af_unix: Don't access successor in unix_del_edges() during GC.

syzbot reported use-after-free in unix_del_edges(). [0]

What the repro does is basically repeat the following quickly.

1. pass a fd of an AF_UNIX socket to itself

socketpair(AF_UNIX, SOCK_DGRAM, 0, [3, 4]) = 0
sendmsg(3, {..., msg_control=[{cmsg_len=20, cmsg_level=SOL_SOCKET,
cmsg_type=SCM_RIGHTS, cmsg_data=[4]}], ...}, 0) = 0

2. pass other fds of AF_UNIX sockets to the socket above

socketpair(AF_UNIX, SOCK_SEQPACKET, 0, [5, 6]) = 0
sendmsg(3, {..., msg_control=[{cmsg_len=48, cmsg_level=SOL_SOCKET,
cmsg_type=SCM_RIGHTS, cmsg_data=[5, 6]}], ...}, 0) = 0

3. close all sockets

Here, two skb are created, and every unix_edge->successor is the first
socket. Then, __unix_gc() will garbage-collect the two skb:

(a) free skb with self-referencing fd
(b) free skb holding other sockets

After (a), the self-referencing socket will be scheduled to be freed
later by the delayed_fput() task.

syzbot repeated the sequences above (1. ~ 3.) quickly and triggered
the task concurrently while GC was running.

So, at (b), the socket was already freed, and accessing it was illegal.

unix_del_edges() accesses the receiver socket as edge->successor to
optimise GC. However, we should not do it during GC.

Garbage-collecting sockets does not change the shape of the rest
of the graph, so we need not call unix_update_graph() to update
unix_graph_grouped when we purge skb.

However, if we clean up all loops in the unix_walk_scc_fast() path,
unix_graph_maybe_cyclic remains unchanged (true), and __unix_gc()
will call unix_walk_scc_fast() continuously even though there is no
socket to garbage-collect.

To keep that optimisation while fixing UAF, let's add the same
updating logic of unix_graph_maybe_cyclic in unix_walk_scc_fast()
as done in unix_walk_scc() and __unix_walk_scc().

Note that when unix_del_edges() is called from other places, the
receiver socket is always alive:

- sendmsg: the successor's sk_refcnt is bumped by sock_hold()
unix_find_other() for SOCK_DGRAM, connect() for SOCK_STREAM

- recvmsg: the successor is the receiver, and its fd is alive

[0]:
BUG: KASAN: slab-use-after-free in unix_edge_successor net/unix/garbage.c:109 [inline]
BUG: KASAN: slab-use-after-free in unix_del_edge net/unix/garbage.c:165 [inline]
BUG: KASAN: slab-use-after-free in unix_del_edges+0x148/0x630 net/unix/garbage.c:237
Read of size 8 at addr ffff888079c6e640 by task kworker/u8:6/1099

CPU: 0 PID: 1099 Comm: kworker/u8:6 Not tainted 6.9.0-rc4-next-20240418-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
Workqueue: events_unbound __unix_gc
Call Trace:
<TASK>
__dump_stack lib/dump_stack.c:88 [inline]
dump_stack_lvl+0x241/0x360 lib/dump_stack.c:114
print_address_description mm/kasan/report.c:377 [inline]
print_report+0x169/0x550 mm/kasan/report.c:488
kasan_report+0x143/0x180 mm/kasan/report.c:601
unix_edge_successor net/unix/garbage.c:109 [inline]
unix_del_edge net/unix/garbage.c:165 [inline]
unix_del_edges+0x148/0x630 net/unix/garbage.c:237
unix_destroy_fpl+0x59/0x210 net/unix/garbage.c:298
unix_detach_fds net/unix/af_unix.c:1811 [inline]
unix_destruct_scm+0x13e/0x210 net/unix/af_unix.c:1826
skb_release_head_state+0x100/0x250 net/core/skbuff.c:1127
skb_release_all net/core/skbuff.c:1138 [inline]
__kfree_skb net/core/skbuff.c:1154 [inline]
kfree_skb_reason+0x16d/0x3b0 net/core/skbuff.c:1190
__skb_queue_purge_reason include/linux/skbuff.h:3251 [inline]
__skb_queue_purge include/linux/skbuff.h:3256 [inline]
__unix_gc+0x1732/0x1830 net/unix/garbage.c:575
process_one_work kernel/workqueue.c:3218 [inline]
process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3299
worker_thread+0x86d/0xd70 kernel/workqueue.c:3380
kthread+0x2f0/0x390 kernel/kthread.c:389
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
</TASK>

Allocated by task 14427:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
unpoison_slab_object mm/kasan/common.c:312 [inline]
__kasan_slab_alloc+0x66/0x80 mm/kasan/common.c:338
kasan_slab_alloc include/linux/kasan.h:201 [inline]
slab_post_alloc_hook mm/slub.c:3897 [inline]
slab_alloc_node mm/slub.c:3957 [inline]
kmem_cache_alloc_noprof+0x135/0x290 mm/slub.c:3964
sk_prot_alloc+0x58/0x210 net/core/sock.c:2074
sk_alloc+0x38/0x370 net/core/sock.c:2133
unix_create1+0xb4/0x770
unix_create+0x14e/0x200 net/unix/af_unix.c:1034
__sock_create+0x490/0x920 net/socket.c:1571
sock_create net/socket.c:1622 [inline]
__sys_socketpair+0x33e/0x720 net/socket.c:1773
__do_sys_socketpair net/socket.c:1822 [inline]
__se_sys_socketpair net/socket.c:1819 [inline]
__x64_sys_socketpair+0x9b/0xb0 net/socket.c:1819
do_syscall_x64 arch/x86/entry/common.c:52 [inline]
do_syscall_64+0xf5/0x240 arch/x86/entry/common.c:83
entry_SYSCALL_64_after_hwframe+0x77/0x7f

Freed by task 1805:
kasan_save_stack mm/kasan/common.c:47 [inline]
kasan_save_track+0x3f/0x80 mm/kasan/common.c:68
kasan_save_free_info+0x40/0x50 mm/kasan/generic.c:579
poison_slab_object+0xe0/0x150 mm/kasan/common.c:240
__kasan_slab_free+0x37/0x60 mm/kasan/common.c:256
kasan_slab_free include/linux/kasan.h:184 [inline]
slab_free_hook mm/slub.c:2190 [inline]
slab_free mm/slub.c:4393 [inline]
kmem_cache_free+0x145/0x340 mm/slub.c:4468
sk_prot_free net/core/sock.c:2114 [inline]
__sk_destruct+0x467/0x5f0 net/core/sock.c:2208
sock_put include/net/sock.h:1948 [inline]
unix_release_sock+0xa8b/0xd20 net/unix/af_unix.c:665
unix_release+0x91/0xc0 net/unix/af_unix.c:1049
__sock_release net/socket.c:659 [inline]
sock_close+0xbc/0x240 net/socket.c:1421
__fput+0x406/0x8b0 fs/file_table.c:422
delayed_fput+0x59/0x80 fs/file_table.c:445
process_one_work kernel/workqueue.c:3218 [inline]
process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3299
worker_thread+0x86d/0xd70 kernel/workqueue.c:3380
kthread+0x2f0/0x390 kernel/kthread.c:389
ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244

The buggy address belongs to the object at ffff888079c6e000
which belongs to the cache UNIX of size 1920
The buggy address is located 1600 bytes inside of
freed 1920-byte region [ffff888079c6e000, ffff888079c6e780)

Reported-by: [e-mail]
Closes: https://syzkaller.appspot.com/bug?extid=f3f3eef1d2100200e593
Fixes: 77e5593aebba ("af_unix: Skip GC if no cycle exists.")
Fixes: fd86344823b5 ("af_unix: Try not to hold unix_gc_lock during accept().")
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

net/unix/garbage.c

···
 	unix_update_graph(unix_edge_successor(edge));
 }
 
+static bool gc_in_progress;
+
 static void unix_del_edge(struct scm_fp_list *fpl, struct unix_edge *edge)
 {
 	struct unix_vertex *vertex = edge->predecessor->vertex;
 
-	unix_update_graph(unix_edge_successor(edge));
+	if (!gc_in_progress)
+		unix_update_graph(unix_edge_successor(edge));
 
 	list_del(&edge->vertex_entry);
 	vertex->out_degree--;
···
 		unix_del_edge(fpl, edge);
 	} while (i < fpl->count_unix);
 
-	receiver = fpl->edges[0].successor;
-	receiver->scm_stat.nr_unix_fds -= fpl->count_unix;
+	if (!gc_in_progress) {
+		receiver = fpl->edges[0].successor;
+		receiver->scm_stat.nr_unix_fds -= fpl->count_unix;
+	}
 	WRITE_ONCE(unix_tot_inflight, unix_tot_inflight - fpl->count_unix);
 out:
 	WRITE_ONCE(fpl->user->unix_inflight, fpl->user->unix_inflight - fpl->count);
···
 
 static void unix_walk_scc_fast(struct sk_buff_head *hitlist)
 {
+	unix_graph_maybe_cyclic = false;
+
 	while (!list_empty(&unix_unvisited_vertices)) {
 		struct unix_vertex *vertex;
 		struct list_head scc;
···
 
 		if (scc_dead)
 			unix_collect_skb(&scc, hitlist);
+		else if (!unix_graph_maybe_cyclic)
+			unix_graph_maybe_cyclic = unix_scc_cyclic(&scc);
 
 		list_del(&scc);
 	}
 
 	list_replace_init(&unix_visited_vertices, &unix_unvisited_vertices);
 }
-
-static bool gc_in_progress;
 
 static void __unix_gc(struct work_struct *work)
 {