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Date: Tue, 12 Dec 2017 19:43:45 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Andy Lutomirski <luto@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
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Subject: Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler
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On Tue, 12 Dec 2017, Peter Zijlstra wrote:
> 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?

Again. load gs does not cause a fault at all like any other segment
load. The fault comes when the segment is accessed the first time or via
LAR. 

Thanks,

	tglx