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tcp: annotate lockless access to sk->sk_err

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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(). Annotate
that helper and the other send-side peeks with READ_ONCE().

On the no-data recv and splice paths, if (sk_err) followed by
sock_error() and an unconditional break can return 0 after another
thread consumes the error. Call sock_error() once and only stop when
it returns a non-zero error.

tcp_bpf_sendmsg() read sk_err twice; fold those unmarked loads into
one READ_ONCE() and use that value as the returned errno. The field
is still not consumed.

MPTCP is handled in the next patch.

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>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/20260925-mptcp-sk-err-net-v5-1-0cac04d6ea48@proton.me
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
This commit is contained in:
Quanye Yang 2026-09-25 05:47:56 -07:00 • committed by Paolo Abeni
commit 3b49f11cc9
4 changed files with 11 additions and 15 deletions
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