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

[Shigeru Yoshida]

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@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+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@xxxxxxxxxxxxxxxxxxxxxxxxx
>> Link: https://syzkaller.appspot.com/bug?id=96eb7f1ce75ef933697f24eeab928c4a716edefe [1]
>> Signed-off-by: Shigeru Yoshida <syoshida@xxxxxxxxxx>
>> ---
>> v2:
>> - Include symbolic stack trace
>> - Add Fixes and Reported-by tags
>> v1: https://lore.kernel.org/all/20221119075615.723290-1-syoshida@xxxxxxxxxx/
>> ---
>>  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...).

Thanks,
Shigeru

> 
> Thanks.
> 
>> +               synchronize_rcu();
>> +               sock_put(&tfile->sk);
>> +       }
>>  }
>>
>>  static void tun_detach_all(struct net_device *dev)
>> --
>> 2.38.1
>>
>