linux-kernel - Re: [syzbot] unexpected kernel reboot (4)

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Date:   Thu, 15 Apr 2021 01:16:13 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Andrey Konovalov <andreyknvl@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Cc:     syzbot <syzbot+9ce030d4c89856b27619@...kaller.appspotmail.com>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>
Subject: Re: [syzbot] unexpected kernel reboot (4)

On 2021/04/15 0:39, Andrey Konovalov wrote:
> On Wed, Apr 14, 2021 at 7:45 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>>
>> On Tue, Apr 13, 2021 at 11:27 PM syzbot
>> <syzbot+9ce030d4c89856b27619@...kaller.appspotmail.com> wrote:
>>>
>>> Hello,
>>>
>>> syzbot found the following issue on:
>>>
>>> HEAD commit:    89698bec Merge tag 'm68knommu-for-v5.12-rc7' of git://git...
>>> git tree:       upstream
>>> console output: https://syzkaller.appspot.com/x/log.txt?x=1243fcfed00000
>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=b234ddbbe2953747
>>> dashboard link: https://syzkaller.appspot.com/bug?extid=9ce030d4c89856b27619
>>> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=173e92fed00000
>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1735da2ed00000
>>>
>>> IMPORTANT: if you fix the issue, please add the following tag to the commit:
>>> Reported-by: syzbot+9ce030d4c89856b27619@...kaller.appspotmail.com
>>>
>>> output_len: 0x000000000e74eb68
>>> kernel_total_size: 0x000000000f226000
>>> needed_size: 0x000000000f400000
>>> trampoline_32bit: 0x000000000009d000
>>> Decompressing Linux... Parsing ELF... done.
>>> Booting the kernel.
>>
>> +linux-input
>>
>> The reproducer connects some USB HID device and communicates with the driver.
>> Previously we observed reboots because HID devices can trigger reboot
>> SYSRQ, but we disable it with "CONFIG_MAGIC_SYSRQ is not set".
>> How else can a USB device reboot the machine? Is it possible to disable it?
>> I don't see any direct includes of <linux/reboot.h> in drivers/usb/*
> 
> This happens when a keyboard sends the Ctrl+Alt+Del sequence, see
> fn_boot_it()->ctrl_alt_del() in drivers/tty/vt/keyboard.c.
> 
> There was a patchset by Tetsuo [1] to suppress this, but I think it
> was abandoned.


Not abandoned; I'm waiting for you to join the discussion.
But for right now I'm trying to merge LOCKDEP's capacity tuning patch for
https://github.com/google/syzkaller/pull/2535 in the next merge window.

I still believe that ctrl_alt_del() etc. should be initially controlled
via kernel config options (despite Linus's objection at
https://lore.kernel.org/lkml/CAHk-=wgz=7MGxxX-tmMmdCsKyYJkuyxNc-4uLP=e_eEV=OzUaw@mail.gmail.com/ ), for
we will need several trials and errors (and an effort to avoid kernel size bloating
like https://lkml.kernel.org/r/YD57hjaSmsYapHnQ@alley still remains) before we can
determine usable units for allow toggling via kernel command line options.

> 
> (This reminds of a somewhat related syzkaller issue:
> https://github.com/google/syzkaller/issues/1824; it relies on a
> similar reproducer.)
> 
> [1] https://groups.google.com/g/syzkaller/c/7wCmrGlLgm0/m/5yG6HVtbBQAJ
>