, but that commit only moved these
pre-existing plain reads into the new helper; the sibling reads left
behind are part of the same defect.  Does the Fixes: attribution still
hold with only the helper annotated?

The added comment states "sk_err can be cleared locklessly by
sock_error()", which reads as a property of the field rather than of this
one call site.  Should the write side conventions be noted too, since
the writers in these files already use WRITE_ONCE() and sock_error()
itself uses data_race() for its own read?

-- 
Sashiko AI review · https://netdev-ai.bots.linux.dev/sashiko/#/patchset/20260909-mptcp-sk-err-net-v2-1-5044abecac90%40proton.me

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

[quanyeyang]

> 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?
> 
Thanks Jiayuan, Matt.

Agreed that eb477fdd6803 only moved the existing load, so I will
drop the Fixes tag and retarget to net-next.

I will also annotate the remaining unmarked sk_err reads on the
TCP/MPTCP recv and splice paths (the other arm of the same loops
that Sashiko pointed out). I would keep this as KCSAN annotations
in the style of e13ec3da05d1, and leave the check-then-sock_error()
TOCTOU as a separate change.

sk_stream_wait_memory() already uses READ_ONCE(sk->sk_err). I will
double-check the sendmsg/bpf sites before expanding further.

Happy to wait if Eric prefers a different scope.

Quanye