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From: Andy Lutomirski <luto@kernel.org>
Date: Fri, 10 Nov 2017 14:06:19 -0800
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Subject: Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL
To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>, moritz.lipp@iaik.tugraz.at,
        Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
        michael.schwarz@iaik.tugraz.at, richard.fellner@student.tugraz.at,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>,
        X86 ML <x86@kernel.org>
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On Thu, Nov 9, 2017 at 10:31 PM, Dave Hansen
<dave.hansen@linux.intel.com> wrote:
> On 11/09/2017 06:25 PM, Andy Lutomirski wrote:
>> Here are two proposals to address this without breaking vsyscalls.
>>
>> 1. Set NX on low mappings that are _PAGE_USER.  Don't set NX on high
>> mappings but, optionally, warn if you see _PAGE_USER on any address
>> that isn't the vsyscall page.
>>
>> 2. Ignore _PAGE_USER entirely and just mark the EFI mm as special so
>> KAISER doesn't muck with it.
>
> These are totally doable.  But, what's the big deal with breaking native
> vsyscall?  We can still do the emulation so nothing breaks: it is just slow.

I have nothing against disabling native.  I object to breaking the
weird binary tracing behavior in the emulation mode, especially if
it's tangled up with KAISER.  I got all kinds of flak in an earlier
version of the vsyscall emulation patches when I broke that use case.
KAISER may get very widely backported -- let's not make changes that
are already known to break things.