stackprotector: Introduce CONFIG_CC_STACKPROTECTOR_STRONG

This changes the stack protector config option into a choice of
"None", "Regular", and "Strong":

   CONFIG_CC_STACKPROTECTOR_NONE
   CONFIG_CC_STACKPROTECTOR_REGULAR
   CONFIG_CC_STACKPROTECTOR_STRONG

"Regular" means the old CONFIG_CC_STACKPROTECTOR=y option.

"Strong" is a new mode introduced by this patch. With "Strong" the
kernel is built with -fstack-protector-strong (available in
gcc 4.9 and later). This option 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 only a small 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 right hand side
      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



Find below a comparison of "size" and "objdump" output when built with
gcc-4.9 in three configurations:

  - defconfig
	11430641 kernel text size
	36110 function bodies

  - defconfig + CONFIG_CC_STACKPROTECTOR_REGULAR
	11468490 kernel text size (+0.33%)
	1015 of 36110 functions are stack-protected (2.81%)

  - defconfig + CONFIG_CC_STACKPROTECTOR_STRONG via this patch
	11692790 kernel text size (+2.24%)
	7401 of 36110 functions are stack-protected (20.5%)

With -strong, ARM's compressed boot code now triggers stack
protection, so a static guard was added. Since this is only used
during decompression and was never used before, the exposure
here is very small. Once it switches to the full kernel, the
stack guard is back to normal.

Chrome OS has been using -fstack-protector-strong for its kernel
builds for the last 8 months with no problems.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Paul Mundt <lethal@linux-sh.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Shawn Guo <shawn.guo@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-mips@linux-mips.org
Cc: linux-arch@vger.kernel.org
Link: http://lkml.kernel.org/r/1387481759-14535-3-git-send-email-keescook@chromium.org


[ Improved the changelog and descriptions some more. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>