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

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Message-ID: <20171212181902.a3dj3haouw3corhq@hirez.programming.kicks-ass.net>
Date:   Tue, 12 Dec 2017 19:19:02 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>, aliguori@...zon.com,
        Will Deacon <will.deacon@....com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [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@...utronix.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?