* 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" and "objdump" output when built with gcc-4.9 in > three configurations: > - defconfig > 11430641 text size > 36110 function bodies > - defconfig + CONFIG_CC_STACKPROTECTOR > 11468490 text size (+0.33%) > 1015 of 36110 functions stack-protected (2.81%) > - defconfig + CONFIG_CC_STACKPROTECTOR_STRONG via this patch > 11692790 text size (+2.24%) > 7401 of 36110 functions stack-protected (20.5%) Ok, these patches now look pretty good to me. One final detail is that I think the information about the percentage of functions affected should be propagated into the help text: > +config CC_STACKPROTECTOR_REGULAR > + bool "Regular" > + select CC_STACKPROTECTOR > + help > + Functions will have the stack-protector canary logic added if they > + have an 8-byte or larger character array on the stack. > + > This feature requires gcc version 4.2 or above, or a distribution > gcc with the feature backported. > > + On an x86 "defconfig" build, this increases the kernel text by 0.3%. > + > +config CC_STACKPROTECTOR_STRONG > + bool "Strong" > + select CC_STACKPROTECTOR > + help > + Functions will have the stack-protector canary logic added in any > + of the following conditions: > + - 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 > + > + This feature requires gcc version 4.9 or above, or a distribution > + gcc with the feature backported. > + > + On an x86 "defconfig" build, this increases the kernel text by 2%. It should say something like: On an x86 "defconfig" build, this feature adds canary checks to about 3% of all kernel functions, which increases kernel code size by about 0.3%. and for the _STRONG option: On an x86 "defconfig" build, this feature adds canary checks to ~20% of all kernel functions, which increases kernel code size by ~2%. this way distibutions and users can make an informed decision about the level of checks they want to employ. Thanks, Ingo
