Re: [PATCH 0/5] expire the use of PcdSetNxForStack

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On 11 September 2018 at 11:13, Ni, Ruiyu <ruiyu.ni@intel.com> wrote:
> On 9/11/2018 4:57 PM, Ard Biesheuvel wrote:
>>
>> On 11 September 2018 at 07:16, Jian J Wang <jian.j.wang@intel.com> wrote:
>>>
>>> BZ#: https://bugzilla.tianocore.org/show_bug.cgi?id=1116
>>>
>>> Since the stack memory is allocated as EfiBootServicesData, its NX
>>> protection
>>> can be covered by BIT4 of PcdDxeNxMemoryProtectionPolicy. To avoid
>>> confusing
>>> in setting related PCDs, PcdSetNxForStack will be expired. Instead, If
>>> BIT4
>>> of PcdDxeNxMemoryProtectionPolicy is set, the DxeIpl will set NX bit in
>>> page
>>> table entries mapping the stack memory.
>>>
>>
>> I disagree. This removes the possibility to map EfiBootServicesData as
>> executable while still mapping the stack NX. As we all know, an
>> executable stack is in a class of its own when it comes to
>> exploitability, and should *never* be mapped executable unless in
>> highly exceptional cases. Mapping all EfiBootServicesData as
>> non-executable may cause backward compatibility problems.
>
> Ard,
> Are you saying you want the capability of setting certain range of BS data
> as executable? Why does ARM need such capability?
>

No, I am saying that mapping all BS data executable should be a
separate decision from mapping the stack executable: if your platform
cannot support the former (for historical reasons) you will likely
still want the latter.