Date: Thu, 10 Aug 2017 10:32:22 -0700 From: Kees Cook <keescook@...omium.org> To: Matt Redfearn <matt.redfearn@...tec.com> Cc: Ralf Baechle <ralf@...ux-mips.org>, Linux MIPS Mailing List <linux-mips@...ux-mips.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>, LKML <linux-kernel@...r.kernel.org>, James Hogan <james.hogan@...tec.com>, Paul Burton <paul.burton@...tec.com>, Josh Poimboeuf <jpoimboe@...hat.com> Subject: Re: [PATCH] MIPS: usercopy: Implement stack frame object validation On Thu, Aug 10, 2017 at 1:24 AM, Matt Redfearn <matt.redfearn@...tec.com> 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@...tec.com> >> 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
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