“Rowhammer” Flaw in DRAM Allows Privilege Escalation: Researchers

Researchers at Google’s Project Zero have demonstrated that a recently documented bug in some dynamic random-access memory (DRAM) chips can be exploited to gain kernel privileges on Linux systems.

The flaw, dubbed “rowhammer,” has been known since at least 2012. However, the first time anyone discussed its security implications was in June 2014, when researchers at Carnegie Mellon University and Intel Labs published an experimental study on DRAM disturbance errors. Now, Google researchers have shown that the vulnerability is exploitable on Linux and possibly other systems for privilege escalation.

Memory cells are arranged in a grid pattern of rows and columns. DRAM chips have become smaller in size, but they have a larger capacity, which has been accomplished by making the cells smaller and placing them closer together.

The problem, according to experts, is that it has become more difficult to prevent cells from electrically interacting with each other. Researchers have demonstrated that repeatedly accessing a row of memory can cause data to become corrupt in nearby rows.

Project Zero has developed two proof-of-concept exploits that leverage this technique, known as bit flips. The first exploit is designed to escalate privileges and escape Native Client (NaCl), a sandboxing system designed for executing compiled code in the browser efficiently and securely.

Google has addressed the issue in Chrome, but researchers said it could have been possible for a NaCl app in the Chrome Web Store to gain kernel privileges by chaining this flaw with a kernel privilege escalation vulnerability.

The second exploit developed by Google researchers runs as an unprivileged x86-64 process on Linux and uses rowhammer-induced bit flips to gain kernel privileges. Researchers believe the exploit could be adapted for other operating systems as well.

“We don’t know for sure how many machines are vulnerable to this attack, or how many existing vulnerable machines are fixable,” Google software engineer Mark Seaborn said in a blog post. “Our exploit uses the x86 CLFLUSH instruction to generate many accesses to the underlying DRAM, but other techniques might work on non-x86 systems too.”

Experts have created a rowhammer testing application (for Linux and Mac OS X), which they have used to analyze a total of 29 x86 laptops with DDR3 DRAM. Roughly half of them exhibited rowhammer-induced bit flips, but the absence of bit flips in this test doesn’t necessarily prove that the devices are not vulnerable, researchers said.

Some DRAM vendors have started implementing rowhammer mitigations in their products, and Google noticed that some of the newer laptop models did not exhibit bit flips. Mitigations are present in LPDDR4 and the attack doesn’t appear to work against ECC (error-correcting code) memory.

ECC memory is capable of correcting single bit flips, but according to Errata Security’s Robert Graham, successful attacks might still be possible if the attacker can get multiple bit flips.

“By itself, this bug may not endanger you. However, it’s much more dangerous when used within conjunction with other bugs. Browsers have ‘sand boxes’ that keep hackers contained even if the first layer of defense breaks. This may provide a way of escaping the sand box (sandbox escape) when used in conjunction with other exploits,” Graham wrote in a blog post.

Google researchers believe that while hardware bugs are less common than software bugs, hardware vendors should not neglect the security impact of flaws that appear to be reliability issues.

“We have shown two ways in which the DRAM rowhammer problem can be exploited to escalate privileges. History has shown that issues that are thought to be ‘only’ reliability issues often have significant security implications, and the rowhammer problem is a good example of this. Many layers of software security rest on the assumption the contents of memory locations don’t change unless the locations are written to,” Seaborn noted.

Cisco has already started investigating some potentially vulnerable products, but the company has pointed out that they contain a number of hardware protections against rowhammer attacks, including ECC memory modules.