linux-hardening - Re: [PATCH v5 00/15] x86: Add support for Clang CFI

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Date:   Wed, 27 Oct 2021 14:17:30 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "x86@...nel.org" <x86@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "llvm@...ts.linux.dev" <llvm@...ts.linux.dev>,
        "ardb@...nel.org" <ardb@...nel.org>
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Wed, Oct 27, 2021 at 12:55:17PM +0000, David Laight wrote:
> From: Mark Rutland
> > Sent: 27 October 2021 13:05
> ...
> > Taking a step back, it'd be nicer if we didn't have the jump-table shim
> > at all, and had some SW landing pad (e.g. a NOP with some magic bytes)
> > in the callees that the caller could check for. Then function pointers
> > would remain callable in call cases, and we could explcitly add landing
> > pads to asm to protect those. I *think* that's what the grsecurity folk
> > do, but I could be mistaken.
> 
> It doesn't need to be a 'landing pad'.
> The 'magic value' could be at 'label - 8'.

Sure; I'd intended to mean the general case of something at some fixed
offset from the entrypoint, either before or after, potentially but not
necessarily inline in the executed instruction stream.

Mark.