Messages in this thread | | | Date | Tue, 17 Nov 2020 18:07:35 +0100 | From | Borislav Petkov <> | Subject | Re: [RFC][PATCH v2 00/21] x86/pti: Defer CR3 switch to C code |
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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.
> 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...
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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