Re: tty: panic in tty_ldisc_restore

[Dmitry Vyukov]

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?


Yes. And it turns out that it's actually super-easy to trigger and we
are not running out of memory.
Vmalloc has a check for fatal_signal_pending. So you just invoke
tiocsetd and kill the process concurrently. Machine is down.