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

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Date:   Tue, 20 Feb 2018 16:31:42 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Andy Lutomirski <luto@...nel.org>, Jann Horn <jannh@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Laura Abbott <labbott@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Sahara <keun-o.park@...kmatter.ae>,
        "Levin, Alexander (Sasha Levin)" <alexander.levin@...izon.com>,
        Michal Hocko <mhocko@...e.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH] fork: Allow stack to be wiped on fork

On Tue, 16 Jan 2018 21:50:15 -0800 Kees Cook <keescook@...omium.org> 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 ;)