Re: [Xen-devel] [PATCH L1TF MDS GT v1 2/3] common/grant_table: harden bound accesses

[Norbert Manthey <nmanthey@xxxxxxxxx>]

On 7/10/19 05:04, Jan Beulich wrote:
> On 08.07.2019 14:58, Norbert Manthey wrote:
>> On 5/24/19 13:10, Jan Beulich wrote:
>>>>>> On 24.05.19 at 11:54, <nmanthey@xxxxxxxxx> wrote:
>>>> On 5/23/19 16:17, Jan Beulich wrote:
>>>>>>>> On 21.05.19 at 09:45, <nmanthey@xxxxxxxxx> wrote:
>>>>>> --- a/xen/common/grant_table.c
>>>>>> +++ b/xen/common/grant_table.c
>>>>>> @@ -988,9 +988,10 @@ map_grant_ref(
>>>>>>           PIN_FAIL(unlock_out, GNTST_bad_gntref, "Bad ref %#x for d%d\n",
>>>>>>                    op->ref, rgt->domain->domain_id);
>>>>>>   
>>>>>> -    act = active_entry_acquire(rgt, op->ref);
>>>>>> +    /* This call also ensures the above check cannot be passed 
>>>>>> speculatively */
>>>>>>       shah = shared_entry_header(rgt, op->ref);
>>>>>>       status = rgt->gt_version == 1 ? &shah->flags : &status_entry(rgt, 
>>>>>> op->ref);
>>>>>> +    act = active_entry_acquire(rgt, op->ref);
>>>>> I know we've been there before, but what guarantees that the
>>>>> compiler won't reload op->ref from memory for either of the
>>>>> latter two accesses? In fact afaict it always will, due to the
>>>>> memory clobber in alternative().
>>>> The compiler can reload op->ref from memory, that is fine here, as the
>>>> bound check happens above, and the shared_entry call comes with an
>>>> lfence() by now, so that we will not continue executing speculatively
>>>> with op->ref being out-of-bounds, independently of whether it's from
>>>> memory or registers.
>>> I don't buy this argumentation: In particular if the cache line got
>>> flushed for whatever reason, the load may take almost arbitrarily
>>> long, opening up a large speculation window again using the
>>> destination register of the load (whatever - not bounds checked -
>>> value that ends up holding).
>> I agree, the given protection does not force the CPU to pick up the
>> fixed value. As you already noticed, the presented fix might not work in
>> all cases, but is among the suitable solutions we have today to prevent
>> simple user controlled out-of-bound accesses during speculation. Relying
>> on the stale value of the register that might be used during speculation
>> makes a potential out-of-bound access much more difficult. Hence, the
>> proposed solution looks good enough to me.
> But using a local variable further reduces the risk afaict: Either
> the compiler puts it into a register, in which case we're entirely
> fine. Or it puts it on the stack, which I assume is more likely to
> stay in cache than a reference to some other data structure (iirc
> also on the stack, but not in the current frame).
If you want me to introduce a local variable, I can do that. I remember
we had discussions around that as well.
>
>>>>>> @@ -3863,6 +3883,9 @@ static int gnttab_get_status_frame_mfn(struct 
>>>>>> domain *d,
>>>>>>               return -EINVAL;
>>>>>>       }
>>>>>>   
>>>>>> +    /* Make sure idx is bounded wrt nr_status_frames */
>>>>>> +    block_speculation();
>>>>>> +
>>>>>>       *mfn = _mfn(virt_to_mfn(gt->status[idx]));
>>>>>>       return 0;
>>>>>>   }
>>>>> Why don't you use array_index_nospec() here? And how come
>>>> There is no specific reason. I will swap.
>>>>> speculation into q() is fine a few lines above?
>>>> I do not see a reason why it would be bad to enter that function
>>>> speculatively. There are no accesses that would have to be protected by
>>>> extra checks, afaict. Otherwise, that function should be protected by
>>>> its own.
>>> Which in fact happens, but only in patch 3. This may be worth saying
>>> explicitly here.
>> Do you want me to explicitly mention this in the commit message, or add
>> a comment here, which I have to drop in patch 3 again? For now, I'd just
>> leave it as is, as the version based fixes are handled in the other commit.
> A commit message remark would both help understand things now and
> in the future, and at the same time avoid me or someone else re-
> raising the question next time round, not the least because of the
> noticable gaps between versions.

I will extend the commit message accordingly.

Best,
Norbert





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