From: Thomas Garnier <thgarnie@google.com>
Subject: Re: x86: PIE support and option to extend KASLR randomization
Date: Mon, 21 Aug 2017 08:57:58 -0700
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Cc: Ingo Molnar <mingo@kernel.org>, Herbert Xu <herbert@gondor.apana.org.au>,
	"David S . Miller" <davem@davemloft.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
	"H . Peter Anvin" <hpa@zytor.com>, Josh Poimboeuf <jpoimboe@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	Matthias Kaehlcke <mka@chromium.org>, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Juergen Gross <jgross@suse.com>, Paolo Bonzini <pbonzini@redhat.com>,
	=?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
	Joerg Roedel <joro@8bytes.org>, Tom Lendacky <thomas.lendacky@amd.com>,
	Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>, Brian Gerst <brgerst@gmail.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>, "Rafael J . Wysocki" <rjw@rjwysocki.net>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>, Tejun Heo <tj@kernel.org>,
	Christoph Lameter <
To: Peter Zijlstra <peterz@infradead.org>
In-Reply-To: <20170821143117.22sekggxizt3mhze@hirez.programming.kicks-ass.net>

On Mon, Aug 21, 2017 at 7:31 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Aug 15, 2017 at 07:20:38AM -0700, Thomas Garnier wrote:
>> On Tue, Aug 15, 2017 at 12:56 AM, Ingo Molnar <mingo@kernel.org> wrote:
>
>> > Have you considered a kernel with -mcmodel=small (or medium) instead of -fpie
>> > -mcmodel=large? We can pick a random 2GB window in the (non-kernel) canonical
>> > x86-64 address space to randomize the location of kernel text. The location of
>> > modules can be further randomized within that 2GB window.
>>
>> -model=small/medium assume you are on the low 32-bit. It generates
>> instructions where the virtual addresses have the high 32-bit to be
>> zero.
>
> That's a compiler fail, right? Because the SDM states that for "CALL
> rel32" the 32bit displacement is sign extended on x86_64.
>

That's different than what I expected at first too.

Now, I think I have an alternative of using mcmodel=large. I could use
-fPIC and ensure modules are never far away from the main kernel
(moving the module section start close to the random kernel end). I
looked at it and that seems possible but will require more work. I
plan to start with the mcmodel=large support and add this mode in a
way that could benefit classic KASLR (without -fPIC) because it
randomize where modules start based on the kernel.

-- 
Thomas