[PATCH 6.7 133/162] af_unix: Drop oob_skb ref before purging queue in GC.

Greg Kroah-Hartman
2024-03-04T21:31:39Z

From: Kuniyuki Iwashima <kuniyu@amazon.com>

commit aa82ac51d63328714645c827775d64dbfd9941f3 upstream.

syzbot reported another task hung in __unix_gc(). [0]

The current while loop assumes that all of the left candidates
have oob_skb and calling kfree_skb(oob_skb) releases the remaining
candidates.

However, I missed a case that oob_skb has self-referencing fd and
another fd and the latter sk is placed before the former in the
candidate list. Then, the while loop never proceeds, resulting
the task hung.

__unix_gc() has the same loop just before purging the collected skb,
so we can call kfree_skb(oob_skb) there and let __skb_queue_purge()
release all inflight sockets.

[0]:
Sending NMI from CPU 0 to CPUs 1:
NMI backtrace for cpu 1
CPU: 1 PID: 2784 Comm: kworker/u4:8 Not tainted 6.8.0-rc4-syzkaller-01028-g71b605d32017 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
Workqueue: events_unbound __unix_gc
RIP: 0010:__sanitizer_cov_trace_pc+0x0/0x70 kernel/kcov.c:200
Code: 89 fb e8 23 00 00 00 48 8b 3d 84 f5 1a 0c 48 89 de 5b e9 43 26 57 00 0f 1f 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 <f3> 0f 1e fa 48 8b 04 24 65 48 8b 0d 90 52 70 7e 65 8b 15 91 52 70
RSP: 0018:ffffc9000a17fa78 EFLAGS: 00000287
RAX: ffffffff8a0a6108 RBX: ffff88802b6c2640 RCX: ffff88802c0b3b80
RDX: 0000000000000000 RSI: 0000000000000002 RDI: 0000000000000000
RBP: ffffc9000a17fbf0 R08: ffffffff89383f1d R09: 1ffff1100ee5ff84
R10: dffffc0000000000 R11: ffffed100ee5ff85 R12: 1ffff110056d84ee
R13: ffffc9000a17fae0 R14: 0000000000000000 R15: ffffffff8f47b840
FS: 0000000000000000(0000) GS:ffff8880b9500000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffef5687ff8 CR3: 0000000029b34000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<NMI>
</NMI>
<TASK>
__unix_gc+0xe69/0xf40 net/unix/garbage.c:343
process_one_work kernel/workqueue.c:2633 [inline]
process_scheduled_works+0x913/0x1420 kernel/workqueue.c:2706
worker_thread+0xa5f/0x1000 kernel/workqueue.c:2787
kthread+0x2ef/0x390 kernel/kthread.c:388
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1b/0x30 arch/x86/entry/entry_64.S:242
</TASK>

Reported-and-tested-by: syzbot+ecab4d36f920c3574bf9@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ecab4d36f920c3574bf9
Fixes: 25236c91b5ab ("af_unix: Fix task hung while purging oob_skb in GC.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
net/unix/garbage.c | 22 +++++++++-------------
1 file changed, 9 insertions(+), 13 deletions(-)

--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -284,9 +284,17 @@ void unix_gc(void)
* which are creating the cycle(s).
*/
skb_queue_head_init(&hitlist);
- list_for_each_entry(u, &gc_candidates, link)
+ list_for_each_entry(u, &gc_candidates, link) {
scan_children(&u->sk, inc_inflight, &hitlist);

+#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
+ if (u->oob_skb) {
+ kfree_skb(u->oob_skb);
+ u->oob_skb = NULL;
+ }
+#endif
+ }
+
/* not_cycle_list contains those sockets which do not make up a
* cycle. Restore these to the inflight list.
*/
@@ -314,18 +322,6 @@ void unix_gc(void)
/* Here we are. Hitlist is filled. Die. */
__skb_queue_purge(&hitlist);

-#if IS_ENABLED(CONFIG_AF_UNIX_OOB)
- while (!list_empty(&gc_candidates)) {
- u = list_entry(gc_candidates.next, struct unix_sock, link);
- if (u->oob_skb) {
- struct sk_buff *skb = u->oob_skb;
-
- u->oob_skb = NULL;
- kfree_skb(skb);
- }
- }
-#endif
-
spin_lock(&unix_gc_lock);

/* There could be io_uring registered files, just push them back to