perf/core: Fix unconditional security_locked_down() call

Currently, the lockdown state is queried unconditionally, even though
its result is used only if the PERF_SAMPLE_REGS_INTR bit is set in
attr.sample_type. While that doesn't matter in case of the Lockdown LSM,
it causes trouble with the SELinux's lockdown hook implementation.

SELinux implements the locked_down hook with a check whether the current
task's type has the corresponding "lockdown" class permission
("integrity" or "confidentiality") allowed in the policy. This means
that calling the hook when the access control decision would be ignored
generates a bogus permission check and audit record.

Fix this by checking sample_type first and only calling the hook when
its result would be honored.

Fixes: b0c8fdc7 ("lockdown: Lock down perf when in confidentiality mode")
Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Paul Moore <paul@paul-moore.com>
Link: https://lkml.kernel.org/r/20210224215628.192519-1-omosnace@redhat.com

kernel/events/core.c

@@ -11833,12 +11833,12 @@ SYSCALL_DEFINE5(perf_event_open,
 			return err;
 	}
 
-	err = security_locked_down(LOCKDOWN_PERF);
-	if (err && (attr.sample_type & PERF_SAMPLE_REGS_INTR))
-		/* REGS_INTR can leak data, lockdown must prevent this */
-		return err;
-
-	err = 0;
+	/* REGS_INTR can leak data, lockdown must prevent this */
+	if (attr.sample_type & PERF_SAMPLE_REGS_INTR) {
+		err = security_locked_down(LOCKDOWN_PERF);
+		if (err)
+			return err;
+	}
 
 	/*
 	 * In cgroup mode, the pid argument is used to pass the fd