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Message-ID: <CAJcbSZHAtxbRzhTcZtBSabW0t+Cj7a1z-xJk8d310a2h8pkG=g@mail.gmail.com>
Date: Thu, 5 Jan 2017 13:08:21 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Andy Lutomirski <luto@...nel.org>
Cc: Arjan van de Ven <arjan@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>,
Borislav Petkov <bp@...en8.de>, Dave Hansen <dave@...1.net>, Chen Yucong <slaoub@...il.com>,
Paul Gortmaker <paul.gortmaker@...driver.com>, Andrew Morton <akpm@...ux-foundation.org>,
Masahiro Yamada <yamada.masahiro@...ionext.com>,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Anna-Maria Gleixner <anna-maria@...utronix.de>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>, Rasmus Villemoes <linux@...musvillemoes.dk>,
Michael Ellerman <mpe@...erman.id.au>, Juergen Gross <jgross@...e.com>,
Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>
Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location
On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@...nel.org> wrote:
> On Thu, Jan 5, 2017 at 11:03 AM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> On Thu, Jan 5, 2017 at 10:58 AM, Arjan van de Ven <arjan@...ux.intel.com> wrote:
>>> On 1/5/2017 9:54 AM, Thomas Garnier wrote:
>>>
>>>>
>>>> That's my goal too. I started by doing a RO remap and got couple
>>>> problems with hibernation. I can try again for the next iteration or
>>>> delay it for another patch. I also need to look at KVM GDT usage, I am
>>>> not familiar with it yet.
>>>
>>>
>>> don't we write to the GDT as part of the TLS segment stuff for glibc ?
>>>
>>
>> Not sure which glibc feature it is.
>>
>> In this design, you can write to the GDT per-cpu variable that will
>> remain read-write. You just need to make the remapping writeable when
>> we load task registers (ltr) then the processor use the current GDT
>> address. At least that the case I know, I might find more through
>> testing.
>
> Hmm. I bet that if we preset the accessed bits in all the segments
> then we don't need it to be writable in general. But your point about
> set_thread_area (TLS) is well taken. However, I strongly suspect that
> we could make set_thread_area unconditionally set the accessed bit and
> no one would ever notice.
Not sure I fully understood and I don't want to miss an important
point. Do you mean making GDT (remapping and per-cpu) read-only and
switch the writeable flag only when we write to the per-cpu entry?
--
Thomas
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