Messages in this thread | | | Subject | Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code | From | Alexandre Chartre <> | Date | Tue, 17 Nov 2020 19:24:42 +0100 |
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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.
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