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

[Greg Kroah-Hartman]

On Tue, Dec 14, 2021 at 05:20:48PM +0100, Alexander Potapenko wrote:
> 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@xxxxxxxxxx>
> ---
> 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

So this means KMSAN is a developer debugging feature only and should
never be turned on on a real device/server that has users?

thanks,

greg k-h