Date: Tue, 5 Nov 2019 09:15:51 +0000 From: Mark Rutland <mark.rutland@....com> To: Sami Tolvanen <samitolvanen@...gle.com> Cc: Will Deacon <will@...nel.org>, Catalin Marinas <catalin.marinas@....com>, Steven Rostedt <rostedt@...dmis.org>, Masami Hiramatsu <mhiramat@...nel.org>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Dave Martin <Dave.Martin@....com>, Kees Cook <keescook@...omium.org>, Laura Abbott <labbott@...hat.com>, Marc Zyngier <maz@...nel.org>, Nick Desaulniers <ndesaulniers@...gle.com>, Jann Horn <jannh@...gle.com>, Miguel Ojeda <miguel.ojeda.sandonis@...il.com>, Masahiro Yamada <yamada.masahiro@...ionext.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>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH v4 11/17] arm64: disable function graph tracing with SCS On Mon, Nov 04, 2019 at 03:44:39PM -0800, Sami Tolvanen wrote: > On Mon, Nov 4, 2019 at 9:11 AM Mark Rutland <mark.rutland@....com> wrote: > > Can you please elaborate on _how_ this is incompatible in the commit > > message? > > > > For example, it's not clear to me if you mean that's functionally > > incompatible, or if you're trying to remove return-altering gadgets. > > > > If there's a functional incompatibility, please spell that out a bit > > more clearly. Likewise if this is about minimizing the set of places > > that can mess with control-flow outside of usual function conventions. > > Sure, I'll add a better description in v5. In this case, the return > address is modified in the kernel stack, which means the changes are > ignored with SCS. Ok, that makes sense to me. I'd suggest something like: | The graph tracer hooks returns by modifying frame records on the | (regular) stack, but with SCS the return address is taken from the | shadow stack, and the value in the frame record has no effect. As we | don't currently have a mechanism to determine the corresponding slot | on the shadow stack (and to pass this through the ftrace | infrastructure), for now let's disable the graph tracer when SCS is | enabled. ... as I suspect with some rework of the trampoline and common ftrace code we'd be able to correctly manipulate the shadow stack for this. Similarly, if clang gained -fpatchable-funciton-etnry, we'd get that for free. Thanks, Mark.
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