[gentoo-user] Re: Spectre-NG

[Martin Vaeth <martin@mvath.de>]

Rich Freeman <rich0@gentoo.org> wrote:
> On Tue, May 8, 2018 at 4:19 AM Martin Vaeth <martin@mvath.de> wrote:
>
>> Rich Freeman <rich0@gentoo.org> wrote:
>> >
>> > Higher-level languages will probably become nearly immune to Spectre
> just
>> > as most are nearly immune to buffer overflows.
>
>> Quite the opposite: Higher-level languages *always* do some checks
>> for array-length etc, and it is the _checks_ which are vulnerable.
>> You can only make them non-vulnerable by making them horribly slow
>> (by omitting speculative execution completely for the corresponding
>> conditionals).
>
> Sure, but my point is that you CAN make them non-vulnerable by changing the
> compiler.

Which would be the horribly slow case I mentioned above.

> On the other hand, if somebody manually does a range check in C the only
> way to fix it is to either fix the source code

If slowness is not the issue, one could fix the C compiler in the same way
by avoiding speculative exection for every conditional jump.

As mentioned, I wonder why gcc/clang do not yet support this
horribly slow but spectre-safe option. It can't be that hard to
implement in the actual code-producing back-end. Only possible
optimization of that code (catching unnecessary cases or
preferring alternative code-paths if there are two many bad
cases) might be tricky, but perhaps this could be postponed
in a first implementation: safety first.