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

[Jason Wang]

在 2022/11/23 02:47, Eric Dumazet 写道:
> On Tue, Nov 22, 2022 at 10:10 AM Shigeru Yoshida <syoshida@xxxxxxxxxx> 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@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...).
> 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