Re: [PATCH] kernel: sys: fix potential Spectre v1

[Gustavo A. R. Silva]

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