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

On 07/29/2015 06:46 PM, David Vrabel wrote:

On 29/07/2015 23:11, 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());

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.
Changing this one use of update_va_mapping to use mmu_update_normal_pt
is the correct fix to unblock this LDT series.  I see no reason why this
cannot be backported.

To properly fix it should include batching and that is not something that I think we should target for stable.


We can address any other potential update_va_mapping calls at a later
date (if they are shown to be problematic).


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