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

[Shigeru Yoshida <syoshida@redhat.com>]

Hi Jason,

On Wed, 23 Nov 2022 12:20:47 +0800, Jason Wang wrote:
> 
> 在 2022/11/23 02:47, Eric Dumazet 写道:
>> On Tue, Nov 22, 2022 at 10:10 AM Shigeru Yoshida <syoshida@redhat.com>
>> wrote:
>>> Hi Eric,
>>>
>>> On Mon, 21 Nov 2022 08:47:17 -0800, Eric Dumazet wrote:
>>>> On Sun, Nov 20, 2022 at 1:02 AM Shigeru Yoshida <syoshida@redhat.com>
>>>> 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+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.
>>>>>
>>>>> This patch fixes the issue by calling sock_put() from tun_detach()
>>>>> after all necessary accesses for the struct net has done.
>>>>>
>>>>> Fixes: 83c1f36f9880 ("tun: send netlink notification when the device
>>>>> is modified")
>>>>> Reported-by: syzbot+106f9b687cd64ee70cd1@syzkaller.appspotmail.com
>>>>> Link:
>>>>> https://syzkaller.appspot.com/bug?id=96eb7f1ce75ef933697f24eeab928c4a716edefe
>>>>> [1]
>>>>> Signed-off-by: Shigeru Yoshida <syoshida@redhat.com>
>>>>> ---
>>>>> v2:
>>>>> - Include symbolic stack trace
>>>>> - Add Fixes and Reported-by tags
>>>>> v1:
>>>>> https://lore.kernel.org/all/20221119075615.723290-1-syoshida@redhat.com/
>>>>> ---
>>>>>   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) {
>>>> Would you mind explaining (a comment would be nice) why this barrier
>>>> is needed ?
>>> I thought that tfile is accessed with rcu_lock(), so I put
>>> synchronize_rcu() here.  Please let me know if I misunderstand the
>>> concept of rcu (I'm losing my confidence...).
>>>
>> Addin Jason for comments.
>>
>> If an RCU grace period was needed before commit 83c1f36f9880 ("tun:
>> send netlink notification when the device is modified"),
>> would we need another patch ?
> 
> 
> I think we don't need another synchronization here. __tun_detach() has
> already done the necessary synchronization when it tries to modify
> tun->tfiles array and tfile->tun.

Thank you so much for your comment.  I'll prepare v3 patch to remove
calling synchronize_rcu().

Thanks,
Shigeru

> Thanks
> 
> 
>>
>> Also sock_flag(sk, SOCK_RCU_FREE) would probably be better than adding
>> a synchronize_rcu() (if again a grace period is needed)
>>
>>
>>
>>> Thanks,
>>> Shigeru
>>>
>>>> Thanks.
>>>>
>>>>> +               synchronize_rcu();
>>>>> +               sock_put(&tfile->sk);
>>>>> +       }
>>>>>   }
>>>>>
>>>>>   static void tun_detach_all(struct net_device *dev)
>>>>> --
>>>>> 2.38.1
>>>>>
>