kexec: Disable at runtime if securelevel has been set.

kexec permits the loading and execution of arbitrary code in ring 0, which
permits the modification of the running kernel. Prevent this if securelevel
has been set.

Signed-off-by: Matthew Garrett <mjg59@srcf.ucam.org>

kernel/kexec.c

@@ -17,6 +17,7 @@
 #include <linux/syscalls.h>
 #include <linux/vmalloc.h>
 #include <linux/slab.h>
+#include <linux/security.h>
 
 #include "kexec_internal.h"
 
@@ -134,6 +135,9 @@ SYSCALL_DEFINE4(kexec_load, unsigned long, entry, unsigned long, nr_segments,
 	if (!capable(CAP_SYS_BOOT) || kexec_load_disabled)
 		return -EPERM;
 
+	if (get_securelevel() > 0)
+		return -EPERM;
+
 	/*
 	 * Verify we have a legal set of flags
 	 * This leaves us room for future extensions.