Date: Fri, 18 Oct 2019 10:56:04 -0700 From: Sami Tolvanen <samitolvanen@...gle.com> To: Jann Horn <jannh@...gle.com> Cc: Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Steven Rostedt <rostedt@...dmis.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Dave Martin <Dave.Martin@....com>, Kees Cook <keescook@...omium.org>, Laura Abbott <labbott@...hat.com>, Mark Rutland <mark.rutland@....com>, Nick Desaulniers <ndesaulniers@...gle.com>, clang-built-linux <clang-built-linux@...glegroups.com>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>, kernel list <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 06/18] add support for Clang's Shadow Call Stack (SCS) On Fri, Oct 18, 2019 at 10:42 AM Jann Horn <jannh@...gle.com> wrote: > (As I mentioned in the other thread, the security documentation there > doesn't fit the kernel usecase.) True. I'll add a note about it here too. > Without CONFIG_SHADOW_CALL_STACK_VMAP, after 128 small stack frames, > you overflow into random physmap memory even if the main stack is > vmapped... I guess you can't get around that without making the SCS > instrumentation more verbose. :/ That's correct. In our testing, 128 stack frames is nearly twice the maximum amount that's been used (on an arm64 device), and for many use cases, allocating a full page is simply too costly despite the advantages. > Could you maybe change things so that independent of whether you have > vmapped SCS or slab-allocated SCS, the scs_corrupted() check looks at > offset 1024-8 (where it currently is for the slab-allocated case)? > That way, code won't suddenly stop working when you disable > CONFIG_SHADOW_CALL_STACK_VMAP; and especially if you use > CONFIG_SHADOW_CALL_STACK_VMAP for development and testing but disable > it in production, that would be annoying. Yes, that's a great idea. I'll change this in v2. Sami
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