Messages in this thread | | | Date | Mon, 25 Nov 2013 15:43:40 -0800 | Subject | Re: [PATCH] use -fstack-protector-strong | From | Kees Cook <> |
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On Mon, Nov 25, 2013 at 3:16 PM, H. Peter Anvin <hpa@zytor.com> wrote: > On 11/25/2013 02:14 PM, Kees Cook wrote: >> Build the kernel with -fstack-protector-strong when it is available >> (gcc 4.9 and later). This increases the coverage of the stack protector >> without the heavy performance hit of -fstack-protector-all. > > What is the difference between the various options?
-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
Chrome OS has been using -fstack-protector-strong for its kernel builds for the last 8 months with no problems.
-Kees
-- Kees Cook Chrome OS Security
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