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SubjectRe: [PATCH] mm/mincore: allow for making sys_mincore() privileged
On Wed, 9 Jan 2019, Dave Chinner wrote:

> FWIW, I just realised that the easiest, most reliable way to invalidate
> the page cache over a file range is simply to do a O_DIRECT read on it.

Neat, good catch indeed. Still, it's only the invalidation part, but the
residency check is the crucial one.

> > Rationale has been provided by Daniel Gruss in this thread -- if the
> > attacker is left with cache timing as the only available vector, he's
> > going to be much more successful with mounting hardware cache timing
> > attack anyway.
>
> No, he said:
>
> "Restricting mincore() is sufficient to fix the hardware-agnostic
> part."
>
> That's not correct - preadv2(RWF_NOWAIT) is also hardware agnostic and
> provides exactly the same information about the page cache as mincore.

Yeah, preadv2(RWF_NOWAIT) is in the same teritory as mincore(), it has
"just" been overlooked. I can't speak for Daniel, but I believe he might
be ok with rephrasing the above as "Restricting mincore() and RWF_NOWAIT
is sufficient ...".

> Timed read/mmap access loops for cache observation are also hardware
> agnostic, and on fast SSD based storage will only be marginally slower
> bandwidth than preadv2(RWF_NOWAIT).
>
> Attackers will pick whatever leak vector we don't fix, so we either fix
> them all (which I think is probably impossible without removing caching
> altogether)

We can't really fix the fact that it's possible to do the timing on the HW
caches though.

> or we start thinking about how we need to isolate the page cache so that
> information isn't shared across important security boundaries (e.g. page
> cache contents are per-mount namespace).

Umm, sorry for being dense, but how would that help that particular attack
scenario on a system that doesn't really employ any namespacing? (which I
still believe is a majority of the systems out there, but I might have
just missed the containers train long time ago :) ).

--
Jiri Kosina
SUSE Labs

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