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Date: Tue, 12 Dec 2017 19:19:02 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Greg KH <gregkh@linuxfoundation.org>, Kees Cook <keescook@google.com>,
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        David Laight <David.Laight@aculab.com>,
        Eduardo Valentin <eduval@amazon.com>, aliguori@amazon.com,
        Will Deacon <will.deacon@arm.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler
Message-ID: <20171212181902.a3dj3haouw3corhq@hirez.programming.kicks-ass.net>
References: <20171212173221.496222173@linutronix.de>
 <20171212173334.345422294@linutronix.de>
 <CALCETrWHQW19G2J2hCS4ZG_U5knG-0RBzruioQzojqWr6ceTBg@mail.gmail.com>
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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?