mm: init: report memory auto-initialization features at boot time

Print the currently enabled stack and heap initialization modes.

Stack initialization is enabled by a config flag, while heap
initialization is configured at boot time with defaults being set in the
config.  It's more convenient for the user to have all information about
these hardening measures in one place at boot, so the user can reason
about the expected behavior of the running system.

The possible options for stack are:
 - "all" for CONFIG_INIT_STACK_ALL;
 - "byref_all" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL;
 - "byref" for CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF;
 - "__user" for CONFIG_GCC_PLUGIN_STRUCTLEAK_USER;
 - "off" otherwise.

Depending on the values of init_on_alloc and init_on_free boottime options
we also report "heap alloc" and "heap free" as "on"/"off".

In the init_on_free mode initializing pages at boot time may take a while,
so print a notice about that as well.  This depends on how much memory is
installed, the memory bandwidth, etc.  On a relatively modern x86 system,
it takes about 0.75s/GB to wipe all memory:

  [    0.418722] mem auto-init: stack:byref_all, heap alloc:off, heap free:on
  [    0.419765] mem auto-init: clearing system memory may take some time...
  [   12.376605] Memory: 16408564K/16776672K available (14339K kernel code, 1397K rwdata, 3756K rodata, 1636K init, 11460K bss, 368108K reserved, 0K cma-reserved)

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Alexander Potapenko <[email protected]>
Suggested-by: Kees Cook <[email protected]>
Acked-by: Kees Cook <[email protected]>
Cc: Christoph Lameter <[email protected]>
Cc: Dmitry Vyukov <[email protected]>
Cc: James Morris <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Kostya Serebryany <[email protected]>
Cc: Laura Abbott <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Randy Dunlap <[email protected]>
Cc: Sandeep Patil <[email protected]>
Cc: "Serge E. Hallyn" <[email protected]>
Cc: Souptick Joarder <[email protected]>
Cc: Marco Elver <[email protected]>
Cc: Kaiwan N Billimoria <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

init/main.c

@@ -520,6 +520,29 @@ static inline void initcall_debug_enable(void)
 }
 #endif
 
+/* Report memory auto-initialization states for this boot. */
+static void __init report_meminit(void)
+{
+	const char *stack;
+
+	if (IS_ENABLED(CONFIG_INIT_STACK_ALL))
+		stack = "all";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF_ALL))
+		stack = "byref_all";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_BYREF))
+		stack = "byref";
+	else if (IS_ENABLED(CONFIG_GCC_PLUGIN_STRUCTLEAK_USER))
+		stack = "__user";
+	else
+		stack = "off";
+
+	pr_info("mem auto-init: stack:%s, heap alloc:%s, heap free:%s\n",
+		stack, want_init_on_alloc(GFP_KERNEL) ? "on" : "off",
+		want_init_on_free() ? "on" : "off");
+	if (want_init_on_free())
+		pr_info("mem auto-init: clearing system memory may take some time...\n");
+}
+
 /*
  * Set up kernel memory allocators
  */
@@ -530,6 +553,7 @@ static void __init mm_init(void)
 	 * bigger than MAX_ORDER unless SPARSEMEM.
 	 */
 	page_ext_init_flatmem();
+	report_meminit();
 	mem_init();
 	kmem_cache_init();
 	pgtable_init();