Re: [PATCH] LDT improvements

[Andy Lutomirski]

> On Dec 7, 2017, at 9:23 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
>> On Thu, 7 Dec 2017, Andy Lutomirski wrote:
>> 
>>> On Thu, Dec 7, 2017 at 4:43 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
>>>> On Wed, Dec 06, 2017 at 11:22:21PM -0800, Andy Lutomirski wrote:
>>>> I think I like this approach.  I also think it might be nice to move the
>>>> whole cpu_entry_area into this new pgd range so that we can stop mucking
>>>> around with the fixmap.
>>> 
>>> Yeah, and also, I don't like the idea of sacrificing a whole PGD
>>> only for the LDT crap which is optional, even. Frankly - and this
>>> is just me - I'd make CONFIG_KERNEL_PAGE_TABLE_ISOLATION xor
>>> CONFIG_MODIFY_LDT_SYSCALL and don't give a rat's *ss about the LDT.
>> 
>> The PGD sacrifice doesn't bother me.  Putting a writable LDT map at a
>> constant address does bother me.  We could probably get away with RO
>> if we trapped and handled the nasty faults, but that could be very
>> problematic.
> 
> Where is the problem? You can map it RO into user space with the USER bit
> cleared. The kernel knows how to access the real stuff.

Blows up when the CPU tries to set the accessed bit.

> 
>> The version here:
>> 
>> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/pti&id=a74d1009ac72a1f04ec5bcd338a4bdbe170ab776
>> 
>> actually seems to work.
> 
> The approach I've taken is to create a VMA and map it into user space with
> the USER bit cleared. A little bit more effort code wise, but that avoids
> all the page table muck and keeps it straight attached to the process.
> 
> Will post once in a bit.

I don't love mucking with user address space.  I'm also quite nervous about putting it in our near anything that could pass an access_ok check, since we're totally screwed if the bad guys can figure out how to write to it.

> 
> Thanks,
> 
>    tglx
>