SELinux is a security enhancement to Linux which allows users and administrators more control over access control.
Access can be constrained on such variables as which users and applications can access which resources. These resources may take the form of files. Standard Linux access controls, such as file modes (-rwxr-xr-x) are modifiable by the user and the applications which the user runs. Conversely, SELinux access controls are determined by a policy loaded on the system which may not be changed by careless users or misbehaving applications.
SELinux also adds finer granularity to access controls. Instead of only being able to specify who can read, write or execute a file, for example, SELinux lets you specify who can unlink, append only, move a file and so on. SELinux allows you to specify access to many resources other than files as well, such as network resources and interprocess communication (IPC).
So you think its easy writing a selinux policy? It actually is - I mean .. technically.
There are tools which auto generate policies from a log but are they good to use? It depends. and here it stops being easy as you, the integrator have to have a deep understanding what the IMPACT of each policy has.
To be able to go on you should really go through this first:
- SELinux FAQ
- https://www.billauer.co.il/selinux-policy-module-howto.html
- https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/9/html-single/using_selinux/index
- and really make use of your favorite search engine!
as it helps a lot to understand how it works. and why is that important? because only if you know how it works you can make a decision if an auto generated policy is doing any harm or not.
On Ubuntu you can install audit2allow like this: apt install policycoreutils-python-utils
the process usally is:
- fetching the denials:
- using Linux:
adb logcat -b all |grep avc: > /tmp/denies - using Windows:
adb shell "logcat -b all |grep avc: > /sdcard/Download/denies"+adb pull /sdcard/Download/denies
- using Linux:
- creating the current policy in your android sources dir
source build/envsetup.sh+lunch <device>as usualmka libselinux sepolicy
- autogen policies:
cat /tmp/denies |audit2allow -p out/target/product/<device-codename>/root/sepolicy - check the output + add the policies to their according filename (ensure you don't open security holes)
- start to build the sepolicies (
mka sepolicy) - fix neverallows, etc occurring during the build
- if all is fixed build the full OS as usual
Step 6 in particular will have you tearing your hair out.. and that's why it is so important to understand the whole thing or learn by doing - keep in mind, however, that workarounds or too wide policies open holes that negate the security benefits of SELinux.
!!! You do NOT need to solve ALL denials you find !!!
Denials in your log are not necessarily something you should fix, really. The rule of thumb is that you fix denials ONLY when they fix a problem - and if you are sure (enough) it is worth to write an allow (i.e. no breaking the security).