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Message-Id: <20180220163142.15366a82a7bece715b997597@linux-foundation.org>
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 ;)
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