[llvm-dev] Spectre V1 Mitigation - Internals?

Craig Topper via llvm-dev
Tue, 17 Sep 2019 15:59:54 -0000

The reverting of state doesn’t occur when the condition is available. The
processor has to “execute” the branch uop and see that it was mispredicted.
This occurs at least one cycle after the condition is available. The
conditional move on the predicate state can execute at the same time as the
branch. Or it can execute before it if the branch unit is busy. The load
can do the same.

On Tue, Sep 17, 2019 at 7:57 AM Praveen Velliengiri via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
> Thanks for your email, I understand that the execution stalls until the
> predicated state is computed, then we mask pointers with all_zeros_mask if
> there is a mis-prediction. But I understand that as soon as the condition
> value is available, the processor can check about it's assumptions and
> revert back.
>
> That is,
> If the branch prediction is correct during speculation, we mask with
> all_ones, the processor can follow the predicted branch to retire.
> But if the processor mispredicted the branch, it will revert back as soon
> as condition become available if this is the case then we don't execute
> speculatively the operations : pointer1 &= predicate_state - (if
branch)
> and *pointer2 & predicted_state - (else branch) right? Or
> out-of-processor's allow such access?
>
> Plus, why we are masking with all_zeros_mask during mis-prediction. Is
> there any reason for choosing all_zeros_mask?
>
> Cheers,
> Praveen
>
>
>
> On Tue, 17 Sep 2019 at 17:08, Jeremy Lakeman <Jeremy.Lakeman at
gmail.com>
> wrote:
>
>> There's a difference between control flow speculation and data
flow.
>> By entangling the control flow & data, the cpu should stall the
memory
>> load until the condition is known, since the pointer value also
isn't known.
>>
>> I assume that actual experts (not me) tested this, and / or asked intel
>> if this was sufficient to prevent the speculative memory load, given
how
>> speculation is actually implemented.
>>
>> On Tue, 17 Sep 2019 at 02:04, Praveen Velliengiri via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi all,
>>>
>>> I understand how the speculative information flow attack works.
I'm
>>> trying get my head around the spectre v1 mitigation of LLVM.
>>> In the design document here :
>>>
https://llvm.org/docs/SpeculativeLoadHardening.html#speculative-load-hardening.
>>>
<https://llvm.org/docs/SpeculativeLoadHardening.html#speculative-load-hardening>
>>>
>>> Example:
>>>
>>> void leak(int data);void example(int* pointer1, int* pointer2) {
>>>   if (condition)
>>>     leak(*pointer1);
>>>   else
>>>     leak(*pointer2);}
>>>
>>> After the applying the mitigation the code resembles like:
>>>
>>> void leak(int data);
>>> void example(int* pointer1, int* pointer2) {
>>>   uintptr_t predicate_state = all_ones_mask;
>>>   if (condition) {
>>>     predicate_state = !condition ? all_zeros_mask :
predicate_state;
>>>     pointer1 &= predicate_state;
>>>     leak(*pointer1);
>>>   } else {
>>>     int value2 = *pointer2 & predicate_state;
>>>     leak(value2);
>>>   }
>>> }
>>>
>>> Let's assume that the branch is mispredicted and if body is
taken. The value predicate_state mask is depend on the "result of the
condition" but which is not yet available hence
>>>
>>> speculative execution. My question whether the value of
predicate_state is also guessed by the processor? If it is correct, then the
value of predicate_state will be
>>> predicate_state, if the processor mis-predicts the condition as
true. Is my assumption is correct? i.e predicate_state = ! 1 ? all_zeros_mask :
predicate_state, where processor predicts
>>> condition as true.
>>>
>>> Or Whether the execution stalls at "predicate_state =
!condition ? all_zeros_mask : predicate_state until the result of condition
became available? If this is true, why we have to
>>>
>>> harden the pointers in the first place, because after the condition
is actually computed, the processors will revert back right the execution trace
of mis-prediction.
>>>
>>> I know that I'm missing something fundamental here, I would
highly appreciate your help on this? Please let me know if you more info!
>>>
>>> Cheeers,
>>>
>>> Praveen
>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>> _______________________________________________
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>
--
~Craig
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