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

[Shigeru Yoshida]

Hi Eric,

On Sat, 19 Nov 2022 10:31:38 -0800, Eric Dumazet wrote:
> On Fri, Nov 18, 2022 at 11:56 PM Shigeru Yoshida <syoshida@xxxxxxxxxx> wrote:
>>
>> syzbot reported use-after-free in tun_detach() [1].  This causes call
>> trace like below:
>>
>> ==================================================================
>> BUG: KASAN: use-after-free in notifier_call_chain+0x1da/0x1e0
>> ...
>> Call Trace:
> 
> Please include a symbolic stack trace, I think syzbot has them.

Thank you so much for your comment.  I got it.

>>  <TASK>
>>  dump_stack_lvl+0x100/0x178
>>  print_report+0x167/0x470
>>  ? __virt_addr_valid+0x5e/0x2d0
>>  ? __phys_addr+0xc6/0x140
>>  ? notifier_call_chain+0x1da/0x1e0
>>  ? notifier_call_chain+0x1da/0x1e0
>>  kasan_report+0xbf/0x1e0
>>  ? notifier_call_chain+0x1da/0x1e0
>>  notifier_call_chain+0x1da/0x1e0
>>  call_netdevice_notifiers_info+0x83/0x130
>>  netdev_run_todo+0xc33/0x11b0
>>  ? generic_xdp_install+0x490/0x490
>>  ? __tun_detach+0x1500/0x1500
>>  tun_chr_close+0xe2/0x190
>>  __fput+0x26a/0xa40
>>  task_work_run+0x14d/0x240
>>  ? task_work_cancel+0x30/0x30
>>  do_exit+0xb31/0x2a40
>>  ? reacquire_held_locks+0x4a0/0x4a0
>>  ? do_raw_spin_lock+0x12e/0x2b0
>>  ? mm_update_next_owner+0x7c0/0x7c0
>>  ? rwlock_bug.part.0+0x90/0x90
>>  ? lockdep_hardirqs_on_prepare+0x17f/0x410
>>  do_group_exit+0xd4/0x2a0
>>  __x64_sys_exit_group+0x3e/0x50
>>  do_syscall_64+0x38/0xb0
>>  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.
>>
>> This patch fixes the issue by calling sock_put() from tun_detach()
>> after all necessary accesses for the struct net has done.
>>
>> Link: https://syzkaller.appspot.com/bug?id=96eb7f1ce75ef933697f24eeab928c4a716edefe [1]
>> Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
> 
> Please add a Fixes: tag, once you identified which commit added this bug.

I got it too.  I'll send v2 patch.

Thanks,
Shigeru

>> ---
>>  drivers/net/tun.c | 6 +++++-
>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>> index 7a3ab3427369..ce9fcf4c8ef4 100644
>> --- a/drivers/net/tun.c
>> +++ b/drivers/net/tun.c
>> @@ -686,7 +686,6 @@ static void __tun_detach(struct tun_file *tfile, bool clean)
>>                 if (tun)
>>                         xdp_rxq_info_unreg(&tfile->xdp_rxq);
>>                 ptr_ring_cleanup(&tfile->tx_ring, tun_ptr_free);
>> -               sock_put(&tfile->sk);
>>         }
>>  }
>>
>> @@ -702,6 +701,11 @@ static void tun_detach(struct tun_file *tfile, bool clean)
>>         if (dev)
>>                 netdev_state_change(dev);
>>         rtnl_unlock();
>> +
>> +       if (clean) {
>> +               synchronize_rcu();
>> +               sock_put(&tfile->sk);
>> +       }
>>  }
>>
>>  static void tun_detach_all(struct net_device *dev)
>> --
>> 2.38.1
>>
>