This is a note to let you know that I've just added the patch titled af_unix: Fix some data-races around unix_sk(sk)->oob_skb. to the 5.15-stable tree which can be found at: http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary The filename of the patch is: af_unix-fix-some-data-races-around-unix_sk-sk-oob_sk.patch and it can be found in the queue-5.15 subdirectory. If you, or anyone else, feels it should not be added to the stable tree, please let <stable@xxxxxxxxxxxxxxx> know about it. commit 1f0077adc8c029f03aaa2107c62729a483e37a2b Author: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx> Date: Thu Mar 17 12:08:08 2022 +0900 af_unix: Fix some data-races around unix_sk(sk)->oob_skb. [ Upstream commit e82025c623e2bf04d162bafceb66a59115814479 ] Out-of-band data automatically places a "mark" showing wherein the sequence the out-of-band data would have been. If the out-of-band data implies cancelling everything sent so far, the "mark" is helpful to flush them. When the socket's read pointer reaches the "mark", the ioctl() below sets a non zero value to the arg `atmark`: The out-of-band data is queued in sk->sk_receive_queue as well as ordinary data and also saved in unix_sk(sk)->oob_skb. It can be used to test if the head of the receive queue is the out-of-band data meaning the socket is at the "mark". While testing that, unix_ioctl() reads unix_sk(sk)->oob_skb locklessly. Thus, all accesses to oob_skb need some basic protection to avoid load/store tearing which KCSAN detects when these are called concurrently: - ioctl(fd_a, SIOCATMARK, &atmark, sizeof(atmark)) - send(fd_b_connected_to_a, buf, sizeof(buf), MSG_OOB) BUG: KCSAN: data-race in unix_ioctl / unix_stream_sendmsg write to 0xffff888003d9cff0 of 8 bytes by task 175 on cpu 1: unix_stream_sendmsg (net/unix/af_unix.c:2087 net/unix/af_unix.c:2191) sock_sendmsg (net/socket.c:705 net/socket.c:725) __sys_sendto (net/socket.c:2040) __x64_sys_sendto (net/socket.c:2048) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:113) read to 0xffff888003d9cff0 of 8 bytes by task 176 on cpu 0: unix_ioctl (net/unix/af_unix.c:3101 (discriminator 1)) sock_do_ioctl (net/socket.c:1128) sock_ioctl (net/socket.c:1242) __x64_sys_ioctl (fs/ioctl.c:52 fs/ioctl.c:874 fs/ioctl.c:860 fs/ioctl.c:860) do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80) entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:113) value changed: 0xffff888003da0c00 -> 0xffff888003da0d00 Reported by Kernel Concurrency Sanitizer on: CPU: 0 PID: 176 Comm: unix_race_oob_i Not tainted 5.17.0-rc5-59529-g83dc4c2af682 #12 Hardware name: Red Hat KVM, BIOS 1.11.0-2.amzn2 04/01/2014 Fixes: 314001f0bf92 ("af_unix: Add OOB support") Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxxxx> Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c index b0bfc78e421c..826ac391a7a4 100644 --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -1996,7 +1996,7 @@ static int queue_oob(struct socket *sock, struct msghdr *msg, struct sock *other if (ousk->oob_skb) consume_skb(ousk->oob_skb); - ousk->oob_skb = skb; + WRITE_ONCE(ousk->oob_skb, skb); scm_stat_add(other, skb); skb_queue_tail(&other->sk_receive_queue, skb); @@ -2514,9 +2514,8 @@ static int unix_stream_recv_urg(struct unix_stream_read_state *state) oob_skb = u->oob_skb; - if (!(state->flags & MSG_PEEK)) { - u->oob_skb = NULL; - } + if (!(state->flags & MSG_PEEK)) + WRITE_ONCE(u->oob_skb, NULL); unix_state_unlock(sk); @@ -2551,7 +2550,7 @@ static struct sk_buff *manage_oob(struct sk_buff *skb, struct sock *sk, skb = NULL; } else if (sock_flag(sk, SOCK_URGINLINE)) { if (!(flags & MSG_PEEK)) { - u->oob_skb = NULL; + WRITE_ONCE(u->oob_skb, NULL); consume_skb(skb); } } else if (!(flags & MSG_PEEK)) { @@ -3006,11 +3005,10 @@ static int unix_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg) case SIOCATMARK: { struct sk_buff *skb; - struct unix_sock *u = unix_sk(sk); int answ = 0; skb = skb_peek(&sk->sk_receive_queue); - if (skb && skb == u->oob_skb) + if (skb && skb == READ_ONCE(unix_sk(sk)->oob_skb)) answ = 1; err = put_user(answ, (int __user *)arg); }