Re: [Xen-devel] [PATCH v4 0/3] x86: modify_ldt improvement, test, and config option

[Boris Ostrovsky]

On 07/29/2015 06:11 PM, Andrew Cooper wrote:
> On 29/07/2015 23:05, Andy Lutomirski wrote:
> > On Wed, Jul 29, 2015 at 2:37 PM, Andrew Cooper
> > <andrew.cooper3@xxxxxxxxxx> wrote:
> > > On 29/07/2015 22:26, Andy Lutomirski wrote:
> > > > On Wed, Jul 29, 2015 at 2:23 PM, Boris Ostrovsky
> > > > <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > > > On 07/29/2015 03:03 PM, Andrew Cooper wrote:
> > > > > > On 29/07/15 15:43, Boris Ostrovsky wrote:
> > > > > > > FYI, I have got a repro now and am investigating.
> > > > > > Good and bad news.  This bug has nothing to do with LDTs themselves.
> > > > > >
> > > > > > I have worked out what is going on, but this:
> > > > > >
> > > > > > diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> > > > > > index 5abeaac..7e1a82e 100644
> > > > > > --- a/arch/x86/xen/enlighten.c
> > > > > > +++ b/arch/x86/xen/enlighten.c
> > > > > > @@ -493,6 +493,7 @@ static void set_aliased_prot(void *v, pgprot_t prot)
> > > > > >             pte = pfn_pte(pfn, prot);
> > > > > >    +       (void)*(volatile int*)v;
> > > > > >           if (HYPERVISOR_update_va_mapping((unsigned long)v, pte, 0)) {
> > > > > >                   pr_err("set_aliased_prot va update failed w/ lazy mode
> > > > > > %u\n", paravirt_get_lazy_mode());
> > > > > >                   BUG();
> > > > > >
> > > > > > Is perhaps not the fix we are looking for, and every use of
> > > > > > HYPERVISOR_update_va_mapping() is susceptible to the same problem.
> > > > > I think in most cases we know that page is mapped so hopefully this is the
> > > > > only site that we need to be careful about.
> > > > Is there any chance we can get some kind of quick-and-dirty fix that
> > > > can go to x86/urgent in the next few days even if a clean fix isn't
> > > > available yet?
> > > Quick and dirty?
> > >
> > > Reading from v is the most obvious and quick way, for areas where we are
> > > certain v exists, is kernel memory and is expected to have a backing
> > > page.  I don't know offhand how many of current
> > > HYPERVISOR_update_va_mapping() callsites this applies to.
> > __get_user((char *)v, tmp), perhaps, unless there's something better
> > in the wings.  Keep in mind that we need this for -stable, and it's
> > likely to get backported quite quickly due to CVE-2015-5157.
> Hmm - something like that tucked inside HYPERVISOR_update_va_mapping()
> would probably work, and certainly be minimal hassle for -stable.
>
> Altering the hypercall used is certainly not something to backport, nor
> are we sure it is a viable fix at this time.

OK, I'll test tonight this quick fix and will defer a more proper patch 
for 4.3 then.

-boris


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