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Message-ID: <88643033-45e1-3078-cb41-d7255ef874ad@redhat.com>
Date:   Wed, 23 Nov 2022 12:20:47 +0800
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Subject: Re: [PATCH v2] net: tun: Fix use-after-free in tun_detach()
Content-Language: en-US
To:     Eric Dumazet <edumazet@google.com>,
        Shigeru Yoshida <syoshida@redhat.com>
Cc:     davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        syzkaller-bugs@googlegroups.com,
        syzbot+106f9b687cd64ee70cd1@syzkaller.appspotmail.com
References: <20221120090213.922567-1-syoshida@redhat.com>
 <CANn89iLy3zBDN-y0JB_FJ9Mnmr5N0OguvHRfjVhyXELEpLREMw@mail.gmail.com>
 <20221123.031005.476714651315933198.syoshida@redhat.com>
 <CANn89iKQVvaHN+QXxmvk+Cm2vauHNcPRyh3ee_F=JH8coUQnnA@mail.gmail.com>
From:   Jason Wang <jasowang@redhat.com>
In-Reply-To: <CANn89iKQVvaHN+QXxmvk+Cm2vauHNcPRyh3ee_F=JH8coUQnnA@mail.gmail.com>
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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.

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
>>>>