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Date: Tue, 27 Feb 2018 11:28:53 -0800
From: Kees Cook <>
To: P J P <>
Cc: Kernel Hardening <>, 
	Florian Weimer <>, P J P <>
Subject: Re: [PATCH 0/1] Zero initialise kernel stack variables

On Tue, Feb 27, 2018 at 3:15 AM, P J P <> wrote:
> Hello,


> Please see:
>   ->
> This experimental patch by Florian Weimer(CC'd) adds an option
> '-finit-local-vars' to gcc(1) compiler. When a program(or kernel)
> is built using this option, its automatic(local) variables are
> initialised with zero(0). This could significantly reduce the kernel
> information leakage issues.
> A dnf(8) repository of the latest gcc-7.3.1 package built with the above
> patch and kernel-4.15.5 package built using '-finit-local-vars' option
> on Fedora-27 is available below
>   ->
> This same kernel is running on my F27 test machine as I write this.
> There is no slowness or notice-able performance impact as such.

Unfortunately "noticeable" isn't going to be a viable metric. You'll
need to do some real-world benchmarks (i.e. kernel builds, hackbench,
etc), and compare the results. Even just initializing
passed-by-reference variables (GCC_PLUGIN_STRUCTLEAK_BYREF_ALL) had
measurable performance impact.

It would be nice to have four options/features available from the
compiler, from least to most performance impact:

- initialize padding to zero when static initializers are used (this
would make  foo = { .field = something }; identical to memset(&foo, 0,
sizeof(foo)); foo.field = something for all structures, but now, any
structures with padding _must_ use the latter to be safe, which is
highly error-prone).

- initialize all uninitialized variables that contain a structure
marked with a special attribute (e.g.
__attribute__((force_initialize)) ).

- initialize all uninitialized variables that are passed by reference

- initialize all uninitialized variables (-finit-local-vars seems to do this)

> The patch here adds a kbuild menu option to enable/disable '-finit-local-vars'
> compiler flag while building the Linux kernel.

Since this is a single patch, I think it'd be better to fold this
entire cover letter into patch 1.

> I'd appreciate your review and/or inputs to test this option further.

Thanks for sending this!


Kees Cook
Pixel Security

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