rxrpc: fix a race in rxrpc_exit_net()

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

rxrpc: fix a race in rxrpc_exit_net()

Current code can lead to the following race:

CPU0 CPU1

rxrpc_exit_net()
rxrpc_peer_keepalive_worker()
if (rxnet->live)

rxnet->live = false;
del_timer_sync(&rxnet->peer_keepalive_timer);

timer_reduce(&rxnet->peer_keepalive_timer, jiffies + delay);

cancel_work_sync(&rxnet->peer_keepalive_work);

rxrpc_exit_net() exits while peer_keepalive_timer is still armed,
leading to use-after-free.

syzbot report was:

ODEBUG: free active (active state 0) object type: timer_list hint: rxrpc_peer_keepalive_timeout+0x0/0xb0
WARNING: CPU: 0 PID: 3660 at lib/debugobjects.c:505 debug_print_object+0x16e/0x250 lib/debugobjects.c:505
Modules linked in:
CPU: 0 PID: 3660 Comm: kworker/u4:6 Not tainted 5.17.0-syzkaller-13993-g88e6c0207623 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: netns cleanup_net
RIP: 0010:debug_print_object+0x16e/0x250 lib/debugobjects.c:505
Code: ff df 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 af 00 00 00 48 8b 14 dd 00 1c 26 8a 4c 89 ee 48 c7 c7 00 10 26 8a e8 b1 e7 28 05 <0f> 0b 83 05 15 eb c5 09 01 48 83 c4 18 5b 5d 41 5c 41 5d 41 5e c3
RSP: 0018:ffffc9000353fb00 EFLAGS: 00010082
RAX: 0000000000000000 RBX: 0000000000000003 RCX: 0000000000000000
RDX: ffff888029196140 RSI: ffffffff815efad8 RDI: fffff520006a7f52
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff815ea4ae R11: 0000000000000000 R12: ffffffff89ce23e0
R13: ffffffff8a2614e0 R14: ffffffff816628c0 R15: dffffc0000000000
FS: 0000000000000000(0000) GS:ffff8880b9c00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe1f2908924 CR3: 0000000043720000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__debug_check_no_obj_freed lib/debugobjects.c:992 [inline]
debug_check_no_obj_freed+0x301/0x420 lib/debugobjects.c:1023
kfree+0xd6/0x310 mm/slab.c:3809
ops_free_list.part.0+0x119/0x370 net/core/net_namespace.c:176
ops_free_list net/core/net_namespace.c:174 [inline]
cleanup_net+0x591/0xb00 net/core/net_namespace.c:598
process_one_work+0x996/0x1610 kernel/workqueue.c:2289
worker_thread+0x665/0x1080 kernel/workqueue.c:2436
kthread+0x2e9/0x3a0 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
</TASK>