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

Pavel Machek
2018-01-04T19:26:54Z

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Hi!

> >> > What remains to be seen is if there are other patterns that affect
> >> > different processors.
> >> >
> >> > In the longer term the compiler itself needs to know what is and isn't
> >> > safe (ie you need to be able to write things like
> >> >
> >> > void foo(tainted __user int *x)
> >> >
> >> > and have the compiler figure out what level of speculation it can do and
> >> > (on processors with those features like IA64) when it can and can't do
> >> > various kinds of non-trapping loads.
> >> >
> >>
> >> It would be great if coccinelle and/or smatch could be taught to catch
> >> some of these case at least as a first pass "please audit this code
> >> block" type of notification.
> >>
> >
> > 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?

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

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