Re: [PATCH v2] net: tun: Fix use-after-free in tun_detach()

On Sun, Nov 20, 2022 at 2:49 AM Hillf Danton [off-list ref] wrote:

On 20 Nov 2022 18:02:13 +0900 Shigeru Yoshida [off-list ref]

quoted

syzbot reported use-after-free in tun_detach() [1].  This causes call
trace like below:

==================================================================
BUG: KASAN: use-after-free in notifier_call_chain+0x1ee/0x200 kernel/notifier.c:75
Read of size 8 at addr ffff88807324e2a8 by task syz-executor.0/3673

CPU: 0 PID: 3673 Comm: syz-executor.0 Not tainted 6.1.0-rc5-syzkaller-00044-gcc675d22e422 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xd1/0x138 lib/dump_stack.c:106
 print_address_description mm/kasan/report.c:284 [inline]
 print_report+0x15e/0x461 mm/kasan/report.c:395
 kasan_report+0xbf/0x1f0 mm/kasan/report.c:495
 notifier_call_chain+0x1ee/0x200 kernel/notifier.c:75
 call_netdevice_notifiers_info+0x86/0x130 net/core/dev.c:1942
 call_netdevice_notifiers_extack net/core/dev.c:1983 [inline]
 call_netdevice_notifiers net/core/dev.c:1997 [inline]
 netdev_wait_allrefs_any net/core/dev.c:10237 [inline]
 netdev_run_todo+0xbc6/0x1100 net/core/dev.c:10351
 tun_detach drivers/net/tun.c:704 [inline]
 tun_chr_close+0xe4/0x190 drivers/net/tun.c:3467
 __fput+0x27c/0xa90 fs/file_table.c:320
 task_work_run+0x16f/0x270 kernel/task_work.c:179
 exit_task_work include/linux/task_work.h:38 [inline]
 do_exit+0xb3d/0x2a30 kernel/exit.c:820
 do_group_exit+0xd4/0x2a0 kernel/exit.c:950
 get_signal+0x21b1/0x2440 kernel/signal.c:2858
 arch_do_signal_or_restart+0x86/0x2300 arch/x86/kernel/signal.c:869
 exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
 exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
 __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
 syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
 do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

The cause of the issue is that sock_put() from __tun_detach() drops
last reference count for struct net, and then notifier_call_chain()
from netdev_state_change() accesses that struct net.

Correct. IOW the race between netdev_run_todo() and cleanup_net() is behind
the uaf report from syzbot.

quoted

This patch fixes the issue by calling sock_put() from tun_detach()
after all necessary accesses for the struct net has done.

Thanks for your fix.

But tun is not special wrt netdev_run_todo() and call_netdevice_notifiers(),
so the correct fix should be making netdev grab another hold on net and
invoking put_net() in the path of netdev_run_todo().

Well, this is not going to work. Unless I am missing something.

1) A netns is destroyed when its refcount reaches zero.

   if (refcount_dec_and_test(&net->ns.count))
        __put_net(net);

    This transition is final.

    (It is illegal to attempt a refcount_inc() from this state)

2) All its netdevices are unregistered.

   Trying to get a reference on netns in netdevice dismantle path
would immediately trigger a refcount_t warning.