Re: [PATCH] fork: Allow stack to be wiped on fork

[Andrew Morton]

On Tue, 16 Jan 2018 21:50:15 -0800 Kees Cook <keescook@xxxxxxxxxxxx> wrote:

> One of the classes of kernel stack content leaks is exposing the contents
> of prior heap or stack contents when a new process stack is allocated.
> Normally, those stacks are not zeroed, and the old contents remain in
> place. With some types of stack content exposure flaws, those contents
> can leak to userspace. Kernels built with CONFIG_CLEAR_STACK_FORK will
> no longer be vulnerable to this, as the stack will be wiped each time
> a stack is assigned to a new process. There's not a meaningful change
> in runtime performance; it almost looks like it provides a benefit.
> 
> Performing back-to-back kernel builds before:
> 	Run times: 157.86 157.09 158.90 160.94 160.80
> 	Mean: 159.12
> 	Std Dev: 1.54
> 
> With CONFIG_CLEAR_STACK_FORK=y:
> 	Run times: 159.31 157.34 156.71 158.15 160.81
> 	Mean: 158.46
> 	Std Dev: 1.46
> 
> ...
>
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -904,6 +904,14 @@ config VMAP_STACK
>  	  the stack to map directly to the KASAN shadow map using a formula
>  	  that is incorrect if the stack is in vmalloc space.
>  
> +config CLEAR_STACK_FORK
> +	bool "Clear the kernel stack at each fork"
> +	help
> +	  To resist stack content leak flaws, this clears newly allocated
> +	  kernel stacks to keep previously freed heap or stack contents
> +	  from being present in the new stack. This has almost no
> +	  measurable performance impact.
> +

It would be much nicer to be able to control this at runtime rather
than compile-time.  Why not a /proc tunable?  We could always use more
of those ;)