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SubjectRe: [patch 13/16] x86/ldt: Introduce LDT write fault handler
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@linutronix.de> 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?

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