HammerKit

Background

HammerKit is an open-source library for inducing and characterizing rowhammer that provides out-of-the-box support for Chrome OS platforms.

The goal behind this project is to provide a platform to share our findings with the wider community, and provide another tool to the mix, so that others have starting points for their own investigations. We feel that democratization of row hammer research is essential for creating an environment where the problem will be taken seriously by the various parties.

What is Row Hammer? Why should I care?

The Wikipedia article for Row Hammer is a good starting point. To summarize: DRAM is composed of banks, which are themselves composed of rows. When a row is repeatedly accessed, bits located in its neighbouring rows may change their values. As you can imagine, the ability to flip bits that you don't have direct access to has potential security implications. Row hammer exploitation has already been demonstrated from Javascript. Since the initial discovery of row hammer, the DRAM vendors have made some efforts to mitigate row hammer. These efforts have proven unsuccessful, as demonstrated by the TRResspass work. The result is that bits can still be flipped via Row Hammer, but more sophisticated access patterns are required. As the DRAM moves to smaller and smaller process nodes, the underlying Row Hammer phenomenon becomes worse, and the mitigation trade offs become harder. This is a very pressing issue for computer security, going forward.

In early 2020, the TRRespass work was produced as a result of a large collaboration between VU, Qualcomm and ETH Zurich. This once again highlighted that Row Hammer is not fixed. This present work is greatly inspired by the TRRespass work.

Compiling

The Makefile that is provided is intended for the Chrome OS environment. A good starting point for Chrome OS development is the Developer's Guide. After checking out the Chrome OS source tree, make a directory under src and place the contents of this package there. After that, make sure that you enter the Chrome OS build chroot and setup the appropriate board. Then, in the directory where this package is kept, run the following command:

  
  
           cros_sdk --working-dir . -- make CC=aarch64-cros-linux-gnu-clang

assuming you are compiling for an ARM64 platform. If the chosen board is x86_64, you can do this instead:

  
  

          cros_sdk --working-dir . -- make CC=x86_64-cros-linux-gnu-clang

Measure Tool

Usage: measure config step_size length

config signifies a config file, which contains several entries, one per line. Each entry has is a name/value pair separated by '='.

measure provides timing data for a range of memory. The idea is to pick a reference memory address, let's say start and establish the time it takes when a loop, such as the following, is executed for each candidate address:

  
  

        for (i = 0; i < count; i++) {
          access *start*
          access *candidate*
        }

The candidate addresses are chosen using step_size, length and offset0 specified in the config file.

candidate address is start + offset0 + i * step_size. The timing data is based on the number of iterations equal to the value of measure_loops in the config, which defaults to 250000. The theory is that any candidate address that is in the same bank as start is going to take a longer time to access, due to bank conflict. By analyzing the resulting data, one can arrive at the conflict_th_us value, which is used for row finding in the hammer tool. offset0 is needed to avoid starting in the same row as start.

Hammer Tool

Usage: hammer config

The sections that follow will discuss the details of operation of the hammer tool.

Basic Row Finder

If either alt_row_find is unspecified or 0 in hammer config, hammer will resort to the basic row finder. It will sample every find_step bytes, and in a similar manner to the measure tool, measure how long it takes to alternatingly access that address and the start measure_loops times. If the time exceed conflict threshold, then it has found a new row.

Optionally, max_fuzz and fuzz_step parameters can be used to introduce a fudge factor into this method. For each find_step, it will try increments of fuzz_step until max_fuzz. As soon as it finds a row, it will simply go to the next find_step increment. This fudging feature is considered deprecated. The preferred way is to use the alternative row finder, which is more flexible (but slower).

Alternative Row Finder

Alternatively, alt_row_find can be set to 1.

Rather than expecting to find entire rows bounded by some step size, the alternative row finding method allows the use of a smaller unit called atomic_unit to compose the rows. The definition of an atomic_unit is the largest naturally aligned region that must belong to exactly one row. Additionally, row_size specifies the number of atomic_unit in a row. The atomic row finder will find atomic units until it has enough of them to compose a row. Then, it will assemble a row and then move onto finding atomic units for the next one.

Additionally, this method permits finding rows in multiple banks simultaneously. The rows within the same bank will be contiguously numbered. The parameters used for this are

• nr_banks : number of banks
• bank_find_step : step size used for bank-finding heuristic
• rows_per_set : number of rows in each bank before we round robin to the next bank.

The bank finder runs before the core code of the alternative row finder, and finds the specified number of banks. The rows are then found in those banks. The algorithm for bank-finding is by exclusion from known banks. The first bank can be represented by an arbitrary address. Subsequent banks are found by taking a distant address and proving that it is not found in any of the existing banks, by showing that the measurement with respect to addresses in those banks always fall below conflict_th_us.

What follows are the various other configuration options that can be placed in a hammer config.

contig={0|1} and cached={0|1}

The android ION driver allows allocation of contiguous uncached memory. The contiguity is useful for locating rows and the uncached memory is useful for hammering without explicit flushes. Depending on the platform, the flushes might have an associated cost. If contig=0 and cached=1, a normal system allocation will be made and then possibly linearized. See pagemap={0|1}.

pagemap={0|1}

Setting pagemap=1 allows using /proc//pagemap to convert virtual addresses to physical addresses. This allows sorting rows (see sort_rows below). Additionally, it is generally useful to be able to inspect the physical addresses of bit flips. pagemap requires a small kernel patch to work with memory from the ION driver. When using contig=0, pagemap=1 allows the linearlization of a non-contiguous allocation. This involves using mremap to reorganize the allocation so that it is clumped into physically contiguous regions, with the largest region first.

size=sz[K|M|G]

Specifies the size of the allocation.

offset0=off[K|M|G]

Specifies the distance between the refrence row and the first address to be tested. This should be large enough to guarantee that the two addresses are not on the same row.

cpu=cpu

Specifies the CPU to pin to. Can be used multiple times for multithreaded mode, which isn't particularly well-tested or encouraged.

sched_fifo=priority

Specified that sched_fifo should be used with the given priority.

n_rows=rows

n_tries=tries

For each set of rows, run the specified number of tries.

min_aggrs=min and max_aggrs=max

Specifies the minimum and maximum number of aggressors generated by the pattern. The individual pattern will decide the placement of those aggressors.

hammer_loops=iterations

Total number of hammer iterations, distributed across all aggressors. For example, if 10M is specified and there are 4 aggressors, each will get 2.5M.

sort_rows={0|1}

Sorts the rows for certain known row ordering schemes in DRAM banks. Without this, the order in which rows are numbered in hammer does not match the adjacency order in the DRAM chip. sort_rows_shift parameter specifies the lowest order row bit.

fill_mode={random|mod}

When fill_mode is random, the rows are filled with random data prior to hammering and verification.

When fill_mode is mod, a modular pattern is used for filling rows. The parameter mod specifies the divisor for the modulus. The parameter victim_mask specifies the mask of the victims in this modulus. For every bit k set in victim_mask, all rows k mod mod are considered victim rows. Victim rows are filled with victim_data_pattern. All other rows are filled with its inverse. By default, the modular pattern is tried with all mod alignments: with the outer loop being the alignment. This behaviour can be disabled by setting mod_stride to 0. If mod_stride is zero, the modular pattern will only be run naturally aligned.

check_rest={0|1}

Enabling check_rest allows covering uncertainty about row finding. It checks the entire memory area instead of just the rows in the current set.

always_refill={0|1}

Normally, we only refill if we detect bit flips. However, some patterns are intentionally designed (for performance or experimental reasons) to not detect all bit flips, but to focus on designated victim rows. For the code to work properly with these patterns, always_refill=1 must be specified.

repeat_flips=count

When a random pattern creates a bit flip, it is often useful to know if the flip is repeatable. repeat_flips allows using exactly the same random numbers to repeat the experiment count times, only if a flip happens in the original attempt.

step=step:count

Introduces an outer loop to hammering. The loop increments by step until it reaches count. The address is simultaneously incremented by step. When multiple step parameters are specified, they represent loops starting with the outermost and proceeding inwards.

pattern=even

Even numbered aggressors.

pattern=random

A uniformly random row is selected from n_rows for each aggressor in each trial.

pattern=trrespass_assisted_double

Assisted double as defined in the trrespass paper. The parameter assisted_double_dist determines the distance from the end of the block of rows to the assistant row.