It's not a firmware partition, it's an interface to the nvram variables provided by the firmware. There's a bunch of reasons why these should be writable - you need to be able to modify some of them to do things like reboot into the firmware settings UI, they're used for secure boot key management, you want to be able to choose which OS to boot into on the next reboot or which network interface to PXE off, that kind of thing.

As for why userspace couldn't remount it itself - it could. It doesn't. Changing the behaviour of systemd without changing the behaviour of the rest of userspace would result in userspace being broken, and making that kind of incompatible change is annoying - especially when fixing it in the kernel allows us to avoid that breakage.

> Can you explain this a bit? I'm not familiar with the particulars of firmware, but I'm having a hard time imagining why any userspace program would expect a firmware partition to be writable.

Best example of why you need to have it RW: if uefi is in fast boot, the only way to actually enter uefi is to boot an OS and have the OS set the uefi variables so that it goes into the firmware screen on next restart. This is (on Linux) done by changing something in that virtual filesystem.

> certainly I'd have a hard time believing that they would need it to be mounted and writable all the time.

You could probably remount it, but that also means that you all the sudden have concurrency problems with that operation. So not really ideal.

For this particular example, wouldn't this be best solved with a separate runlevel for "reboot into UEFI"? Stop all daemons, remount all normal filesystems read-only, remount UEFI RW, write variable and reboot.

Possibly, but what happens if a faulty script does a `rm` too much in that short time window?

The kernel fix prevents that, using mount flags alone only restrict the vulnerability but it doesn't make it go away.