Re: [gentoo-user] Re: Spectre-NG

[Rich Freeman]

On Thu, May 10, 2018 at 1:34 AM Martin Vaeth <martin@×××××.de> wrote:

> As a simple example, assume that you have read a password file
> into a string of your language and now access a single password.
> No matter, how you mark the end of the password (fixed-length, \0, \n,
> ...) speculative execution might always access the next password(s)
> unless you prevent it globally. Whether it is exploitable depends
> of course on other things. There is no difference to C.


I'll just reply to this because the rest of the email is just a natural
consequence of it.

I will confess up-front I'm not an expert in Spectre, but I don't see how
having the next password speculatively read would on its own create a
vulnerability.  You would also need some kind of indirect memory access
based on the speculatively-accessed data to create a timing channel.

And of course it is entirely possible that the loop wouldn't be
sequentially reading in data in a way that could cause a bound to be
exceeded, and in this situation I don't see why protection would be
required.  Consider:

for x = 1 to 10 : y=5+3 : next x

This is obviously trivial, but I don't see how executing that loop a few
too many times would cause problems, so blocking speculation is just going
to hurt performance for no benefit.

I still tend to think that the additional context around these memory
accesses that is available in a high-level language could be used by a
compiler to determine when protection is required.

Certainly a skilled programmer might be able to do an even better job if
they were vigilant, but not all programmers are equally skilled...

-- 
Rich