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

Pavel Machek
2018-01-04T22:44:59Z

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Hi!

> >> > What should one be looking for. Do you have a typical example?
> >> >
> >>
> >> See "Exploiting Conditional Branch Misprediction" from the paper [1].
> >>
> >> The typical example is an attacker controlled index used to trigger a
> >> dependent read near a branch. Where an example of "near" from the
> >> paper is "up to 188 simple instructions inserted in the source code
> >> between the ‘if’ statement and the line accessing array...".
> >>
> >> if (attacker_controlled_index < bound)
> >> val = array[attacker_controlled_index];
> >> else
> >> return error;
> >>
> >> ...when the cpu speculates that the 'index < bound' branch is taken it
> >> reads index and uses that value to read array[index]. The result of an
> >> 'array' relative read is potentially observable in the cache.
> >
> > You still need
> >
> > (void) array2[val];
> >
> > after that to get something observable, right?
>
> As far as I understand the presence of array2[val] discloses more
> information, but in terms of the cpu taking an action that it is
> observable in the cache that's already occurred when "val =
> array[attacker_controlled_index];" is speculated. Lets err on the

Well yes, attacker can observe val =
array[attacker_controlled_index]; . But that's not something he's
interested in. So the CPU cheats and attacker has a proof. But he knew
that before.

>side
> of caution and shut down all the observable actions that are already
> explicitly gated by an input validation check. In other words, a low
> bandwidth information leak is still a leak.

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.

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

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