Date: Fri, 18 Oct 2019 19:42:06 +0200 From: Jann Horn <jannh@...gle.com> To: Sami Tolvanen <samitolvanen@...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@...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 6:14 PM Sami Tolvanen <samitolvanen@...gle.com> wrote: > This change adds generic support for Clang's Shadow Call Stack, which > uses a shadow stack to protect return addresses from being overwritten > by an attacker. Details are available here: > > https://clang.llvm.org/docs/ShadowCallStack.html (As I mentioned in the other thread, the security documentation there doesn't fit the kernel usecase.) [...] > +config SHADOW_CALL_STACK_VMAP > + def_bool n > + depends on SHADOW_CALL_STACK > + help > + Use virtually mapped shadow call stacks. Selecting this option > + provides better stack exhaustion protection, but increases per-thread > + memory consumption as a full page is allocated for each shadow stack. 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. :/ 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.
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