Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

[Peter Zijlstra]

On Tue, Dec 12, 2017 at 09:58:58AM -0800, Andy Lutomirski wrote:
> On Tue, Dec 12, 2017 at 9:32 AM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> > +bool __ldt_write_fault(unsigned long address)
> > +{
> > +       struct ldt_struct *ldt = current->mm->context.ldt;
> > +       unsigned long start, end, entry;
> > +       struct desc_struct *desc;
> > +
> > +       start = (unsigned long) ldt->entries;
> > +       end = start + ldt->nr_entries * LDT_ENTRY_SIZE;
> > +
> > +       if (address < start || address >= end)
> > +               return false;
> > +
> > +       desc = (struct desc_struct *) ldt->entries;
> > +       entry = (address - start) / LDT_ENTRY_SIZE;
> > +       desc[entry].type |= 0x01;
> 
> You have another patch that unconditionally sets the accessed bit on
> installation.  What gives?

Right, initially we didn't set that unconditionally. But even when we
did do that, we've observed the CPU generating these write faults.

> Also, this patch is going to die a horrible death if IRET ever hits
> this condition.  Or load gs.

Us touching the CS/SS descriptors with LAR should avoid IRET going off
the rails, I'm not familiar with the whole gs thing, but we could very
easily augment refresh_ldt_segments() I suppose.

Would you care to be a little more specific and or propose a testcase
for this situation?