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Date:   Wed, 9 Jan 2019 11:08:57 +0100 (CET)
From:   Jiri Kosina <>
To:     Dave Chinner <>
cc:     Linus Torvalds <>,
        Matthew Wilcox <>,
        Jann Horn <>,
        Andrew Morton <>,
        Greg KH <>,
        Peter Zijlstra <>,
        Michal Hocko <>, Linux-MM <>,
        kernel list <>,
        Linux API <>
Subject: Re: [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

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