Friday, October 30, 2015

Speculation: What the next row hammer style exploit could look like

Speculation:  What the next row hammer style exploit could look like

One advantage of being a blogger is that I can speculate wildly and I’ll do this in this blog.
A few years ago I was part of a team that developed video codecs. H.264 and mpeg2 essentially. To test these we had a student aid and around 15 computers. The student aid checked that test where running 24/7 and that the results wasn’t garbage. In this way the poor PC’s ragged up months worth of CPU time. Now occasionally we had to change components on these computers – often RAM. Resonantly I had written what was essentially a file copy routine and for some reason it ended up being tested on one of these old computers. The result was lots of binary compare errors – always a single bit in a random byte. I spend hours trying to find the bug in my code, to no avail. The bug it turned out was that the DRAM modules where broken and produced enough bit errors that copying a 8gig set of files would produce compare errors, but not enough that the system seemed to run unstably. It had been less than a month after the laptop we were supposed to do my black hat presentation on died while row hammer testing. Probably the RAM went dead. Now video encoding and row hammer has something in common. They produce lots of cache misses. Row hammer by design – it doesn’t work without them-  and video codecs simply because motion search and motion compensation routines plows through lots and lots of memory and back then there was no 3rd level cache. And finally came an article about causing ageing in a Sparc processor: KARIMI et al(2015):.

Now the speculation of mine is now that memory circuits age. Which seems to be true.See: Schroeder, Pinhiero & Weber(xxxx). Further we know that memory circuits that are used are more prone to error than idle ones. Not really surprising. What we don’t know is if using memory causes it to age.  For static memory like a usb-stick it’s relatively easy to produce. But for DRAM I found nothing. I speculate that our video codec testing caused ageing in memory and that row hammer testing possibly did the same on our presentation laptop for black hat. The point is normally the cache will catch the majority of memory access, which isn’t much on most computers anyway since they spend most of their life idling. However as we know from row hammer we can force memory access in an attempt to age the memory. A rough guess is that most addresses in a random computer see only one access per 6 milliseconds or so. With malicious code we can do an access around every 250 nano seconds. In case you wonder it’s factor 24000 – and DRAM age in normal computers too. If this works we’re basically doing hardware destruction – nothing new under the sky here. Virus tried to destroy hard drives and floppy drives in the 90ies.

The new thing is row hammer.  Seaborn and Dullien (2015) forcefully illustrated that what is normally a reliability issue can become a security issue very fast. The way that worked was simply to spray the memory with PTE’s, hammer away and wait until read write access was given to a page owned by the attacker. Now my file copying incidence produced random bit flips during normal execution – I was definitely not hammering anything. So bit flips without hammering could be possible in aged chips.

 If we are so unfortunate that I’m right about the DRAM ageing then there is a good chance that it’s exploitable. Read /Write memory patterns for a couple of months non-stop, then when errors start to occur spray the memory with PTE’s and wait for the local privilege escalation take place.

Granted this is a pretty rare scenario and it’s real world importance probably shouldn’t be over estimated. But it is a pretty nifty idea. I share it, instead of developing it because I do not have the resources (time and hardware) to pursue this at all. And before you accuse me of selling the skin before the bear has been shot: There is so much speculation in here. Last if you have some old hardware you can mistreat and you are willing to put up with the electricity cost to test this. Please get in contact and I’ll write up a small test program in a jiffy – remember even if my speculation is wrong these kinds of experiments can shed some light on ageing in DRAM – and interesting endeavor all together. Thus even if I’m wrong we might learn something.


KARIMI et al(2015): MAGIC: Malicious Aging in Circuits/Cores:

Seaborn & Dullien (2015): Exploiting the DRAM rowhammer bug to gain kernel privileges.

Schroeder, Pinhiero & Weber(xxxx): DRAM Errors in the Wild: A Large-Scale Field Study: