mm/init: Add 'rodata=off' boot cmdline parameter to disable read-only kernel mappings

It may be useful to debug writes to the readonly sections of memory,
so provide a cmdline "rodata=off" to allow for this. This can be
expanded in the future to support "log" and "write" modes, but that
will need to be architecture-specific.

This also makes KDB software breakpoints more usable, as read-only
mappings can now be disabled on any kernel.

Suggested-by: H. Peter Anvin <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: David Brown <[e-mail]>
Cc: Denys Vlasenko <[e-mail]>
Cc: Emese Revfy <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Mathias Krause <[e-mail]>
Cc: Michael Ellerman <[e-mail]>
Cc: PaX Team <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Cc: [e-mail]
Cc: linux-arch <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Ingo Molnar <[e-mail]>

Documentation/kernel-parameters.txt

···
 
 	ro		[KNL] Mount root device read-only on boot
 
+	rodata=		[KNL]
+		on	Mark read-only kernel memory as read-only (default).
+		off	Leave read-only kernel memory writable for debugging.
+
 	root=		[KNL] Root filesystem
 			See name_to_dev_t comment in init/do_mounts.c.
 

init/main.c

···
 extern void init_IRQ(void);
 extern void fork_init(void);
 extern void radix_tree_init(void);
-#ifndef CONFIG_DEBUG_RODATA
-static inline void mark_rodata_ro(void) { }
-#endif
 
 /*
  * Debug helper: via this flag we know that we are in 'early bootup code'
···
 
 static noinline void __init kernel_init_freeable(void);
 
+#ifdef CONFIG_DEBUG_RODATA
+static bool rodata_enabled = true;
+static int __init set_debug_rodata(char *str)
+{
+	return strtobool(str, &rodata_enabled);
+}
+__setup("rodata=", set_debug_rodata);
+
+static void mark_readonly(void)
+{
+	if (rodata_enabled)
+		mark_rodata_ro();
+	else
+		pr_info("Kernel memory protection disabled.\n");
+}
+#else
+static inline void mark_readonly(void)
+{
+	pr_warn("This architecture does not have kernel memory protection.\n");
+}
+#endif
+
 static int __ref kernel_init(void *unused)
 {
 	int ret;
···
 	/* need to finish all async __init code before freeing the memory */
 	async_synchronize_full();
 	free_initmem();
-	mark_rodata_ro();
+	mark_readonly();
 	system_state = SYSTEM_RUNNING;
 	numa_default_policy();
 

kernel/debug/kdb/kdb_bp.c

···
 	} else {
 		kdb_printf("%s: failed to set breakpoint at 0x%lx\n",
 			   __func__, bp->bp_addr);
-#ifdef CONFIG_DEBUG_RODATA
 		if (!bp->bp_type) {
 			kdb_printf("Software breakpoints are unavailable.\n"
-				   "  Change the kernel CONFIG_DEBUG_RODATA=n\n"
+				   "  Boot the kernel with rodata=off\n"
 				   "  OR use hw breaks: help bph\n");
 		}
-#endif
 		return 1;
 	}
 	return 0;