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

From: Kees Cook
Date: Mon Aug 07 2017 - 17:47:14 EST

On Mon, Aug 7, 2017 at 2:46 PM, Ard Biesheuvel
<ard.biesheuvel@xxxxxxxxxx> wrote:
> On 7 August 2017 at 22:44, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>> On Mon, Aug 7, 2017 at 2:27 PM, Ard Biesheuvel
>> <ard.biesheuvel@xxxxxxxxxx> wrote:
>>> On 7 August 2017 at 21:39, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>>>> 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@xxxxxxxxxxxx>
>>>> ---
>>>> 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?
> Ah yes, so we will fault. We should probably double check whether we
> will not misidentify the fault because of the subtraction we do first,
> but that should be trivial to add.

Okay, cool. I'd be curious to see what the lkdtm tests show for you.


Kees Cook
Pixel Security