linux-kernel - Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

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Date:   Thu, 9 Nov 2017 11:26:34 -0800
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     "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, richard.fellner@...dent.tugraz.at,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>, X86 ML <x86@...nel.org>
Subject: Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

On 11/09/2017 11:04 AM, Andy Lutomirski wrote:
> On Wed, Nov 8, 2017 at 11:47 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
>>
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>>
>> The VSYSCALL page is mapped by kernel page tables at a kernel address.
>> It is troublesome to support with KAISER in place, so disable the
>> native case.
>>
>> Also add some help text about how KAISER might affect the emulation
>> case as well.
> 
> Can you re-explain why this is helpful?

How about this?

The KAISER code attempts to "poison" the user portion of the kernel page
tables.  It detects the entries pages that it wants that it wants to
poison in two ways:
 * Looking for addresses >= PAGE_OFFSET
 * Looking for entries without _PAGE_USER set

But, to allow the _PAGE_USER check to work, we stopped it from being
set on all init_mm entries.

The VDSO is at a address >= PAGE_OFFSET and it is also mapped by the
init_mm.  The fact that we remove _PAGE_USER from the page tables makes
it unreadable to userspace.

This makes the "NATIVE" case totally unusable since userspace can not
even see the memory any more.  Disable it whenever KAISER is enabled.