Re: tty: panic in tty_ldisc_restore

[Dmitry Vyukov]

On Tue, Feb 7, 2017 at 11:51 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> 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)
>
> ?


Or can we just kill the task? Still better than kernel panic.