Re: [PATCH net-next v3] tcp: annotate lockless access to sk->sk_err

[Simon Horman <horms@kernel.org>]

On Sat, Sep 12, 2026 at 11:43:34PM +0800, Quanye Yang wrote:
> 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() and getsockopt(SO_ERROR) call sock_error() without the
> socket lock. sock_error() clears sk_err with xchg(), which races with
> unmarked loads of the same field.
> 
> KCSAN reported the unmarked peek in tcp_recv_should_stop(). The same
> lockless writer races with the other plain sk_err reads on the
> TCP/MPTCP send, recv and splice paths.
> 
> Annotate those peeks with READ_ONCE(). No extra ordering is needed:
> the value is only used to decide whether I/O should stop. This does
> not consume sk_err; the check-then-sock_error() TOCTOU on the no-data
> paths is a separate issue.
> 
> Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> Link: https://lore.kernel.org/netdev/8bbee583-6f21-4817-bfeb-2d60057380a3@linux.dev/
> 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>
> ---
> Changes in v3:
> - drop the Fixes tag and retarget to net-next
> - annotate the remaining unmarked sk_err peeks on the TCP/MPTCP
>   send, recv and splice paths
> - Link to v2: https://patch.msgid.link/20260909-mptcp-sk-err-net-v2-1-5044abecac90@proton.me
> 
> 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

Reviewed-by: Simon Horman <horms@kernel.org>