Making Lektor work with grsecurity

Posted on Sat 29 October 2016 in Tech

I started using grsecurity on my servers in 2015, and there's always a bit of tuning required.

I was recently playing with Lektor (before I swapped to Pelican), and I had a bit of trouble with my grsec kernel. In particular, Lektor and Pelican are both run within a virtualenv Python environment, and grsec eats it like popcorn in two different ways:

1) TPE (Trusted Path Execution) throws a wobbly:

[253241.370019] grsec: From {ssh-origin-ip}: denied untrusted exec (due to file in world-writable directory) of /tmp/#50 by /usr/local/lib/lektor/bin/lektor[lektor:60593] uid/euid:1000/1000 gid/egid:1000/1000, parent /bin/bash[bash:60581] uid/euid:1000/1000 gid/egid:1000/1000

This block occurs because the virtualenv violates grsecurity's Trusted Path Execution protection - the Python executable isn't under a trusted path, and the user isn't in the TPE-bypass group.

To resolve this one, add your user to the TPE group. This will allow the user to execute binaries which aren't in trusted locations:

sudo usermod -aG grsec-tpe $USER

2) RWX protection. This is a common problem with Python-based apps, even though Python has an exception by default.

[ 207.534876] grsec: From {ssh-source-ip}: denied RWX mmap of <anonymous mapping> by /usr/local/lib/lektor/bin/lektor[lektor:534] uid/euid:1000/1000 gid/egid:1000/1000, parent /bin/bash[bash:477] uid/euid:1000/1000 gid/egid:1000/1000

First, create your PaX ELF headers, and then disable grsec's MEMPROTECT extension on the included python2 binary, while you enable EMUTRAMP:

sudo paxctl /usr/local/lib/lektor/bin/python2 -c
sudo paxctl /usr/local/lib/lektor/bin/python2 -Em

Result!