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

From: Andy Lutomirski
Date: Tue Nov 21 2017 - 18:33:18 EST

On Tue, Nov 21, 2017 at 3:17 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> 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()
> calls.

I'm hoping we can remove kaiser_add_mapping() entirely. Maybe that's
silly optimism.

> 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'm not sure I'm understanding the issue. We'd promise to map the
cpu_entry_area without using large pages, but I'm not sure I know what
you're referring to. The only issue I see is that we'd have to be
quite careful when tearing down the user tables to avoid freeing the
shared part.

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

I definitely like that part.