[PATCH 4.9 31/71] net/af_unix: fix a data-race in unix_dgram_sendmsg / unix_release_sock

Sasha Levin
2021-06-28T15:19:34Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit a494bd642d9120648b06bb7d28ce6d05f55a7819 ]

While unix_may_send(sk, osk) is called while osk is locked, it appears
unix_release_sock() can overwrite unix_peer() after this lock has been
released, making KCSAN unhappy.

Changing unix_release_sock() to access/change unix_peer()
before lock is released should fix this issue.

BUG: KCSAN: data-race in unix_dgram_sendmsg / unix_release_sock

write to 0xffff88810465a338 of 8 bytes by task 20852 on cpu 1:
unix_release_sock+0x4ed/0x6e0 net/unix/af_unix.c:558
unix_release+0x2f/0x50 net/unix/af_unix.c:859
__sock_release net/socket.c:599 [inline]
sock_close+0x6c/0x150 net/socket.c:1258
__fput+0x25b/0x4e0 fs/file_table.c:280
____fput+0x11/0x20 fs/file_table.c:313
task_work_run+0xae/0x130 kernel/task_work.c:164
tracehook_notify_resume include/linux/tracehook.h:189 [inline]
exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
exit_to_user_mode_prepare+0x156/0x190 kernel/entry/common.c:209
__syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:302
do_syscall_64+0x56/0x90 arch/x86/entry/common.c:57
entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88810465a338 of 8 bytes by task 20888 on cpu 0:
unix_may_send net/unix/af_unix.c:189 [inline]
unix_dgram_sendmsg+0x923/0x1610 net/unix/af_unix.c:1712
sock_sendmsg_nosec net/socket.c:654 [inline]
sock_sendmsg net/socket.c:674 [inline]
____sys_sendmsg+0x360/0x4d0 net/socket.c:2350
___sys_sendmsg net/socket.c:2404 [inline]
__sys_sendmmsg+0x315/0x4b0 net/socket.c:2490
__do_sys_sendmmsg net/socket.c:2519 [inline]
__se_sys_sendmmsg net/socket.c:2516 [inline]
__x64_sys_sendmmsg+0x53/0x60 net/socket.c:2516
do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0xffff888167905400 -> 0x0000000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20888 Comm: syz-executor.0 Not tainted 5.13.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/unix/af_unix.c | 7 ++++---
1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index bcd6ed6e7e25..8bbaa35937dd 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -534,12 +534,14 @@ static void unix_release_sock(struct sock *sk, int embrion)
u->path.mnt = NULL;
state = sk->sk_state;
sk->sk_state = TCP_CLOSE;
+
+ skpair = unix_peer(sk);
+ unix_peer(sk) = NULL;
+
unix_state_unlock(sk);

wake_up_interruptible_all(&u->peer_wait);

- skpair = unix_peer(sk);
-
if (skpair != NULL) {
if (sk->sk_type == SOCK_STREAM || sk->sk_type == SOCK_SEQPACKET) {
unix_state_lock(skpair);
@@ -554,7 +556,6 @@ static void unix_release_sock(struct sock *sk, int embrion)

unix_dgram_peer_wake_disconnect(sk, skpair);
sock_put(skpair); /* It may now die */
- unix_peer(sk) = NULL;
}

/* Try to flush out this socket. Throw out buffers at least */
--
2.30.2