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Re: [Xen-devel] [PATCH SpectreV1+L1TF v5 8/9] common/grant_table: block speculative out-of-bound accesses


  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Norbert Manthey <nmanthey@xxxxxxxxx>
  • Date: Thu, 7 Feb 2019 11:20:23 +0100
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  • Cc: Juergen Gross <jgross@xxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>, Dario Faggioli <dfaggioli@xxxxxxxx>, Martin Pohlack <mpohlack@xxxxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, David Woodhouse <dwmw@xxxxxxxxxxxx>, "Martin Mazein\(amazein\)" <amazein@xxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Julian Stecklina <jsteckli@xxxxxxxxx>, Bjoern Doebel <doebel@xxxxxxxxx>
  • Delivery-date: Thu, 07 Feb 2019 10:20:59 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
  • Openpgp: preference=signencrypt

On 2/7/19 10:50, Norbert Manthey wrote:
> On 2/6/19 16:53, Jan Beulich wrote:
>>>>> On 06.02.19 at 16:06, <nmanthey@xxxxxxxxx> wrote:
>>> On 2/6/19 15:52, Jan Beulich wrote:
>>>>>>> On 29.01.19 at 15:43, <nmanthey@xxxxxxxxx> wrote:
>>>>> @@ -963,6 +965,9 @@ map_grant_ref(
>>>>>          PIN_FAIL(unlock_out, GNTST_bad_gntref, "Bad ref %#x for d%d\n",
>>>>>                   op->ref, rgt->domain->domain_id);
>>>>>  
>>>>> +    /* Make sure the above check is not bypassed speculatively */
>>>>> +    op->ref = array_index_nospec(op->ref, nr_grant_entries(rgt));
>>>>> +
>>>>>      act = active_entry_acquire(rgt, op->ref);
>>>>>      shah = shared_entry_header(rgt, op->ref);
>>>>>      status = rgt->gt_version == 1 ? &shah->flags : &status_entry(rgt, 
>>>>> op->ref);
>>>> Just FTR - this is a case where the change, according to prior
>>>> discussion, is pretty unlikely to help at all. The compiler will have
>>>> a hard time realizing that it could keep the result in a register past
>>>> the active_entry_acquire() invocation, as that - due to the spin
>>>> lock acquired there - acts as a compiler barrier. And looking at
>>>> generated code (gcc 8.2) confirms that there's a reload from the
>>>> stack.
>>> I could change this back to a prior version that protects each read
>>> operation.
>> That or use block_speculation() with a comment explaining why.
>>
>> Also - why are there no changes at all to the unmap_grant_ref() /
>> unmap_and_replace() call paths? Note in particular the security
>> related comment next to the bounds check of op->ref there. I've
>> gone through earlier review rounds, but I couldn't find an indication
>> that this might have been the result of review feedback.
> You are right. I am not sure whether I had a fix placed there in the
> beginning. I will replace the first "smp_rmb();" in function
> unmap_common for the next iteration with the "block_speculation" macro.
I just checked this one more time. The maptrack_entry macro has been
extended with the array_index_nospec macro already, so that the
assignment to the map variable is in bound. Therefore, I actually will
not introduce the block_speculation macro.
>
> The other check unlikely(op->ref >= nr_grant_entries(rgt)) can only
> reach out-of-bounds for the unmap case, in case the map->ref entry has
> been out-of-bounds beforehand. I did not find an assignment that is not
> protected by a bound check and a speculation barrier or array_nospec_index.
>
> Best,
> Norbert
>
>



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