linux-kernel - Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard pages

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Message-ID: <20170808092348.GA19207@leverpostej>
Date:   Tue, 8 Aug 2017 10:23:48 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Laura Abbott <labbott@...hat.com>
Cc:     Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org,
        ard.biesheuvel@...aro.org, catalin.marinas@....com,
        james.morse@....com, luto@...capital.net, matt@...eblueprint.co.uk,
        will.deacon@....com, kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard
 pages

On Mon, Aug 07, 2017 at 03:55:22PM -0700, Laura Abbott wrote:
> On 08/07/2017 03:00 PM, Mark Rutland wrote:
> > On Mon, Aug 07, 2017 at 01:39:48PM -0700, Kees Cook wrote:

> >> Do these tests both trip with the new arm64 VMAP_STACK code?
> > 
> >> +/* Test that VMAP_STACK is actually allocating with a leading guard page */
> >> +void lkdtm_STACK_GUARD_PAGE_LEADING(void)

> >> +/* Test that VMAP_STACK is actually allocating with a trailing guard page */
> >> +void lkdtm_STACK_GUARD_PAGE_TRAILING(void)

> > I can give these a go tomorrow.
> > 
> > These *should* fault, and IIUC should trigger the usual "Unable to handle
> > kernel %s at virtual address %08lx\n" splat from arm64's __do_kernel_fault(),
> > which should end up with an Oops().

> I gave these a quick test in QEMU and they seem to do the correct thing:

> # echo STACK_GUARD_PAGE_LEADING > /sys/kernel/debug/provoke-crash/DIRECT 
> [   24.593306] lkdtm: Performing direct entry STACK_GUARD_PAGE_LEADING
> [   24.593780] lkdtm: attempting bad read from page below current stack
> [   24.594289] Unable to handle kernel paging request at virtual address ffff000009b77fff
> [   24.594747] swapper pgtable: 4k pages, 48-bit VAs, pgd = ffff000009050000
> [   24.595443] [ffff000009b77fff] *pgd=000000007effe003, *pud=000000007effd003, *pmd=000000007cc43003, *pte=0000000000000000
> [   24.596743] Internal error: Oops: 96000007 [#1] PREEMPT SMP

> # echo STACK_GUARD_PAGE_TRAILING > /sys/kernel/debug/provoke-crash/DIRECT 
> [  103.144313] lkdtm: Performing direct entry STACK_GUARD_PAGE_TRAILING
> [  103.144749] lkdtm: attempting bad read from page above current stack
> [  103.145100] Unable to handle kernel paging request at virtual address ffff000009c2c000
> [  103.145477] swapper pgtable: 4k pages, 48-bit VAs, pgd = ffff000009050000
> [  103.145836] [ffff000009c2c000] *pgd=000000007effe003, *pud=000000007effd003, *pmd=000000007d014003, *pte=0000000000000000
> [  103.146445] Internal error: Oops: 96000007 [#2] PREEMPT SMP

Great! thanks for giving those a spin.

I can confirm likewise on Juno with 64K pages (not that this should make
any difference).

Mark.