Re: [patch 00/16] x86/ldt: Use a VMA based read only mapping

[Andy Lutomirski]

On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> Peter and myself spent quite some time to figure out how to make CPUs cope
> with a RO mapped LDT.
>
> While the initial trick of writing the ACCESS bit in a special fault
> handler covers most cases, the tricky problem of CS/SS in return to user
> space (IRET ...) was giving us quite some headache.
>
> Peter finally found a way to do so. Touching the CS/SS selectors with LAR
> on the way out to user space makes it work w/o trouble.
>
> Contrary to the approach Andy was taking with storing the LDT in a special
> map area, the following series uses a special mapping which is mapped
> without the user bit and read only. This just ties the LDT to the process
> which is the most natural way to do it, removes the requirement for special
> pagetable code and works independent of pagetable isolation.
>
> This was tested on quite a range of Intel and AMD machines, but the test
> coverage on 32bit is quite meager. I'll resurrect a few dust bricks
> tomorrow.

I think it's neat that you got this working.  But it's like three
times the size of my patch, is *way* more intrusive, and isn't
obviously correct WRT IRET and load_gs_index().  So... how is it
better than my patch?