On Thu, Aug 10, 2017 at 1:24 AM, Matt Redfearn <matt.redfearn@xxxxxxxxxx> wrote: > Hi Kees, > > > On 08/08/17 20:11, Kees Cook wrote: >> >> On Tue, Aug 8, 2017 at 5:23 AM, Matt Redfearn <matt.redfearn@xxxxxxxxxx> >> wrote: >>> >>> This implements arch_within_stack_frames() for MIPS that validates if an >>> object is wholly contained by a kernel stack frame. >>> >>> With CONFIG_HARDENED_USERCOPY enabled, MIPS now passes the LKDTM tests >>> USERCOPY_STACK_FRAME_TO, USERCOPY_STACK_FRAME_FROM and >>> USERCOPY_STACK_BEYOND on a Creator Ci40. >>> >>> Since the MIPS kernel does not use frame pointers, we re-use the MIPS >>> kernels stack frame unwinder which uses instruction inspection to deduce >>> the stack frame size. As such it introduces a larger performance penalty >>> than on arches which use the frame pointer. >> >> Hmm, given x86's plans to drop the frame pointer, I wonder if the >> inter-frame checking code should be gated by a CONFIG. This (3%) is a >> rather high performance hit to take for a relatively small protection >> (it's mainly about catching too-large-reads, since most >> too-large-writes will be caught by the stack canary). >> >> What do you think? > > > If x86 is going to move to a more expensive stack unwinding method than the > frame pointer then I guess it may end up seeing a similar performance hit to > what we see on MIPS. In that case it might make sense to add a CONFIG for > this such that only those who wish to make the trade off of performance for > the added protection need enable it. Sounds good. Can you send a v2 that adds a CONFIG, maybe something like CONFIG_HARDENED_USERCOPY_UNWINDER with a description of the trade-offs? Then x86 can do this too when it drops frame pointers. Thanks! -Kees -- Kees Cook Pixel Security
