[bug report] [regression?] bpf lsm breaks /proc/*/attr/current with security= on commandline

[Filip Hejsek <filip.hejsek@gmail.com>]

Hello,

TLDR: because of bpf lsm, putting security=selinux on commandline
      results in /proc/*/attr/current returning errors.

When the legacy security= commandline option is used, the specified lsm
is added to the end of the lsm list. For example, security=apparmor
results in the following order of security modules:

   capability,landlock,lockdown,yama,bpf,apparmor

In particular, the bpf lsm will be ordered before the chosen major lsm.

This causes reads and writes of /proc/*/attr/current to fail, because
the bpf hook overrides the apparmor/selinux hook.

As you can see in the code below, only the first registered hook is
called (when reading attr/current, lsmid is 0):

int security_getprocattr(struct task_struct *p, int lsmid, const char *name,
			 char **value)
{
	struct lsm_static_call *scall;

	lsm_for_each_hook(scall, getprocattr) {
		if (lsmid != 0 && lsmid != scall->hl->lsmid->id)
			continue;
		return scall->hl->hook.getprocattr(p, name, value);
	}
	return LSM_RET_DEFAULT(getprocattr);
}

Even though the bpf lsm doesn't allow attaching bpf programs to this
hook, it still prevents the other hooks from being called.

This is maybe a regression, because with the same commandline, reading
from /proc/*/attr/current probably worked before the introduction of
bpf lsm.

Regards,
Filip Hejsek