af_unix: Drop oob_skb ref before purging queue in GC.

In the Linux kernel, the following vulnerability has been resolved:

af_unix: Drop oob_skb ref before purging queue in GC.

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>