af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.

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

af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.

syzbot reported a warning [0] in __unix_gc() with a repro, which

creates a socketpair and sends one socket's fd to itself using the

peer.

socketpair(AF_UNIX, SOCK_STREAM, 0, [3, 4]) = 0

sendmsg(4, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="\360", iov_len=1}],

msg_iovlen=1, msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET,

cmsg_type=SCM_RIGHTS, cmsg_data=[3]}],

msg_controllen=24, msg_flags=0}, MSG_OOB|MSG_PROBE|MSG_DONTWAIT|MSG_ZEROCOPY) = 1

This forms a self-cyclic reference that GC should finally untangle

but does not due to lack of MSG_OOB handling, resulting in memory

leak.

Recently, commit 11498715f266 ("af_unix: Remove io_uring code for

GC.") removed io_uring's dead code in GC and revealed the problem.

The code was executed at the final stage of GC and unconditionally

moved all GC candidates from gc_candidates to gc_inflight_list.

That papered over the reported problem by always making the following

WARN_ON_ONCE(!list_empty(&gc_candidates)) false.

The problem has been there since commit 2aab4b969002 ("af_unix: fix

struct pid leaks in OOB support") added full scm support for MSG_OOB

while fixing another bug.

To fix this problem, we must call kfree_skb() for unix_sk(sk)->oob_skb

if the socket still exists in gc_candidates after purging collected skb.

Then, we need to set NULL to oob_skb before calling kfree_skb() because

it calls last fput() and triggers unix_release_sock(), where we call

duplicate kfree_skb(u->oob_skb) if not NULL.

Note that the leaked socket remained being linked to a global list, so

kmemleak also could not detect it. We need to check /proc/net/protocol

to notice the unfreed socket.

[0]:

WARNING: CPU: 0 PID: 2863 at net/unix/garbage.c:345 __unix_gc+0xc74/0xe80 net/unix/garbage.c:345

Modules linked in:

CPU: 0 PID: 2863 Comm: kworker/u4:11 Not tainted 6.8.0-rc1-syzkaller-00583-g1701940b1a02 #0

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024

Workqueue: events_unbound __unix_gc

RIP: 0010:__unix_gc+0xc74/0xe80 net/unix/garbage.c:345

Code: 8b 5c 24 50 e9 86 f8 ff ff e8 f8 e4 22 f8 31 d2 48 c7 c6 30 6a 69 89 4c 89 ef e8 97 ef ff ff e9 80 f9 ff ff e8 dd e4 22 f8 90 <0f> 0b 90 e9 7b fd ff ff 48 89 df e8 5c e7 7c f8 e9 d3 f8 ff ff e8

RSP: 0018:ffffc9000b03fba0 EFLAGS: 00010293

RAX: 0000000000000000 RBX: ffffc9000b03fc10 RCX: ffffffff816c493e

RDX: ffff88802c02d940 RSI: ffffffff896982f3 RDI: ffffc9000b03fb30

RBP: ffffc9000b03fce0 R08: 0000000000000001 R09: fffff52001607f66

R10: 0000000000000003 R11: 0000000000000002 R12: dffffc0000000000

R13: ffffc9000b03fc10 R14: ffffc9000b03fc10 R15: 0000000000000001

FS: 0000000000000000(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00005559c8677a60 CR3: 000000000d57a000 CR4: 00000000003506f0

DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

Call Trace:

<TASK>

process_one_work+0x889/0x15e0 kernel/workqueue.c:2633

process_scheduled_works kernel/workqueue.c:2706 [inline]

worker_thread+0x8b9/0x12a0 kernel/workqueue.c:2787

kthread+0x2c6/0x3b0 kernel/kthread.c:388

ret_from_fork+0x45/0x80 arch/x86/kernel/process.c:147

ret_from_fork_asm+0x1b/0x30 arch/x86/entry/entry_64.S:242

</TASK>