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

From: Quanye Yang <redacted>

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 <redacted>
---
Link: https://lore.kernel.org/all/14749060-d011-41e7-9a4b-754eb5bd9d5b@redhat.com/ (local)
---
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 [off-list ref]