[PATCH] use -fstack-protector-strong

[Kees Cook]

On Mon, Nov 25, 2013 at 3:16 PM, H. Peter Anvin <hpa at 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