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Re: [PATCH v3 2/2] provide -fstack-protector-strong build option

To: Kees Cook <keescook@chromium.org>
Subject: Re: [PATCH v3 2/2] provide -fstack-protector-strong build option
From: Ingo Molnar <mingo@kernel.org>
Date: Wed, 18 Dec 2013 10:59:05 +0100
Cc: linux-kernel@vger.kernel.org, Michal Marek <mmarek@suse.cz>, Russell King <linux@arm.linux.org.uk>, Ralf Baechle <ralf@linux-mips.org>, Paul Mundt <lethal@linux-sh.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Andrew Morton <akpm@linux-foundation.org>, James Hogan <james.hogan@imgtec.com>, Stephen Rothwell <sfr@canb.auug.org.au>, Shawn Guo <shawn.guo@linaro.org>, x86@kernel.org, linux-kbuild@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-sh@vger.kernel.org
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* Kees Cook <keescook@chromium.org> wrote:

> This changes the stack protector config option into a choice of "None",
> "Regular", and "Strong". For "Strong", the kernel is built with
> -fstack-protector-strong (gcc 4.9 and later). This options increases
> the coverage of the stack protector without the heavy performance hit
> of -fstack-protector-all.
> 
> For reference, the stack protector options available in gcc are:
> 
> -fstack-protector-all:
> Adds the stack-canary saving prefix and stack-canary checking suffix to
> _all_ function entry and exit. Results in substantial use of stack space
> for saving the canary for deep stack users (e.g. historically xfs), and
> measurable (though shockingly still low) performance hit due to all the
> saving/checking. Really not suitable for sane systems, and was entirely
> removed as an option from the kernel many years ago.
> 
> -fstack-protector:
> Adds the canary save/check to functions that define an 8
> (--param=ssp-buffer-size=N, N=8 by default) or more byte local char
> array. Traditionally, stack overflows happened with string-based
> manipulations, so this was a way to find those functions. Very few
> total functions actually get the canary; no measurable performance or
> size overhead.
> 
> -fstack-protector-strong
> Adds the canary for a wider set of functions, since it's not just those
> with strings that have ultimately been vulnerable to stack-busting. With
> this superset, more functions end up with a canary, but it still
> remains small compared to all functions with no measurable change in
> performance. Based on the original design document, a function gets the
> canary when it contains any of:
> - local variable's address used as part of the RHS of an assignment or
>   function argument
> - local variable is an array (or union containing an array), regardless
>   of array type or length
> - uses register local variables
> https://docs.google.com/a/google.com/document/d/1xXBH6rRZue4f296vGt9YQcuLVQHeE516stHwt8M9xyU
> 
> Comparison of "size" output when built with gcc-4.9 in three configurations:
> - defconfig
> - defconfig + CONFIG_CC_STACKPROTECTOR (+0.33%)
> - defconfig + CONFIG_CC_STACKPROTECTOR_STRONG via this patch (+2.24%)
> 
> text      data     bss      dec       hex     filename
> 11430641  1457584  1191936  14080161  d6d8a1  vmlinux
> 11468490  1457584  1191936  14118010  d76c7a  vmlinux.stackprotector
> 11692790  1457584  1191936  14342310  dad8a6  vmlinux.stackprotector-strong

Beyond the kernel size calculation, could you please also provide an 
estimation about the _number_ of functions affected, out of N kernel 
functions, so that the user has a rough picture about the scope and 
distribution of these variants?

I.e. something like:

                                                 # of canary checks
 
..................................................................................
 - defconfig                                     0 functions out of 100k 
functions
 - defconfig + STACKPROTECTOR                   1k functions out of 100k 
functions  
 - defconfig + STACKPROTECTOR_STRONG           20k functions out of 100k 
functions
        
Thanks,

        Ingo

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