So you are safe, because you updated your bash, run your policy in enforcing mode, enabled NX and ASLR and boot using a cryptographically signed shim bootloader.
Well, you actually failed by the first step.
Here is why:
Even if thats hard to believe, but there might be
remotely exploitable buffer overflows in your secure
bootloader. The 90's party of today just happen
downstairs. Bootloaders make use of UEFI network stacks
to implement PXE and PXEv6 and sometimes fail to do so:
The issue however is not so severe because a lot of UEFI
firmwares fail to verify what they get via PXEv6 anyway,so
delivering an overflow payload is overkill. :)
Thats actually why you see 'schlimm' debug output in
secure mode in the PoC demo screenshot at all.
There were some twists necesarry to actually achieve *ptr = value
as seen above, mainly due to protocol specifics. If you are interested, we can discuss this privately.