Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables

[Kees Cook]

On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> On Wed, 1 Nov 2017, Linus Torvalds wrote:
>> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>> > On 11/01/2017 02:28 PM, Thomas Gleixner wrote:
>> >> On Wed, 1 Nov 2017, Andy Lutomirski wrote:
>> >>> The vsyscall page is _PAGE_USER and lives in init_mm via the fixmap.
>> >>
>> >> Groan, forgot about that abomination, but still there is no point in having
>> >> it marked PAGE_USER in the init_mm at all, kaiser or not.
>> >
>> > So shouldn't this patch effectively make the vsyscall page unusable?
>> > Any idea why that didn't show up in any of the x86 selftests?
>>
>> I actually think there may be two issues here:
>>
>>  - vsyscall isn't even used much - if any - any more
>
> Only legacy user space uses it.
>
>>  - the vsyscall emulation works fine without _PAGE_USER, since the
>> whole point is that we take a fault on it and then emulate.
>>
>> We do expose the vsyscall page read-only to user space in the
>> emulation case, but I'm not convinced that's even required.
>
> I don't see a reason why it needs to be mapped at all for emulation.
>
>> Nobody who configures KAISER enabled would possibly want to have the
>> actual native vsyscall page enabled. That would be an insane
>> combination.
>>
>> So the only possibly difference would be a user mode program that
>> actually looks at the vsyscall page, which sounds unlikely to be an
>> issue.  It's legacy and not really used.
>
> Right, and we can either disable the NATIVE mode when KAISER is on or just
> rip the native mode out completely. Most distros have native mode disabled
> anyway, so you cannot even enable it on the kernel command line.
>
> I'm all for ripping it out or at least removing the config switch to enable
> native mode as a first step.

I would like to see NATIVE removed too.

-Kees

-- 
Kees Cook
Pixel Security