Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code

Alexandre Chartre
2020-11-17T18:25:34Z

On 11/17/20 6:07 PM, Borislav Petkov wrote:
> On Tue, Nov 17, 2020 at 09:19:01AM +0100, Alexandre Chartre wrote:
>> We are not reversing PTI, we are extending it.
>
> You're reversing it in the sense that you're mapping more kernel memory
> into the user page table than what is mapped now.
>
>> PTI removes all kernel mapping from the user page-table. However there's
>> no issue with mapping some kernel data into the user page-table as long as
>> these data have no sensitive information.
>
> I hope that is the case.
>
>> Actually, PTI is already doing that but with a very limited scope. PTI adds
>> into the user page-table some kernel mappings which are needed for userland
>> to enter the kernel (such as the kernel entry text, the ESPFIX, the
>> CPU_ENTRY_AREA_BASE...).
>>
>> So here, we are extending the PTI mapping so that we can execute more kernel
>> code while using the user page-table; it's a kind of PTI on steroids.
>
> And this is what bothers me - someone else might come after you and say,
> but but, I need to map more stuff into the user pgt because I wanna do
> X... and so on.

Agree, any addition should be strictly checked. I have been careful to expand
it to the minimum I needed.

>> The minimum size would be 1 page (4KB) as this is the minimum mapping size.
>> It's certainly enough for now as the usage of the PTI stack is limited, but
>> we will need larger stack if we won't to execute more kernel code with the
>> user page-table.
>
> So on a big machine with a million tasks, that's at least a million
> pages more which is what, ~4 Gb?
>
> There better be a very good justification for the additional memory
> consumption...

Yeah, adding a per-task allocation is my main concern, hence this RFC.

alex.