CVE-2024-26676

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>