Re: [PATCH 12/30] x86, kaiser: map GDT into user page tables

From: Dave Hansen
Date: Tue Nov 21 2017 - 18:17:22 EST

On 11/21/2017 02:46 PM, Andy Lutomirski wrote:
>> GDT: R/O TSS: R/W at least because of trampoline stack entry code:
>> EXEC+R/O exception stacks: R/W
> Can you avoid code duplication by adding some logic right after the
> kernel cpu_entry_area is set up to iterate page by page over the PTEs
> in the cpu_entry_area for that CPU and just install exactly the same
> PTEs into the kaiser table? E.g. just call kaiser_add_mapping once
> per page but with the parameters read out from the fixmap PTEs
> instead of hard coded?

Yes, we could do that. But, what's the gain? We end up removing
effectively three (long) lines of code from three kaiser_add_mapping()

To do this, we need to special-case the kernel page table walker to deal
with PTEs only since we can't just grab PMD or PUD flags and stick them
in a PTE. We would only be able to use this path when populating things
that we know are 4k-mapped in the kernel.

I guess the upside is that we don't open-code the permissions in the
KAISER code that *have* to match the permissions that the kernel itself

It also means that theoretically you could not touch the KAISER code the
next time we expand the cpu entry area.