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From:   tedheadster <tedheadster@gmail.com>
Date:   Tue, 13 Feb 2018 18:14:13 -0500
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Subject: Re: x86/stack protector: X86_32_LAZY_GS=n hangs kernel on old processors
To:     Kees Cook <keescook@chromium.org>
Cc:     Tejun Heo <tj@kernel.org>, jeremy@xensource.com,
        Rusty Russell <rusty@rustcorp.com.au>,
        Ingo Molnar <mingo@elte.hu>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Feb 13, 2018 at 4:37 PM, Kees Cook <keescook@chromium.org> wrote:
> On Tue, Feb 13, 2018 at 8:57 AM, tedheadster <tedheadster@gmail.com> wrote:
>> Changing X86_32_LAZY_GS to 'y' does not cause the kernel to hang.
>>
>>> On Tue, Feb 13, 2018 at 11:40:17AM -0500, tedheadster wrote:
>>>>   in your patch "x86: make lazy %gs optional on x86_32" were you able
>>>> to test it on really old processors? In 4.16.0-rc1, X86_32_LAZY_GS got
>>>> toggled from 'y' to 'n' in my default config because of changes to the
>>>> stack protector code. It hangs my ancient i486 test machine right
>>>> after 'Booting the kernel'.
>>>
>>> I didn't have access to an i486 at the time or ever since, so it
>>> wasn't tested there.  If this is specific to i486, flagging it so in
>>> the config probably isn't the end of the world at this point.
>>
>> Tejun,
>>   I will be able to test this on other 32bit hardware at the end of
>> the week. Hopefully I'll be able to identify which processors it does
>> not work on (i486/i586/i686).
>
> So, this is the exact opposite of my tests: if I had X86_32_LAZY_GS=y
> and stack protector enabled (via _AUTO), the boot would hang. This
> change solved that for me:
>
>  config X86_32_LAZY_GS
>         def_bool y
> -       depends on X86_32 && !CC_STACKPROTECTOR
> +       depends on X86_32 && CC_STACKPROTECTOR_NONE
>
> since stack-protector in _AUTO mode had to reorganize that logic. It
> seemed LAZY_GS isn't compatible with stack-protector, so I made sure
> to retain that. What are your other configs?
>

Kees,
  I am running the 4.16-rc1 release. My arch/x86/Kconfig has this logic:

config X86_32_LAZY_GS
        def_bool y
        depends on X86_32 && CC_STACKPROTECTOR_NONE

I get a hang on i486 when I choose any of these configuration options:

CONFIG_CC_STACKPROTECTOR_AUTO=y
CONFIG_CC_STACKPROTECTOR_REGULAR=y
CONFIG_CC_STACKPROTECTOR_STRONG=y

Also, CONFIG_X86_32_LAZY_GS does not appear in any of these config
files, not even as "# CONFIG_X86_32_LAZY_GS is not set", which I
thought was strange.

The only configuration that works on i486 is:

CONFIG_CC_STACKPROTECTOR_NONE=y
CONFIG_X86_32_LAZY_GS=y

Now it gets interesting. All four of these configurations boots
successfully when compiled for, and run on, a Pentium 4 M
(CONFIG_PENTIUM4). So it certainly is related to what version of the
processor you use.

I will continue to try other configuration combinations and report back.

- Matthew