linux-kernel - Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP

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Date:   Fri, 14 Jul 2017 16:25:03 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Kees Cook <keescook@...omium.org>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        James Morse <james.morse@....com>,
        Takahiro Akashi <akashi.takahiro@...aro.org>,
        Dave Martin <dave.martin@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Laura Abbott <labbott@...oraproject.org>
Subject: Re: [kernel-hardening] Re: [RFC PATCH 6/6] arm64: add VMAP_STACK and
 detect out-of-bounds SP

On Fri, Jul 14, 2017 at 04:03:51PM +0100, Robin Murphy wrote:
> On 14/07/17 15:39, Robin Murphy wrote:
> > On 14/07/17 15:06, Mark Rutland wrote:

> >> 	add	sp, sp, x0	// sp = orig_sp + orig_x0
> >> 	sub	x0, sp, x0	// x0 = orig_sp
> >> 	< check > 
> >> 	sub	x0, sp, x0	// x0 = orig_x0
> > 
> > Haven't you now forcibly cleared the top bit of x0 thanks to overflow?
> 
> ...or maybe not. I still can't quite see it, but I suppose it must
> cancel out somewhere, since Mr. Helpful C Program[1] has apparently
> proven me mistaken :(
> 
> I guess that means I approve!
> 
> Robin.
> 
> [1]:
> #include <assert.h>
> #include <stdint.h>
> 
> int main(void) {
>         for (int i = 0; i < 256; i++) {
>                 for (int j = 0; j < 256; j++) {
>                         uint8_t x = i;
>                         uint8_t y = j;
>                         y = y + x;
>                         x = y - x;
>                         x = y - x;
>                         y = y - x;
>                         assert(x == i && y == j);
>                 }
>         }
> }

I guess we have our first Tested-by for this series. :)

Thanks for taking a look!

Mark.