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Subject[PATCH 5.4 293/320] af_unix: annote lockless accesses to unix_tot_inflight & gc_in_progress
Date
From: Eric Dumazet <edumazet@google.com>

commit 9d6d7f1cb67cdee15f1a0e85aacfb924e0e02435 upstream.

wait_for_unix_gc() reads unix_tot_inflight & gc_in_progress
without synchronization.

Adds READ_ONCE()/WRITE_ONCE() and their associated comments
to better document the intent.

BUG: KCSAN: data-race in unix_inflight / wait_for_unix_gc

write to 0xffffffff86e2b7c0 of 4 bytes by task 9380 on cpu 0:
unix_inflight+0x1e8/0x260 net/unix/scm.c:63
unix_attach_fds+0x10c/0x1e0 net/unix/scm.c:121
unix_scm_to_skb net/unix/af_unix.c:1674 [inline]
unix_dgram_sendmsg+0x679/0x16b0 net/unix/af_unix.c:1817
unix_seqpacket_sendmsg+0xcc/0x110 net/unix/af_unix.c:2258
sock_sendmsg_nosec net/socket.c:704 [inline]
sock_sendmsg net/socket.c:724 [inline]
____sys_sendmsg+0x39a/0x510 net/socket.c:2409
___sys_sendmsg net/socket.c:2463 [inline]
__sys_sendmmsg+0x267/0x4c0 net/socket.c:2549
__do_sys_sendmmsg net/socket.c:2578 [inline]
__se_sys_sendmmsg net/socket.c:2575 [inline]
__x64_sys_sendmmsg+0x53/0x60 net/socket.c:2575
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffffff86e2b7c0 of 4 bytes by task 9375 on cpu 1:
wait_for_unix_gc+0x24/0x160 net/unix/garbage.c:196
unix_dgram_sendmsg+0x8e/0x16b0 net/unix/af_unix.c:1772
unix_seqpacket_sendmsg+0xcc/0x110 net/unix/af_unix.c:2258
sock_sendmsg_nosec net/socket.c:704 [inline]
sock_sendmsg net/socket.c:724 [inline]
____sys_sendmsg+0x39a/0x510 net/socket.c:2409
___sys_sendmsg net/socket.c:2463 [inline]
__sys_sendmmsg+0x267/0x4c0 net/socket.c:2549
__do_sys_sendmmsg net/socket.c:2578 [inline]
__se_sys_sendmmsg net/socket.c:2575 [inline]
__x64_sys_sendmmsg+0x53/0x60 net/socket.c:2575
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000002 -> 0x00000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9375 Comm: syz-executor.1 Not tainted 5.16.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: 9915672d4127 ("af_unix: limit unix_tot_inflight")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Link: https://lore.kernel.org/r/20220114164328.2038499-1-eric.dumazet@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/unix/garbage.c | 14 +++++++++++---
net/unix/scm.c | 6 ++++--
2 files changed, 15 insertions(+), 5 deletions(-)

--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -192,8 +192,11 @@ void wait_for_unix_gc(void)
{
/* If number of inflight sockets is insane,
* force a garbage collect right now.
+ * Paired with the WRITE_ONCE() in unix_inflight(),
+ * unix_notinflight() and gc_in_progress().
*/
- if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
+ if (READ_ONCE(unix_tot_inflight) > UNIX_INFLIGHT_TRIGGER_GC &&
+ !READ_ONCE(gc_in_progress))
unix_gc();
wait_event(unix_gc_wait, gc_in_progress == false);
}
@@ -213,7 +216,9 @@ void unix_gc(void)
if (gc_in_progress)
goto out;

- gc_in_progress = true;
+ /* Paired with READ_ONCE() in wait_for_unix_gc(). */
+ WRITE_ONCE(gc_in_progress, true);
+
/* First, select candidates for garbage collection. Only
* in-flight sockets are considered, and from those only ones
* which don't have any external reference.
@@ -299,7 +304,10 @@ void unix_gc(void)

/* All candidates should have been detached by now. */
BUG_ON(!list_empty(&gc_candidates));
- gc_in_progress = false;
+
+ /* Paired with READ_ONCE() in wait_for_unix_gc(). */
+ WRITE_ONCE(gc_in_progress, false);
+
wake_up(&unix_gc_wait);

out:
--- a/net/unix/scm.c
+++ b/net/unix/scm.c
@@ -59,7 +59,8 @@ void unix_inflight(struct user_struct *u
} else {
BUG_ON(list_empty(&u->link));
}
- unix_tot_inflight++;
+ /* Paired with READ_ONCE() in wait_for_unix_gc() */
+ WRITE_ONCE(unix_tot_inflight, unix_tot_inflight + 1);
}
user->unix_inflight++;
spin_unlock(&unix_gc_lock);
@@ -79,7 +80,8 @@ void unix_notinflight(struct user_struct

if (atomic_long_dec_and_test(&u->inflight))
list_del_init(&u->link);
- unix_tot_inflight--;
+ /* Paired with READ_ONCE() in wait_for_unix_gc() */
+ WRITE_ONCE(unix_tot_inflight, unix_tot_inflight - 1);
}
user->unix_inflight--;
spin_unlock(&unix_gc_lock);

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