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

[Ard Biesheuvel]

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:
>>> 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@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.