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Date: Sat, 29 Oct 2016 15:06:02 +0200
From: Daniel Gruss <daniel@...ss.cc>
To: "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>
Cc: Pavel Machek <pavel@....cz>, Mark Rutland <mark.rutland@....com>,
Kees Cook <keescook@...omium.org>, Peter Zijlstra <peterz@...radead.org>,
Arnaldo Carvalho de Melo <acme@...hat.com>,
kernel list <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Subject: Re: Re: rowhammer protection [was Re: Getting
interrupt every million cache misses]
I think that this idea to mitigate Rowhammer is not a good approach.
I wrote Rowhammer.js (we published a paper on that) and I had the first
reproducible bit flips on DDR4 at both, increased and default refresh
rates (published in our DRAMA paper).
We have researched the number of cache misses induced from different
applications in the past and there are many applications that cause more
cache misses than Rowhammer (published in our Flush+Flush paper) they
just cause them on different rows.
Slowing down a system surely works, but you could also, as a mitigation
just make this CPU core run at the lowest possible frequency. That would
likely be more effective than the solution you suggest.
Now, every Rowhammer attack exploits not only the DRAM effects but also
the way the operating system organizes memory.
Some papers exploit page deduplication and disabling page deduplication
should be the default also for other reasons, such as information
disclosure attacks. If page deduplication is disabled, attacks like
Dedup est Machina and Flip Feng Shui are inherently not possible anymore.
Most other attacks target page tables (the Google exploit, Rowhammer.js,
Drammer). Now in Rowhammer.js we suggested a very simple fix, that is
just an extension of what Linux already does.
Unless out of memory page tables and user pages are not placed in the
same 2MB region. We suggested that this behavior should be more strict
even in memory pressure situations. If the OS can only find a page table
that resides in the same 2MB region as a user page, the request should
fail instead and the process requesting it should go out of memory. More
generally, the attack surface is gone if the OS never places a page
table in proximity of less than 2MB to a user page.
That is a simple fix that does not cost any runtime performance. It
mitigates all these scary attacks and won't even incur a memory cost in
most situation.
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