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, whichcreates a socketpair and sends one socket's fd to itself using thepeer.  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) = 1This forms a self-cyclic reference that GC should finally untanglebut does not due to lack of MSG_OOB handling, resulting in memoryleak.Recently, commit 11498715f266 ("af_unix: Remove io_uring code forGC.") removed io_uring's dead code in GC and revealed the problem.The code was executed at the final stage of GC and unconditionallymoved all GC candidates from gc_candidates to gc_inflight_list.That papered over the reported problem by always making the followingWARN_ON_ONCE(!list_empty(&gc_candidates)) false.The problem has been there since commit 2aab4b969002 ("af_unix: fixstruct pid leaks in OOB support") added full scm support for MSG_OOBwhile fixing another bug.To fix this problem, we must call kfree_skb() for unix_sk(sk)->oob_skbif the socket still exists in gc_candidates after purging collected skb.Then, we need to set NULL to oob_skb before calling kfree_skb() becauseit calls last fput() and triggers unix_release_sock(), where we callduplicate kfree_skb(u->oob_skb) if not NULL.Note that the leaked socket remained being linked to a global list, sokmemleak also could not detect it.  We need to check /proc/net/protocolto notice the unfreed socket.[0]:WARNING: CPU: 0 PID: 2863 at net/unix/garbage.c:345 __unix_gc+0xc74/0xe80 net/unix/garbage.c:345Modules linked in:CPU: 0 PID: 2863 Comm: kworker/u4:11 Not tainted 6.8.0-rc1-syzkaller-00583-g1701940b1a02 #0Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024Workqueue: events_unbound __unix_gcRIP: 0010:__unix_gc+0xc74/0xe80 net/unix/garbage.c:345Code: 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 e8RSP: 0018:ffffc9000b03fba0 EFLAGS: 00010293RAX: 0000000000000000 RBX: ffffc9000b03fc10 RCX: ffffffff816c493eRDX: ffff88802c02d940 RSI: ffffffff896982f3 RDI: ffffc9000b03fb30RBP: ffffc9000b03fce0 R08: 0000000000000001 R09: fffff52001607f66R10: 0000000000000003 R11: 0000000000000002 R12: dffffc0000000000R13: ffffc9000b03fc10 R14: ffffc9000b03fc10 R15: 0000000000000001FS:  0000000000000000(0000) GS:ffff8880b9400000(0000) knlGS:0000000000000000CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033CR2: 00005559c8677a60 CR3: 000000000d57a000 CR4: 00000000003506f0DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400Call 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>
Key phrases: memoryleak