Re: [PATCH] LDT improvements

[Thomas Gleixner]

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.

> 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.

Thanks,

	tglx