From: Quanye Yang 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 Link: https://lore.kernel.org/netdev/8bbee583-6f21-4817-bfeb-2d60057380a3@linux.dev/ Reported-by: Matthieu Baerts (NGI0) Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/632 Signed-off-by: Quanye Yang --- 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 --- include/net/tcp.h | 3 ++- net/core/stream.c | 2 +- net/ipv4/tcp.c | 6 +++--- net/ipv4/tcp_bpf.c | 7 +++---- net/mptcp/protocol.c | 6 +++--- 5 files changed, 12 insertions(+), 12 deletions(-) diff --git a/include/net/tcp.h b/include/net/tcp.h index 5e5f5f9b89a3..6ee513bdb503 100644 --- a/include/net/tcp.h +++ b/include/net/tcp.h @@ -3093,7 +3093,8 @@ 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); diff --git a/net/core/stream.c b/net/core/stream.c index 2d748581862d..5c0adc3077f0 100644 --- a/net/core/stream.c +++ b/net/core/stream.c @@ -133,7 +133,7 @@ int sk_stream_wait_memory(struct sock *sk, long *timeo_p) while (1) { sk_set_bit(SOCKWQ_ASYNC_NOSPACE, sk); - if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN)) + if (READ_ONCE(sk->sk_err) || (sk->sk_shutdown & SEND_SHUTDOWN)) goto do_error; if (!*timeo_p) goto do_eagain; diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 1c867a302444..9e7fa7e39726 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -843,7 +843,7 @@ ssize_t tcp_splice_read(struct socket *sock, loff_t *ppos, break; if (sock_flag(sk, SOCK_DONE)) break; - if (sk->sk_err) { + if (READ_ONCE(sk->sk_err)) { ret = sock_error(sk); break; } @@ -1228,7 +1228,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size) mss_now = tcp_send_mss(sk, &size_goal, flags); err = -EPIPE; - if (sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN)) + if (READ_ONCE(sk->sk_err) || (sk->sk_shutdown & SEND_SHUTDOWN)) goto do_error; while (msg_data_left(msg)) { @@ -2760,7 +2760,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len, if (sock_flag(sk, SOCK_DONE)) break; - if (sk->sk_err) { + if (READ_ONCE(sk->sk_err)) { copied = sock_error(sk); break; } diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c index 2e234d155b5e..a96323e65758 100644 --- a/net/ipv4/tcp_bpf.c +++ b/net/ipv4/tcp_bpf.c @@ -286,7 +286,7 @@ static int tcp_bpf_recvmsg_parser(struct sock *sk, if (sock_flag(sk, SOCK_DONE)) goto out; - if (sk->sk_err) { + if (READ_ONCE(sk->sk_err)) { copied = sock_error(sk); goto out; } @@ -553,10 +553,9 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size) bool enospc = false; u32 copy, osize; - if (sk->sk_err) { - err = -sk->sk_err; + err = -READ_ONCE(sk->sk_err); + if (err) goto out_err; - } copy = msg_data_left(msg); if (!sk_stream_memory_free(sk)) diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index e1f08f71cdb1..7631fa582834 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -2028,7 +2028,7 @@ static int mptcp_sendmsg(struct sock *sk, struct msghdr *msg, size_t len) } ret = -EPIPE; - if (unlikely(sk->sk_err || (sk->sk_shutdown & SEND_SHUTDOWN))) + if (unlikely(READ_ONCE(sk->sk_err) || (sk->sk_shutdown & SEND_SHUTDOWN))) goto do_error; pfrag = sk_page_frag(sk); @@ -2429,7 +2429,7 @@ static int mptcp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len, !timeo) break; } else { - if (sk->sk_err) { + if (READ_ONCE(sk->sk_err)) { copied = sock_error(sk); break; } @@ -4655,7 +4655,7 @@ static ssize_t mptcp_splice_read(struct socket *sock, loff_t *ppos, break; if (sock_flag(sk, SOCK_DONE)) break; - if (sk->sk_err) { + if (READ_ONCE(sk->sk_err)) { ret = sock_error(sk); break; } --- base-commit: 211f2a875f6f447745d80d5762d6d614503ac84a change-id: 20260908-mptcp-sk-err-net-7ff88ef05044 Best regards, -- Quanye Yang