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SubjectRe: [PATCH] kernel: sys: fix potential Spectre v1
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On 05/18/2018 05:08 PM, Dan Williams wrote:
>>
>> Oh I see now. Just to double check, then something like the following would
>> be broken too, because is basically the same as the code above, and well, it
>> doesn't make much sense to store the value returned by macro
>> array_index_nospec into x, correct?:
>
> Correct, broken:
>
>>
>> bool foo(int x)
>> {
>> if(x >= MAX)
>> return false;
>
> Under speculation we may not return here when x is greater than max.
>
>> x = array_index_nospec(x, MAX);
>
> x is now sanitized under speculation to zero, but the compiler would
> likely just throw this away because nothing consumes it.
>
>> return true;
>> }
>>
>> int vulnerable(int x)
>> {
>> if(!foo(x))
>> return -1;
>
> cpu might speculate that this branch is not taken...
>
>>
>> temp = array[x];
>
> ...so x had better be bounded here, otherwise Spectre.
>

I got it.

I appreciate the feedback.

Thanks, Dan.
--
Gustavo

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