Re: [RFC PATCH] asm/generic: introduce if_nospec and nospec_barrier

Pavel Machek
2018-01-04T23:33:03Z

[-- Attachment #1: Type: text/plain, Size: 1337 bytes --]

Hi!

> > What did it leak? Nothing. Attacker had to know
> > array+attacker_controlled_index, and he now knows
> > (array+attacker_controlled_index)%CACHELINE_SIZE.
> >
> > With (void) array2[val];, the attack gets interesting -- I now know
> > *(array+attacker_controlled_index) % CACHELINE_SIZE ... allowing me to
> > get information from arbitrary place in memory -- which is useful for
> > .. reading ssh keys, for example.
>
> Right, but how far away from "val = array[attacker_controlled_index];"
> in the instruction stream do you need to look before you're
> comfortable there's no 'val' dependent reads in the speculation window
> on all possible architectures. Until we have variable annotations and
> compiler help my guess is that static analysis has an easier time
> pointing us to the first potentially bad speculative access.

Well, you are already scanning for if (attacker_controlled_index <
limit) .... array[attacker_controlled_index] and those can already be
far away from each other....

Anyway, likely in the end human should be creating the patch, and if
there's no array2[val], we do not need the patch after all.

Best regards,

Pavel
--
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 181 bytes --]