This is the first updated version of my original “Row hammer the short summary” blog post. As I had predicted the summer was going to be interesting in terms of row hammer and it certainly appears I was right about that. With so much going on I found it worthwhile updating this blog post to be in line with the latest developments and to fix up a few minor details.
Short version of how dram works.
Current DRAM comes in modules called DIMM’s. If you buy a modern memory module for your PC, you’re buying a DIMM. If you look at the DIMM most DIMMs will have chips on both sides. Each side of the DIMM is a rank. Each rank again consists of a number of banks. The banks are in the physical individual chips you can see. Inside a bank you’d find a two dimensional matrix of memory cells. There are 32k rows in the matrix and 16k or 512k cells per row. Each cell stores one bit and consists of a transistor for control and a capacitor which stores charge to signify bit is equal to 1 and no charge when bit is equal to 0 (on some chips the encoding is reversed). Thus a row stores 8kb or 64kb of data depending on the exact kind of DRAM you have in front of you. When you read or write from/to DRAM an entire row is first read into a so called so called row buffer. This is because for reading automatically discharges the capacitor and since writes rarely rewrite the entire row. Reading a row into the row buffer is called activating the row. An active row is thus cached in the row buffer. If a row is already active, it is not reactivated on requests. Also to prevent the capacitors loose charge overtime they are refreshed regularly (typically every 64 ms) by activating the rows.
Row hammer introduction
This section is based on  Kim et Al. where not otherwise noted.
When a row is activated a small effect is caused on the neighboring row due to so called cross talk effects. The effect can be caused by electromagnetic interference, so called conductive bridges where there is minor electric conductivity in dram modules where it shouldn’t be. And finally, so called hot-carrier-injection may play a role where an electron reaches sufficient kinetic energy where it leaks from the system or even permanently damage parts of the circuitry. The net effect is a loss of charge in the DRAM cell which if large enough will cause a bit to flip.
Consequently, it’s possible to cause bits to flip in DRAM by reading or writing repeatedly and systematically from/to two rows in DRAM to (active the rows) bit flips can be introduced in rows up to 9 rows away from these two “aggressor rows”. The 9 rows are called victim rows. The most errors happen in the row immediately next to an aggressor row. Picking the aggressor rows so they bracket a victim row is called double sided row hammering and is far more efficient that normal row hammering. Using two adjacent rows to hammer surrounding rows is called amplified single sided hammering and can be useful in exploitation scenarios. If the victim rows are refreshed before enough cross talk can be generated no bit flips is incurred. As a rule of thumb the higher the frequency of row activation the higher the probability of flipping bits.
It has been shown that bits can be flipped in less than 9 milliseconds and typically requires around 128k row activations.  Seaborn & Dullien has reported bit flips with as little as 98k row activations.
The problem occurs primarily with RAM produced after 2010. In a sample of 129 RAM modules from 3 manufacturers over 80% where vulnerable. With all modules produced after 2012 being vulnerable.  Lanteigne showed that DDR4 ram is vulnerable too with 8 out of 12 sampled DRAMs was subject to bit flips. Further this paper showed that certain patterns in the DRAM rows where more likely to cause bit flips.
 Lateigne concludes that AMD and Intel CPU’s are both capable of row hammering, but that the most bit flips are encountered when the methodlogy is adapted to the underlying memory controller in the attacked system.
Physical addressing and finding banks and row
Obviously to cause row hammering one needs two addresses belonging to rows in the same bank.  showed that repeatedly choosing two random addresses in a large buffer would in a practical amount of time belong to the same bank and thus be useful for hammering in software.
An optimal solution requires that the attacker has knowledge of physical addresses. Even with a physical address an attacker would need to know how they map to dimm, banks and rows to optimally row hammer.  Seaborn used row hammer itself to derive the complex function that determines physical address to dram location for a sandy bridge computer.  Pessl et al. showed how to use “row buffer side channel attacks” to derive the complex addressing function generically and provided maps for many modern intel CPU’s.
To obtain physical addresses the /proc/$PID/pagemap could provide this information. However, /proc/$PID/pagemap, which is not available in all operating systems and no longer affords unprivileged access in most operating systems that do support it. This problem for an attacker remains to be definitively solved.
Row hammering with Software
To cause row hammer from software you need to activate memory rows, that is cause reads or writes to physical memory. However modern processors are equipped with caches, so that they don’t incur serious speed penalties when memory is read or written. Thus to cause row hammering bit flips it’s required to bypass the caches.
 Aweke posted on a forum he was able to flip bits without clflush – he did not say how, but it was likely using the same method as  which systematically accessed memory in a pattern that causes the processor to evict the address of the attacker row from the cache causing the next read to go to physical memory. Unfortunately, how to evict caches is CPU dependent and undocumented and despite  Gruss, Maurice & Mangard mapping out how to optimally evict on most modern CPU’s it’s not the most trivial process. Typically, this requires knowledge of the physical address discussed above as well as a complex mapping function for cache sets. It is however possible to approximate this either through using large pages or through timing side channels. Also it is slower and thus less efficient than the clflush version above. Since this vector does not require special instructions, it’s applicable to native code (including sandboxed code), java script and potentially other JIT compiled languages.
 Qiao & Seaborn found out that the movnti instruction is capable of by passing the caches on it’s own. Further this instruction is commonly used – including as memcpy/memset in common libraries and thus difficult to ban in validator sandboxes and lowers the burden for future row hammer as a code reuse attack. It remains to be seen if JIT compiled languages can make use of it.
Finally,  Fogh showed that the policies that maintains the coherency of multiple caches on the CPU can be used to cause row activations and speculated it would be fast enough for row hammering. Since the coherency policies are subject to much less change than cache eviction policies and does not require any special instructions this method may solve problems with the other methods should it be fast enough. This remain to be researched.
Exploiting row hammer
 showed that row hammer could be used to break out of the NaCL chrome sandbox. The NaCL sandbox protects itself against by verifying all code paths before execution and block the use of undesired activities. To avoid new unintended code paths to be executed the sandbox enforces a 32 bit aligned address for relative jumps and adding a base address. In code it looks like this:
and rax, ~31a
add rax, r15 //(base address of sandbox)
Bit flips in these instructions often cause other registers to be used and thus bypass the sandbox enforcing limits on relative jumps. By spraying code like the above then row hammer, check for usable bit flips and finally use one of these relative jumps to execute a not validated code path and thus exit the sandbox. Not validated code path can be entered through code reuse style gadgets.
The second and more serious privilege escalation attack demonstrated by  was from ring 3 user privileges to ring 0. Since adjacent physical addresses has a tendency to be used at the same time, CPU vendors map adjacent physical addresses to different parts of RAM as this offers the possibility of memory request being handled by DRAM in parallel. This has the effect that banks are often shared between different software across trust boundaries. This allows an attacker to flip bits in page table entries (PTE). PTE’s is central in the security on x86 and controls access writes to memory as well as mapping between virtual and physical addresses. By repeatedly memory mapping a file many new PTE’s are sprayed and the attack has a good chance of hitting by random using row hammer. The attacker hopes that a bit flips so that a PTE with write privileges maps to a new physical address where another PTE is located. By strategically modifying this second PTE the attacker has read & write access to the entire memory.
 Bhattacharya & Mukhopadhyay. Used to Row Hammer to extract a private RSA 1024 bit key. Their attack used a combination of PAGEMAP, a cache side channel attack (prime+probe) and a row buffer side channel attack to find the approximate location in physical memory of the private key. Once located row hammer is used to introduce a fault in the private key, and fault analysis is then used to derive the real private key. This make the attack somewhat unique in that it’s the only attack so far that does not rely on any kind of spraying.
 Xiao et al. The content of this paper is unknown to me, yet the title suggests that cross-VM and a new kind of privilege escalation is possible with row hammer.
It is likely that row hammer can be exploited in other ways too.
Row hammer mitigation
Most hardware mitigations suggest focuses on refreshing victim rows thus leaving less time for row hammer to do its work. Unfortunately, during the refresh ram is unable to respond to requests from the CPU and thus it comes at a performance penalty.
The simplest suggestion is increase the refresh rate for all ram. Much hardware support this already for high-temperatures. Usually the refresh rate is doubled, however to perfectly rule out row one would need to increase the refresh rate more than 7 fold . Which in term is a steep performance penalty and a power consumption issue.
TTR  is a method that keeps track of used rows and cause targeted refresh of neighbors to minimize the penalty. The method needs to be supported in both CPU as well as RAM modules. MAC also known as maximum activation count keeps tracks of how many times a given row was activated. pTTR does this only statistically and thus cheaper to build into hardware. PARA  is another suggested hardware mitigation to statistically refresh victim rows. ARMOR  a solution that keep tracks of row activation in the memory interface.
It has been suggested that ECC ram can be used as a mitigation. Unfortunately, ECC ram will not to detect or correct bit flips in all instances where there are multiple bit flips in a row. Thus this leaves room for an attack to be successful even with ECC ram. Also ECC ram may cause the attacked system to reset, turning row hammer into a Denial of Service attack.  Suggests this problem persists in real life experiments. Keeping track of ECC errors may however serve as an indication that a system was under attack and could be used to trigger other counter measures.
Nishat Herath and I suggested using the LLC miss performance counter to detect row hammering here  Fogh & Nishat. LLC Misses are rare in real usage, but abundant in row hammering scenarios.  Gruss et al. , Payer refined the method respectively with correcting for generally activity in the memory subsystem. The methods are likely to present false positives in some cases and  and  therefore suggested only slowing down offenders to prevent bit flips.  Aweke et al. presented a method that first detected row hammering as above, then verified using PEBS performance monitoring, which has the advantage of delivering an address related to a cache miss and thus grants the ability to selectively read neighbor rows and thus doing targeted row refresh in a software implementation.  Fogh speculated that this method could be effectively bypassed by an attacker.
 Yoongu Kim, R. Daly, J. Kim, C. Fallin, Ji Hye Lee, Donghyuk Lee, C. Wilkerson, K. Lai, and O. Mutlu. Flipping Bits in Memory Without Accessing Them: An Experimental Study of DRAM Disturbance Errors. In Computer Architecture (ISCA), 2014 ACM/IEEE 41st International Symposium on, pages 361–372, June 2014.
 Mark Seaborn and Thomas Dullien. Exploiting the DRAM rowhammer bug to gain kernel privileges. March 2015.
 Mark Seaborn and Thomas Dullien. “Exploiting the DRAM rowhammer bug to gain kernel privileges”. https://www.blackhat.com/docs/us-15/materials/us-15-Seaborn-Exploiting-The-DRAM-Rowhammer-Bug-To-Gain-Kernel-Privileges.pdf
 Mark Lanteigne. “How Rowhammer Could Be Used to Exploit Weaknesses in Computer Hardware
“. Third I/O. http://www.thirdio.com/rowhammer.pdf
 Mark Seaborn.” How physical addresses map to rows and banks in DRAM”;
 Peter Pessl, Daniel Gruss, Clémentine Maurice, Michael Schwarz, Stefan Mangard: „Reverse Engineering Intel DRAM Addressing and Exploitation“
 Zelalem Birhanu Aweke, “Rowhammer without CLFLUSH,” https://groups.google.com/forum/#!topic/rowhammer-discuss/ojgTgLr4q M, May 2015, retrieved on July 16, 2015.
 Rui Qiao, Mark Seaborn: “A New Approach for Rowhammer Attacks”.http://seclab.cs.sunysb.edu/seclab/pubs/host16.pdf
 Anders Fogh: “Row hammer, java script and MESI”http://dreamsofastone.blogspot.de/2016/02/row-hammer-java-script-and-mesi.html
 Anders Fogh, Nishat Herath. “These Are Not Your Grand Daddys CPU Performance Counters”. Black Hat 2015. http://dreamsofastone.blogspot.de/2015/08/speaking-at-black-hat.html
 Daniel Gruss, Clémentine Maurice, Klaus Wagner, Stefan Mangard: “Flush+Flush: A Fast and Stealthy Cache Attack”
 Mathias Payer: “HexPADS: a platform to detect “stealth” attacks“. https://nebelwelt.net/publications/files/16ESSoS.pdf
 Zelalem Birhanu Aweke, Salessawi Ferede Yitbarek, Rui Qiao, Reetuparna Das, Matthew Hicks, Yossi Oren, Todd Austin:”ANVIL: Software-Based Protection Against Next-Generation Rowhammer Attacks”
 Anders Fogh: “Anvil & Next generation row hammer attacks”. http://dreamsofastone.blogspot.de/2016/03/anvil-next-generation-row-hammer-attacks.html
 Sarani Bhattacharya, Debdeep Mukhopadhyay: “Curious case of Rowhammer: Flipping Secret Exponent Bits using Timing Analysis”. http://eprint.iacr.org/2016/618.pdf
 Erik Bosman, Kaveh Razavi, Herbert Bos, Cristiano Giuffrida “Dedup Est Machina: Memory Deduplication as an Advanced Exploitation Vector”
 Yuan Xiao, Xiaokuan Zhang, Yinqian Zhang, and Mircea-Radu Teodorescu:”One Bit Flips, One Cloud Flops: Cross-VM Row Hammer Attacks and Privilege Escalation”. To be published
 Mark Lateigne: “A Tale of Two Hammers. A Brief Rowhammer Analysis of AMD vs. Intel”. http://www.thirdio.com/rowhammera1.pdf