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From: Andy Lutomirski <luto@kernel.org>
Date: Thu, 7 Dec 2017 09:08:34 -0800
Message-ID: <CALCETrVrQRUn6PoBBGSD8+-wRrWp3mQtKY1sYnCRFyDBte5usA@mail.gmail.com>
Subject: Re: [PATCH] LDT improvements
To: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>
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On Thu, Dec 7, 2017 at 4:43 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Wed, Dec 06, 2017 at 11:22:21PM -0800, Andy Lutomirski wrote:
>> I think I like this approach.  I also think it might be nice to move the
>> whole cpu_entry_area into this new pgd range so that we can stop mucking
>> around with the fixmap.
>
> Yeah, and also, I don't like the idea of sacrificing a whole PGD
> only for the LDT crap which is optional, even. Frankly - and this
> is just me - I'd make CONFIG_KERNEL_PAGE_TABLE_ISOLATION xor
> CONFIG_MODIFY_LDT_SYSCALL and don't give a rat's *ss about the LDT.

The PGD sacrifice doesn't bother me.  Putting a writable LDT map at a
constant address does bother me.  We could probably get away with RO
if we trapped and handled the nasty faults, but that could be very
problematic.

The version here:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=a74d1009ac72a1f04ec5bcd338a4bdbe170ab776

actually seems to work.