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

[Eric Dumazet]

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 ?

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
> >>
> >
>