docs: warn about limitations of landlock

[kmk3]

And mark it as experimental.

Relates to #6078.

[glitsj16]

@kmk3 Although the Firejail/Landlock status is made much clearer in this PR it might still be nice to note that without having landlock in the lsm=x,y,z kernel parameter (cfr. apparmor) Firejail will ignore landlock commands.

Example bootloader lsm param for good Firejail support: lsm=landlock,lockdown,yama,integrity,apparmor,bpf

[kmk3]

Although the Firejail/Landlock status is made much clearer in this PR it
might still be nice to note that without having landlock in the lsm=x,y,z
kernel parameter (cfr. apparmor) Firejail will ignore landlock commands.

Example bootloader lsm param for good Firejail support:
lsm=landlock,lockdown,yama,integrity,apparmor,bpf

Landlock should work by default without needing to enable it.

Is there any distribution that disables it?

Example to check if it works:

$ firejail --quiet --noprofile true
$ firejail --quiet --noprofile --landlock.enforce --landlock.fs.read=/foo true
Cannot start application: Permission denied

[glitsj16]

Is there any distribution that disables it?

Not that I know of. I wasn't clear enough though, let me try to explain what I mean. If a user enables AppArmor as default security model on every boot via the lsm kernel parameter and landlock isn't part of that param, it does get disabled.

[kmk3]

If a user enables AppArmor as default security model on every boot via the
lsm kernel parameter and landlock isn't part of that param, it does get
disabled.

I see it now; added an item about it.

[netblue30]

all set, thanks!