linux-kernel - Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location

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Date:   Thu, 5 Jan 2017 13:58:17 -0800
From:   Thomas Garnier <thgarnie@...gle.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Andy Lutomirski <luto@...nel.org>,
        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 1:19 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jan 5, 2017 at 1:08 PM, Thomas Garnier <thgarnie@...gle.com> wrote:
>> 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?
>>
>
> What I mean is: write to the GDT through normal percpu access (or
> whatever the normal mapping is) but load a read-only alias into the
> GDT register.  As long as nothing ever tries to write through the GDTR
> alias, no page faults will be generated.  So we just need to make sure
> that nothing ever writes to it through GDTR.  AFAIK the only reason
> the CPU ever writes to the address in GDTR is to set an accessed bit.
>

A write is made when we use load_TR_desc (ltr). I didn't see any other yet.

> --Andy



-- 
Thomas