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From: Andy Lutomirski <luto@amacapital.net>
Date: Thu, 4 Jan 2018 21:33:12 -0800
Message-ID: <CALCETrUYxkLUkCNPQu7=z9LRjLQwKND5QZmEK-77Rz1T7+FHQA@mail.gmail.com>
Subject: Re: [PATCH 4.4 00/37] 4.4.110-stable review
To: Linus Torvalds <torvalds@linux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@oracle.com>,
        Kees Cook <keescook@chromium.org>
Cc: Thomas Voegtle <tv@lio96.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Guenter Roeck <linux@roeck-us.net>,
        Shuah Khan <shuahkh@osg.samsung.com>, patches@kernelci.org,
        Ben Hutchings <ben.hutchings@codethink.co.uk>,
        lkft-triage@lists.linaro.org, stable <stable@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Thu, Jan 4, 2018 at 12:43 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
>> On Jan 4, 2018, at 12:29 PM, Linus Torvalds <torvalds@linux-foundation.org> wrote:
>>
>>> On Thu, Jan 4, 2018 at 12:16 PM, Thomas Voegtle <tv@lio96.de> wrote:
>>>
>>> Attached a screenshot.
>>> Is that useful? Are there some debug options I can add?
>>
>> Not much of an oops, because the SIGSEGV happens in user space. The
>> only reason you get any kernel stack printout at all is because 'init'
>> dying will make the kernel print that out.
>>
>> The segfault address for init looks like the fixmap area to me (first
>> byte in the last page of the fixmap?). "Error 5" means that it's a
>> user-space read that got a protection fault. So it's not a LDT of GDT
>> update or anything like that, it's a normal access from user space (or
>> a qemu emulation bug, but that sounds unlikely).
>>
>> Is that the vsyscall page?
>>
>> Adding Luto to the participants. I think he noticed one of the
>> vsyscall patches missing earlier in the 4.9 series. Maybe the 4.4
>> series had something similar..
>>
>
> That's almost certainly it.
>
> I'll try to find some time today or tomorrow to add a proper selftest.
>

Give this a shot:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=17c5ebeb2e00879b0af1a9c32bf37ecdd9b9b31b

Boot with each of vsyscall=none, vsyscall=native, and vsyscall=emulate
and run both the 32-bit and 64-bit variants of that test.  All six
combinations should pass.  But I bet they don't on 4.4.