0xadb/lsprobe

A toy permissive Linux security module for security ops research.

lsprobe

A toy permissive Linux security module for security ops research.

Building

Prerequisites

• About 23 GB of the hard drive space (about 2.5 GB for sources plus 20 GB to build packages)
• 2-3 hours on 2 x86 cores with 4Gb RAM (see Notes)

Dependencies

• Ubuntu 16.04:

sudo apt-get install git build-essential kernel-package fakeroot libncurses5-dev libssl-dev 

• Debian 9

apt-get install git build-essential fakeroot libncurses5-dev libssl-dev libelf-dev bison flex

Sources

Get kernel sources and put module sourcess into security folder:

git clone git://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git
cd ./linux-stable
git checkout -b stable v5.0
cd ./security/
git clone https://github.com/0xADB/lsprobe.git
cd ..

Integrate lsprobe to build process:

• Add to security/Kconfig:

source "security/integrity/Kconfig"
source "security/lsprobe/Kconfig" # <-- this line

• Add to security/Makefile:

subdir-$(CONFIG_SECURITY_YAMA)         += yama
subdir-$(CONFIG_SECURITY_LSPROBE)      += lsprobe # <--this line
...
obj-$(CONFIG_SECURITY_YAMA)            += yama/
obj-$(CONFIG_SECURITY_LSPROBE)         += lsprobe/ # <-- and this line

Integrate lsprobe to boot process:

• add to include/linux/lsm_hooks.h

#ifdef CONFIG_SECURITY_YAMA
extern void __init yama_add_hooks(void);
#else
static inline void __init yama_add_hooks(void) { }
#endif
#ifdef CONFIG_SECURITY_LSPROBE			      // these lines
extern void __init lsprobe_add_hooks(void);	      //
#else						      //
static inline void __init lsprobe_add_hooks(void) { } //
#endif						      //

• insert into security_init function in security/security.c:

yama_add_hooks();
lsprobe_add_hooks(); // this line

Configuration

Go to the linux-stable directory and clone current config and adapt it choosing default answers:

cp /boot/config-$(uname -r) .config
yes '' | make oldconfig

Customize it by

make menuconfig

selecting in the Security options directory the Security probe entry.

Building package

In the linux-stable directory:

rm vmlinux-gdb.py # see Notes
make -j $(getconf _NPROCESSORS_ONLN) deb-pkg LOCALVERSION="-lsprobed"

On success the .deb package will be in the directory above:

ls ../*.deb

Building just kernel

In the linux-stable directory:

make -j $(getconf _NPROCESSORS_ONLN) LOCALVERSION="-lsprobed"
sudo make modules_install install
sudo update-grub2

Notes

• Before building dep-pkg target remove vmlinux-gdb.py symbolic link in linux-stable if any or dpkg-source will complain:

dpkg-source: error: cannot represent change to vmlinux-gdb.py:
dpkg-source: error:   new version is symlink to /mnt/shared_workspace/src/linux-stable/scripts/gdb/vmlinux-gdb.py
dpkg-source: error:   old version is nonexistent
...
dpkg-source: error: unrepresentable changes to source
dpkg-buildpackage: error: dpkg-source -i.git -b linux-stable gave error exit status 1
scripts/package/Makefile:70: recipe for target 'deb-pkg' failed
make[1]: *** [deb-pkg] Error 1
Makefile:1390: recipe for target 'deb-pkg' failed
make: *** [deb-pkg] Error 2

• Kernel building takes considerable time - on a debian 9 kvm guest with 2 x86-cores (and a browser playing music in a background):

$ time make -j $(getconf _NPROCESSORS_ONLN) LOCALVERSION="-lsprobed"
real	96m52.259s
user	137m26.076s
sys	13m59.624s
$ time make -j $(getconf _NPROCESSORS_ONLN) deb-pkg LOCALVERSION="-lsprobed"
real	130m55.841s
user	178m5.568s
sys	15m15.480s

Also note the hard drive space occupied after the build:

$ du -hs ./*
8.0K	./linux-5.0.0-lsprobed_5.0.0-lsprobed-1_amd64.buildinfo
4.0K	./linux-5.0.0-lsprobed_5.0.0-lsprobed-1_amd64.changes
632K	./linux-5.0.0-lsprobed_5.0.0-lsprobed-1.diff.gz
4.0K	./linux-5.0.0-lsprobed_5.0.0-lsprobed-1.dsc
162M	./linux-5.0.0-lsprobed_5.0.0-lsprobed.orig.tar.gz
11M	./linux-headers-5.0.0-lsprobed_5.0.0-lsprobed-1_amd64.deb
43M	./linux-image-5.0.0-lsprobed_5.0.0-lsprobed-1_amd64.deb
552M	./linux-image-5.0.0-lsprobed-dbg_5.0.0-lsprobed-1_amd64.deb
1012K	./linux-libc-dev_5.0.0-lsprobed-1_amd64.deb
21G	./linux-stable

References

• https://blog.ptsecurity.com/2012/09/writing-linux-security-module.html
• https://www.maketecheasier.com/build-custom-kernel-ubuntu/
• https://elixir.bootlin.com
• http://dazuko.dnsalias.org
• https://kernelnewbies.org/KernelBuild
• https://thibaut.sautereau.fr/2017/05/26/linux-security-modules-part-1
• https://thibaut.sautereau.fr/2017/05/26/linux-security-modules-part-2