kexec: Disable at runtime if the kernel enforces module loading restrictions

BugLink: http://bugs.launchpad.net/bugs/1566221

kexec permits the loading and execution of arbitrary code in ring 0, which
is something that module signing enforcement is meant to prevent. It makes
sense to disable kexec in this situation.
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

kernel/kexec.c

@@ -17,6 +17,7 @@
 #include <linux/syscalls.h>
 #include <linux/vmalloc.h>
 #include <linux/slab.h>
+#include <linux/module.h>
 
 #include "kexec_internal.h"
 
@@ -131,7 +132,7 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
 	int result;
 
 	/* We only trust the superuser with rebooting the system. */
-	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
+	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled || secure_modules())
 		return -EPERM;
 
 	/*