Re: [PATCH 41/43] security: kmsan: fix interoperability with auto-initialization

Heap and stack initialization is great, but not when we are trying
uses of uninitialized memory. When the kernel is built with KMSAN,
having kernel memory initialization enabled may introduce false
negatives.

We disable CONFIG_INIT_STACK_ALL_PATTERN and CONFIG_INIT_STACK_ALL_ZERO
under CONFIG_KMSAN, making it impossible to auto-initialize stack
variables in KMSAN builds. We also disable CONFIG_INIT_ON_ALLOC_DEFAULT_ON
and CONFIG_INIT_ON_FREE_DEFAULT_ON to prevent accidental use of heap
auto-initialization.

We however still let the users enable heap auto-initialization at
boot-time (by setting init_on_alloc=1 or init_on_free=1), in which case
a warning is printed.

Signed-off-by: Alexander Potapenko <glider@google.com>
---
Link: https://linux-review.googlesource.com/id/I86608dd867018683a14ae1870f1928ad925f42e9
---
 mm/page_alloc.c            | 4 ++++
 security/Kconfig.hardening | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index fa8029b714a81..4218dea0c76a2 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c

@@ -855,6 +855,10 @@ void init_mem_debugging_and_hardening(void)
 	else
 		static_branch_disable(&init_on_free);
 
+	if (IS_ENABLED(CONFIG_KMSAN) &&
+	    (_init_on_alloc_enabled_early || _init_on_free_enabled_early))
+		pr_info("mem auto-init: please make sure init_on_alloc and init_on_free are disabled when running KMSAN\n");
+
 #ifdef CONFIG_DEBUG_PAGEALLOC
 	if (!debug_pagealloc_enabled())
 		return;

diff --git a/security/Kconfig.hardening b/security/Kconfig.hardening
index d051f8ceefddd..bd13a46024457 100644
--- a/security/Kconfig.hardening
+++ b/security/Kconfig.hardening

@@ -106,6 +106,7 @@ choice
 	config INIT_STACK_ALL_PATTERN
 		bool "pattern-init everything (strongest)"
 		depends on CC_HAS_AUTO_VAR_INIT_PATTERN
+		depends on !KMSAN
 		help
 		  Initializes everything on the stack (including padding)
 		  with a specific debug value. This is intended to eliminate

@@ -124,6 +125,7 @@ choice
 	config INIT_STACK_ALL_ZERO
 		bool "zero-init everything (strongest and safest)"
 		depends on CC_HAS_AUTO_VAR_INIT_ZERO
+		depends on !KMSAN