* Kees Cook <keescook@xxxxxxxxxxxx> 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
