Academics from Vrije University in Amsterdam and ETH Zurich have
published a new research paper describing yet another variation of
the Rowhammer attack.
Dubbed SMASH[1]
(Synchronized MAny-Sided Hammering), the technique can be used to
successfully trigger the attack from JavaScript on modern DDR4 RAM
cards, notwithstanding extensive mitigations that have been put in
place by manufacturers over the last seven years.
“Despite their in-DRAM Target Row Refresh (TRR) mitigations,
some of the most recent DDR4 modules are still vulnerable to
many-sided Rowhammer bit flips,” the researchers said.
“SMASH exploits high-level knowledge of cache replacement
policies to generate optimal access patterns for eviction-based
many-sided Rowhammer. To bypass the in-DRAM TRR mitigations, SMASH
carefully schedules cache hits and misses to successfully trigger
synchronized many-sided Rowhammer bit flips.”
By synchronizing memory requests with DRAM refresh commands, the
researchers developed an end-to-end JavaScript exploit which can
fully compromise the Firefox browser in 15 minutes on average,
proving that web users continue to remain at risk from such
attacks.
What is Rowhammer?
First, a quick primer about Rowhammer[2], an umbrella term
referring to a class of exploits that leverage a hardware design
quirk in DDR4 systems. Memory RAM cards save data inside what’s
called memory cells (each consisting of a capacitor and a
transistor) that are arranged on the RAM’s silicon chip in the form
of a matrix.
But given capacitors’ natural discharge rate, the memory cells
tend to lose their state over time and therefore require a periodic
reading and rewriting of each cell in order to restore the charge
on the capacitor to its original level. On the other hand,
increased densities of DRAM integrated circuits have enabled
increased rates of electromagnetic interactions between memory
cells and a greater possibility of data loss.
In 2014, researchers found that by repeatedly carrying out rapid
read/write operations on a memory row, over and over again — aka
“row hammering” — they could induce an electrical disturbance that
would alter data stored in nearby memory rows.
Since then, multiple methods have been devised, expanding on the
methods and exploitation scenarios of the original Rowhammer
research to bypass protections put in place (ECCploit[3]), launch attacks via
JavaScript (Rowhammer.js[4]), network packets
(Throwhammer[5]), and field-programmable
gate array (FPGA) cards (JackHammer[6]), and even read
sensitive memory data from other processes running on the same
hardware (RAMBleed[7])
In response to the findings, industry-wide countermeasures like
Target Row Refresh (TRR) were billed as the “ultimate solution” for
all the aforementioned Rowhammer attack versions until VU
researchers in March 2020 demonstrated a fuzzing tool called
“TRRespass[8]” that could be used to
make Rowhammer attacks work on the TRR-protected DDR4 cards.
From TRRespass to SMASH
While TRRespass aims to achieve a TRR bypass using native code,
no methods were available to trigger them in the browser from
JavaScript. That’s where SMASH comes in, granting the attacker an
arbitrary read and write primitive in the browser.
Specifically, the exploit chain is initiated when a victim
visits a malicious website under the adversary’s control or a
legitimate website that contains a malicious ad, taking advantage
of the Rowhammer bitflips triggered from within the JavaScript
sandbox to gain control over the victim’s browser.
“The current version of SMASH relies on [transparent huge pages]
for the construction of efficient self-evicting patterns,” the
researchers said. “Disabling THP, while introducing some
performance overhead, would stop the current instance of
SMASH.”
“Furthermore, our exploit relies specifically on corrupting
pointers in the browser to break ASLR and pivot to a counterfeit
object. Protecting the integrity of pointers in software or in
hardware (e.g., using PAC [23]) would stop the current SMASH
exploit.”