linux-kernel - Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init

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Message-Id: <A4F58550-CAA8-4AE2-8DE5-C6970CC47210@amacapital.net>
Date:   Thu, 2 Nov 2017 12:59:22 +0100
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        moritz.lipp@...k.tugraz.at,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        michael.schwarz@...k.tugraz.at, Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 02/23] x86, kaiser: do not set _PAGE_USER for init_mm page tables



> On Nov 2, 2017, at 12:33 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Thu, 2 Nov 2017, Andy Lutomirski wrote:
>>> On Wed, Nov 1, 2017 at 3:20 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
>>>> On Wed, 1 Nov 2017, Linus Torvalds wrote:
>>>>> On Wed, Nov 1, 2017 at 2:52 PM, Dave Hansen <dave.hansen@...ux.intel.com> 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.
>> 
>> At least a couple years ago, the maintainers of some userspace tracing
>> tools complained very loudly at the early versions of the patches.
>> There are programs like pin (semi-open-source IIRC) that parse
>> instructions, make an instrumented copy, and run it.  This means that
>> the vsyscall page needs to contain text that is semantically
>> equivalent to what calling it actually does.
>> 
>> So yes, read access needs to work.  I should add a selftest for this.
>> 
>> This is needed in emulation mode as well as native mode, so removing
>> native mode is totally orthogonal.
> 
> Fair enough. I enabled function tracing with emulate_vsyscall as the filter
> on a couple of machines and so far I have no hit at all. Though I found a
> VM with a real old user space (~2005) and that actually used it.
> 
> So for the problem at hand, I'd suggest we disable the vsyscall stuff if
> CONFIG_KAISER=y and be done with it.

I think that time() on not-so-old glibc uses it.  Even more recent versions of Go use it. :(

> 
> Thanks,
> 
>    tglx
> 
> 
> 
> 
>