Re: [PATCH net-next v3] tcp: annotate lockless access to sk->sk_err
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From: Simon Horman <horms@kernel.org>
Date: 2026-09-17 08:16:40
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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/ (local) 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> --- 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 (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 (local)
Reviewed-by: Simon Horman <horms@kernel.org>