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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 12 Dec 2017 19:43:45 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Andy Lutomirski <luto@...nel.org>,
        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, 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@...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?

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