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

[Norbert Manthey <nmanthey@xxxxxxxxx>]

Sorry for the late reply. I try to pick up where we left the discussion
the last time.

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:
>>>> Guests can issue grant table operations and provide guest controlled
>>>> data to them. This data is used as index for memory loads after bound
>>>> checks have been done. To avoid speculative out-of-bound accesses, we
>>>> use the array_index_nospec macro where applicable, or the macro
>>>> block_speculation. Note, that the block_speculation is always used in
>>> s/always/already/ ?
>> They both work, but the 'always' underlines that a caller can rely on
>> the fact that this will happen on all execution path of that function.
>> Hence, I like to stick to 'always' here.
> Well, I'm not a native speaker, but to me "always" doesn't express
> what you want to express here. What you mean I'd word "... is used
> on all paths of ..."
I will rephrase accordingly.
>
>>>> the calls to shared_entry_header and nr_grant_entries, so that no
>>>> additional protection is required once these functions have been
>>>> called.
> As an aside, your use of "in the calls to" looks also misleading to me,
> because the fences sit in the functions, not at the call sites.
Will fix.
>
>>>> --- 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.
>
>>>> @@ -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.

Best,
Norbert





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