Help on how to configure for user-defined memory protection support (GSoC 2020)

[Utkarsh Rai]

On Sat, May 16, 2020 at 9:16 PM Joel Sherrill <joel at rtems.org> wrote:

>
>
> On Sat, May 16, 2020 at 10:14 AM Gedare Bloom <gedare at rtems.org> wrote:
>
>> Utkarsh,
>>
>> What do you mean by "This would although mean that we would have page
>> tables of  1MB."
>>
>> Check that you use plain text when inlining a reply, or at least that you
>> broke the reply format.
>>
>> Gedare
>>
>> On Fri, May 15, 2020, 6:04 PM Utkarsh Rai <utkarsh.rai60 at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Thu, May 14, 2020 at 10:23 AM Sebastian Huber <
>>> sebastian.huber at embedded-brains.de> wrote:
>>>
>>>> Hello Utkarsh Rai,
>>>>
>>>> On 13/05/2020 14:30, Utkarsh Rai wrote:
>>>> > Hello,
>>>> > My GSoC project,  providing thread stack protection support, has to
>>>> be
>>>> > a user-configurable feature.
>>>> > My question is,  what would be the best way to implement this, my
>>>> idea
>>>> > was to model it based on the existing system configuration
>>>> > <https://docs.rtems.org/branches/master/c-user/config/intro.html>,
>>>> but
>>>> > Dr. Gedare pointed out that configuration is undergoing heavy changes
>>>> > and may look completely different in future releases. Kindly advise
>>>> me
>>>> > as to what would be the best way to proceed.
>>>> before we start with an implementation. It would be good to define what
>>>> a thread stack protection support is supposed to do.
>>>
>>>
>>> The thread stack protection mechanism will protect against stack
>>> overflow errors and will completely isolate the thread stacks from each
>>> other. Sharing of thread stack will be possible only when the user makes
>>> explicit calls to do so. More details about this can be found in this
>>> thread <https://lists.rtems.org/pipermail/devel/2020-May/059768.html>.
>>>
>>>> Then there should
>>>> be a concept for systems with a Memory Protection Unit (MPU) and a
>>>> concept for systems with a Memory Management Unit (MMU). MMUs may
>>>> provide normal 4KiB Pages, large Pages (for example 1MiB) or something
>>>> more flexible. We should identify BSPs which should have support for
>>>> this. For each BSP should be a concept. Then we should think about how
>>>> a
>>>> user can configure this feature.
>>>
>>> For memory protection will have a 1:1 VA-PA address translation that
>>>> means a 4KiB page size will be set for both the MPU and MMU, a 1:1 mapping
>>>> will ensure we will have to do lesser page table walks.This would although
>>>> mean that we would have page tables of  1MB. I will be first providing the
>>>> support for Armv7 based BSPs (RPi , BBB, etc. have MMU support) then when I
>>>> have a working example I will move on to provide the support for RISC-V.
>>>> which has MPU support.
>>>
>>>
> I think Sebastian is asking exactly what I did. What are the processor
> (specific CPU) requirements to support thread stack protection?
>

For thread stack protection the processor should have the option of paging
along with appropriate 'access bits' setting. Both RISC-V and ARMv7-A (the
ones that I will be focusing on my project) have the option of defining
pages of 4KiB size with appropriate access bits.


>
> For example, to be effective, I imagine a 1MB granularity might be
> sufficient to protect code versus data/bss. But it is likely insufficient
> to protect thread stacks.
>
> Similarly, a processor with a limited number of "protection areas" would
> be unsuitable as a basis for implementing thread stack protection. Here I
> am thinking of the PowerPC with a handful of TLB registers. You would have
> to turn on paging.
>

I agree, most of the processors have protection regions between 8 to 16 and
in some cases as less as 4. For stack protection paging with each page of
size 4KiB, as it is applicable for processors with mpu or mmu and is
optimal, in the sense that we would have appropriate number and size of
pages for thread stacks, is the best option.


> This is the general guidance that needs to be provided so anyone can
> evaluate how much protection they really can have on their target.
>
> --joel
>
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