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Date: Fri, 6 Jan 2017 10:02:55 -0800
From: Thomas Garnier <thgarnie@...gle.com>
To: Andy Lutomirski <luto@...capital.net>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>, 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 6:34 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, Jan 5, 2017 at 3:05 PM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Thu, Jan 5, 2017 at 12:18 PM, Andy Lutomirski <luto@...nel.org> wrote:
>>>
>>> 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.
>>
>> I'm not sure that this is architecturally safe.
>>
>
> Hmm. Last time I looked, I couldn't find *anything* in the SDM
> explaining what happened if a GDT access resulted in a page fault. I
> did discover that Xen intentionally (!) lazily populates and maps LDT
> pages. An attempt to access a not-present page results in #PF with
> the error cod e indicating kernel access even if the access came from
> user mode.
>
> SDM volume 3 7.2.2 says "Pages corresponding to the previous task’s
> TSS, the current task’s TSS, and the descriptor table entries for
> each all should be marked as read/write." But I don't see how a CPU
> implementation could possibly care what the page table for the TSS
> descriptor table entries says after LTR is done because the CPU isn't
> even supposed to *read* that memory.
>
> OTOH a valid implementation could easily require that the page table
> says that the page is writable merely to load a segment, especially in
> weird cases (IRET?). That being said, this is all quite easy to test.
>
> Also, Thomas, why are you creating a new memory region? I don't see
> any benefit to randomizing the GDT address. How about just putting it
> in the fixmap? This would be NR_CPUS * 4 pages if do my limit=0xffff
> idea. I'm not sure if the fixmap code knows how to handle this much
> space.
When I looked at the fixmap, you had to define the space you need
ahead of time and I am not sure there was enough space as you said.
--
Thomas
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