Re: tty: panic in tty_ldisc_restore

[Dmitry Vyukov]

On Tue, Feb 7, 2017 at 11:43 AM, Greg Kroah-Hartman
<gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Feb 07, 2017 at 11:24:13AM +0100, Dmitry Vyukov wrote:
>> On Thu, Feb 2, 2017 at 7:23 PM, Greg Kroah-Hartman
>> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Thu, Feb 02, 2017 at 07:03:41PM +0100, Dmitry Vyukov wrote:
>> >> On Thu, Feb 2, 2017 at 6:55 PM, Greg Kroah-Hartman
>> >> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
>> >> > On Thu, Feb 02, 2017 at 06:48:48PM +0100, Dmitry Vyukov wrote:
>> >> >> Hello,
>> >> >>
>> >> >> Syzkaller fuzzer started crashing kernel with the following panics:
>> >> >>
>> >> >> Kernel panic - not syncing: Couldn't open N_TTY ldisc for ircomm0 --- error -12.
>> >> >> CPU: 0 PID: 5637 Comm: syz-executor3 Not tainted 4.9.0 #6
>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine,
>> >> >> BIOS Google 01/01/2011
>> >> >>  ffff8801d4ba7a18 ffffffff8234d0df ffffffff00000000 1ffff1003a974ed6
>> >> >>  ffffed003a974ece 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1
>> >> >>  0000000000000000 0000000000000000 ffff8801d4ba76a8 00000000dabb4fad
>> >> >> Call Trace:
>> >> >>  [<ffffffff8234d0df>] __dump_stack lib/dump_stack.c:15 [inline]
>> >> >>  [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>> >> >>  [<ffffffff818280d4>] panic+0x1fb/0x412 kernel/panic.c:179
>> >> >>  [<ffffffff826bb0d4>] tty_ldisc_restore drivers/tty/tty_ldisc.c:520 [inline]
>> >> >>  [<ffffffff826bb0d4>] tty_set_ldisc+0x704/0x8b0 drivers/tty/tty_ldisc.c:579
>> >> >>  [<ffffffff826a3a93>] tiocsetd drivers/tty/tty_io.c:2667 [inline]
>> >> >>  [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924
>> >> >>  [<ffffffff81a7a22f>] vfs_ioctl fs/ioctl.c:43 [inline]
>> >> >>  [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630 fs/ioctl.c:679
>> >> >>  [<ffffffff81a7b72f>] SYSC_ioctl fs/ioctl.c:694 [inline]
>> >> >>  [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
>> >> >>  [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2
>> >> >>
>> >> >> Kernel panic - not syncing: Couldn't open N_TTY ldisc for ptm2 --- error -12.
>> >> >> CPU: 0 PID: 7844 Comm: syz-executor0 Not tainted 4.9.0 #6
>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine,
>> >> >> BIOS Google 01/01/2011
>> >> >>  ffff8801c3307a18 ffffffff8234d0df ffffffff00000000 1ffff10038660ed6
>> >> >>  ffffed0038660ece 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1
>> >> >>  0000000000000000 0000000000000000 ffff8801c33076a8 00000000dabb4fad
>> >> >> Call Trace:
>> >> >>  [<ffffffff8234d0df>] __dump_stack lib/dump_stack.c:15 [inline]
>> >> >>  [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>> >> >>  [<ffffffff818280d4>] panic+0x1fb/0x412 kernel/panic.c:179
>> >> >>  [<ffffffff826bb0d4>] tty_ldisc_restore drivers/tty/tty_ldisc.c:520 [inline]
>> >> >>  [<ffffffff826bb0d4>] tty_set_ldisc+0x704/0x8b0 drivers/tty/tty_ldisc.c:579
>> >> >>  [<ffffffff826a3a93>] tiocsetd drivers/tty/tty_io.c:2667 [inline]
>> >> >>  [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924
>> >> >>  [<ffffffff81a7a22f>] vfs_ioctl fs/ioctl.c:43 [inline]
>> >> >>  [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630 fs/ioctl.c:679
>> >> >>  [<ffffffff81a7b72f>] SYSC_ioctl fs/ioctl.c:694 [inline]
>> >> >>  [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:685
>> >> >>  [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2
>> >> >>
>> >> >>
>> >> >> In all cases there is a vmalloc failure right before that:
>> >> >>
>> >> >> syz-executor4: vmalloc: allocation failure, allocated 0 of 16384
>> >> >> bytes, mode:0x14000c2(GFP_KERNEL|__GFP_HIGHMEM), nodemask=(null)
>> >> >> syz-executor4 cpuset=/ mems_allowed=0
>> >> >> CPU: 1 PID: 4852 Comm: syz-executor4 Not tainted 4.9.0 #6
>> >> >> Hardware name: Google Google Compute Engine/Google Compute Engine,
>> >> >> BIOS Google 01/01/2011
>> >> >>  ffff8801c41df898 ffffffff8234d0df ffffffff00000001 1ffff1003883bea6
>> >> >>  ffffed003883be9e 0000000041b58ab3 ffffffff84b38180 ffffffff8234cdf1
>> >> >>  0000000000000282 ffffffff84fd53c0 ffff8801dae65b38 ffff8801c41df4d0
>> >> >> Call Trace:
>> >> >>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>> >> >>  [<ffffffff8234d0df>] dump_stack+0x2ee/0x3ef lib/dump_stack.c:51
>> >> >>  [<ffffffff8186530f>] warn_alloc+0x21f/0x360
>> >> >>  [<ffffffff819792c9>] __vmalloc_node_range+0x4e9/0x770
>> >> >>  [<     inline     >] __vmalloc_node mm/vmalloc.c:1749
>> >> >>  [<     inline     >] __vmalloc_node_flags mm/vmalloc.c:1763
>> >> >>  [<ffffffff8197961b>] vmalloc+0x5b/0x70 mm/vmalloc.c:1778
>> >> >>  [<ffffffff826ad77b>] n_tty_open+0x1b/0x470 drivers/tty/n_tty.c:1883
>> >> >>  [<ffffffff826ba973>] tty_ldisc_open.isra.3+0x73/0xd0
>> >> >> drivers/tty/tty_ldisc.c:463
>> >> >>  [<     inline     >] tty_ldisc_restore drivers/tty/tty_ldisc.c:510
>> >> >>  [<ffffffff826bafb4>] tty_set_ldisc+0x5e4/0x8b0 drivers/tty/tty_ldisc.c:579
>> >> >>  [<     inline     >] tiocsetd drivers/tty/tty_io.c:2667
>> >> >>  [<ffffffff826a3a93>] tty_ioctl+0xc63/0x2370 drivers/tty/tty_io.c:2924
>> >> >>  [<ffffffff81a7a22f>] do_vfs_ioctl+0x1bf/0x1630
>> >> >>  [<     inline     >] SYSC_ioctl fs/ioctl.c:698
>> >> >>  [<ffffffff81a7b72f>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>> >> >>  [<ffffffff84377941>] entry_SYSCALL_64_fastpath+0x1f/0xc2
>> >> >> arch/x86/entry/entry_64.S:204
>> >> >>
>> >> >>
>> >> >> I've found that it's even documented in the source code, but it does
>> >> >> not look like a good failure mode for allocation failure:
>> >> >>
>> >> >> static int n_tty_open(struct tty_struct *tty)
>> >> >> {
>> >> >>         struct n_tty_data *ldata;
>> >> >>
>> >> >>         /* Currently a malloc failure here can panic */
>> >> >>         ldata = vmalloc(sizeof(*ldata));
>> >> >
>> >> > How are you running out of vmalloc() memory?
>> >>
>> >>
>> >> I don't know exactly. But it does not seem to represent a problem for
>> >> the fuzzer.
>> >> Is it meant to be very hard to do?
>> >
>> > Yes, do you know of any normal way to cause it to fail?
>>
>>
>> I don't. But I means approximately nothing.
>> Do you mean that it is not possible to trigger?
>> Doesn't simply creating lots of kernel resources (files, sockets,
>> pipe) will do the trick? Or just paging in lots of memory? Even if the
>> process itself will be chosen as OOM kill target, it will still take
>> the machine down with itself due to the panic while returning from the
>> syscall, no?
>
> I'm not saying that it's impossible, just an "almost" impossible thing
> to hit.  Obviously you have hit it, so it can happen :)
>
> But, how to fix it?  I really don't know.  Unwinding a failure at this
> point in time is very tough, as that comment shows.  Any suggestions of
> how it could be resolved are greatly appreciated.

Is it possible to not shutdown the old discipline tty_set_ldisc before
we prepare everything for the new one:

  /* Shutdown the old discipline. */
  tty_ldisc_close(tty, old_ldisc);

Currently it does:

  close(old)
  if (open(new))
    open(old) // assume never fails

it looks inherently problematic.
Couldn't we do:

  if (open(new))
    return -ESOMETHING
  close(old)

?