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

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On Mon, Aug 7, 2017 at 2:27 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> On 7 August 2017 at 21:39, Kees Cook <keescook@chromium.org> wrote:
>> Two new tests STACK_GUARD_PAGE_LEADING and STACK_GUARD_PAGE_TRAILING
>> attempt to read the byte before and after, respectively, of the current
>> stack frame, which should fault under VMAP_STACK.
>>
>> Signed-off-by: Kees Cook <keescook@chromium.org>
>> ---
>> Do these tests both trip with the new arm64 VMAP_STACK code?
>
> Probably not. On arm64, the registers are stacked by software at
> exception entry,  and so we decrement the sp first by the size of the
> register file, and if the resulting value overflows the stack, the
> situation is handled as if the exception was caused by a faulting
> stack access while it may be caused by something else in reality.
> Since the act of handling the exception is guaranteed to overflow the
> stack anyway, this does not really make a huge difference, and it
> prevents the recursive fault from wiping the context that we need to
> produce the diagnostics.
>
> This means an illegal access right above the stack will go undetected.

I thought vmap entries provided guard pages around allocations?
Shouldn't that catch it?

-- 
Kees Cook
Pixel Security

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[PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard pages Kees Cook <keescook@chromium.org> - 2017-08-07 22:40 +0200
  Re: [kernel-hardening] [PATCH] lkdtm: Test VMAP_STACK allocates  leading/trailing guard pages Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-08-07 23:30 +0200
    Re: [kernel-hardening] [PATCH] lkdtm: Test VMAP_STACK allocates  leading/trailing guard pages Kees Cook <keescook@chromium.org> - 2017-08-07 23:50 +0200
      Re: [kernel-hardening] [PATCH] lkdtm: Test VMAP_STACK allocates  leading/trailing guard pages Kees Cook <keescook@chromium.org> - 2017-08-07 23:50 +0200
      Re: [kernel-hardening] [PATCH] lkdtm: Test VMAP_STACK allocates  leading/trailing guard pages Ard Biesheuvel <ard.biesheuvel@linaro.org> - 2017-08-07 23:50 +0200
  Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard  pages Mark Rutland <mark.rutland@arm.com> - 2017-08-08 00:10 +0200
    Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard  pages Laura Abbott <labbott@redhat.com> - 2017-08-08 01:00 +0200
      Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard pages Kees Cook <keescook@chromium.org> - 2017-08-08 01:40 +0200
      Re: [PATCH] lkdtm: Test VMAP_STACK allocates leading/trailing guard  pages Mark Rutland <mark.rutland@arm.com> - 2017-08-08 11:30 +0200

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