CVE-2024-26676: af_unix: Call kfree_skb() for dead unix_(sk)->oob_skb in GC.

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

Description
===========

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>

The Linux kernel CVE team has assigned CVE-2024-26676 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.15.103 with commit f3969427fb06 and fixed in 5.15.149 with commit 4fe505c63aa3
	Issue introduced in 6.1.20 with commit ac1968ac3992 and fixed in 6.1.78 with commit e0e09186d882
	Issue introduced in 6.3 with commit 2aab4b969002 and fixed in 6.6.17 with commit b74aa9ce13d0
	Issue introduced in 6.3 with commit 2aab4b969002 and fixed in 6.7.5 with commit 82ae47c5c3a6
	Issue introduced in 6.3 with commit 2aab4b969002 and fixed in 6.8 with commit 1279f9d9dec2

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-26676
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/unix/garbage.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4fe505c63aa3273135a57597fda761e9aecc7668
	https://git.kernel.org/stable/c/e0e09186d8821ad59806115d347ea32efa43ca4b
	https://git.kernel.org/stable/c/b74aa9ce13d02b7fd37c5325b99854f91b9b4276
	https://git.kernel.org/stable/c/82ae47c5c3a6b27fdc0f9e83c1499cb439c56140
	https://git.kernel.org/stable/c/1279f9d9dec2d7462823a18c29ad61359e0a007d