Re: Re: [kernel-hardening] rowhammer protection [was Re: Getting interrupt every million cache misses]

From: Daniel Gruss
Date: Sat Oct 29 2016 - 09:14:13 EST

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.