linux-kernel - Re: [PATCH v2] use -fstack-protector-strong

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Date:	Tue, 17 Dec 2013 12:29:31 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Kees Cook <keescook@...omium.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	Thomas Gleixner <tglx@...utronix.de>,
	"x86@...nel.org" <x86@...nel.org>,
	Shawn Guo <shawn.guo@...aro.org>,
	Olof Johansson <olofj@...omium.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2] use -fstack-protector-strong


* Kees Cook <keescook@...omium.org> wrote:

> On Wed, Nov 27, 2013 at 10:11 AM, Kees Cook <keescook@...omium.org> wrote:
> > On Wed, Nov 27, 2013 at 9:55 AM, H. Peter Anvin <hpa@...or.com> wrote:
> >> On 11/27/2013 09:54 AM, Ingo Molnar wrote:
> >>>>
> >>>> Looks to be 2% for defconfig. That's way better. Shall I send a v3?
> >>>
> >>> Well, it's better than 9%, but still almost an order of magnitude
> >>> higher than the cost is today, and a lot of distros have
> >>> CONFIG_CC_STACKPROTECTOR=y.
> >>>
> >>> So it would be nice to measure how much the instruction count goes up
> >>> in some realistic system-bound test. How much does something like
> >>> kernel/built-in.o increase, as per 'size' output?
> >
> >    text    data     bss     dec     hex filename
> >  929611   90851  594496 1614958  18a46e built-in.o-gcc-4.9
> >  954648   90851  594496 1639995  19063b built-in.o-gcc-4.9+strong
> >
> > Looks like 3% for defconfg + CONFIG_CC_STACKPROTECTOR
> >
> >>
> >> Do we need CONFIG_CC_STACKPROTECTOR_STRONG?
> >
> > I'm hoping to avoid this since nearly anyone using 
> > CC_STACKPROTECTOR would want strong added, but as a fallback, I'm 
> > happy to implement it as a separate config item.
> 
> Any verdict on this? Should I go with adding ..._STRONG like we used 
> to have for ..._ALL, or is defaulting to -strong best?

I'm not opposed to the feature itself, just to the specific structure 
you presented - as outlined in my review feedback.

The cost of the feature itself appears to be significant (this cost 
should be outlined in the help text btw), while I think the cost of 
adding this as a new _STRONG option is minimal.

So I'd go forward with addressing two issues:

1)

I'd add the new STACKPROTECTOR_STRONG option and maybe rename the old 
one to STACKPROTECTOR_WEAK.

If in a year or two most distros have switched over to the _STRONG 
variant, despite its costs, then we can drop the weak variant.

2)

It would also be nice to see a head to head comparison of the 3 
variants:

	!STACKPROTECTOR
	STACKPROTECTOR_LIGHT
	STACKPROTECTOR_STRONG

of defconfig vmlinux size and estimated number of checks inserted in 
each case - so people/distros can make an informed decision about the 
relative quality differences between these variants and whether they 
want to carry the costs of that.

Thanks,

	Ingo
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