Access Physical Memory

Motivation

Recently, while studying the interrupt vector table and side-channel attacks, I hit a wall when attempting to access Physical memory. This page will serve as a guide for accessing Physical memory.

Configure and Compile Linux Kernel

Follow the instructions outlined in this blog post to being this process.

The important configurations are CONFIG_STRICT_DEVMEM and CONFIG_IO_STRICT_DEVMEM. Set both to 'n' to remove restrictions and allow access to physical memory.

I will include the commands below to support persistence in the event previoulsy linked blog post is removed.

# list of steps

## Install dependencies
$ sudo apt install build-essential libncurses-dev bison flex libssl-dev libelf-dev fakeroot
$ sudo apt install dwarves

# Download target Kernel Version
$ wget https://cdn.kernel.org/pub/linux/kernel/{VERSION_MAJOR}/linux-{VERSION_MINOR}.tar.xz
$ tar -xf linux-{VERSION_MINOR}.tar.gz

# Relocated Configuration File
$ cd linux-{VERSION_MINOR}
$ cp -v /boot/config-$(uname -r) .config

# Filter out existing Modules
$ make localmodconfig

# Disable and Set Config Lines
$ scripts/config --disable SYSTEM_TRUSTED_KEYS
$ scripts/config --disable SYSTEM_REVOCATION_KEYS
$ scripts/config --set-str CONFIG_SYSTEM_TRUSTED_KEYS ""
$ scripts/config --set-str CONFIG_SYSTEM_REVOCATION_KEYS ""

# Remove Physical Memory restrictions
$ sed -ri 's/CONFIG_STRICT_DEVMEM=y/CONFIG_STRICT_DEVMEM=n/g' .config
$ sed -ri 's/CONFIG_IO_STRICT_DEVMEM=y/CONFIG_IO_STRICT_DEVMEM=n/g' .config

# The above configurations may not be present in the configuration file.  If not,
# Simply add the lines to the .config file yourself.

# Compile
$ fakeroot make

# Compile using multiple CPU Cores
## The fakeroot command prints error messages.  Without fakeroot, you must type
## echo $? after an error 2, to get more information.
$ fakeroot make -j$(nproc)

# Verify Successful Compile
$ echo $?

# Installation
$ sudo make modules_install
$ sudo make install
$ sudo reboot

# Final Verification
$ uname -rs