linux-hardening - Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI

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Date:   Mon, 16 May 2022 22:37:23 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Joao Moreira <joao@...rdrivepizza.com>,
        Sedat Dilek <sedat.dilek@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-hardening@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, llvm@...ts.linux.dev
Subject: Re: [RFC PATCH v2 20/21] x86: Add support for CONFIG_CFI_CLANG

On Mon, May 16, 2022 at 12:39:19PM -0700, Sami Tolvanen wrote:

> > > With the current compiler patch, LLVM sets up function arguments after
> > > the CFI check. if it's a problem, we can look into changing that.
> >
> > Yes, please fix that. Again see that same patch for why this is a
> > problem. Objtool can trivially find retpoline calls, but finding this
> > kCFI gadget is going to be hard work. If you ensure they're
> > unconditionally stuck together, then the problem goes away find one,
> > finds the other.
> 
> You can use .kcfi_traps to locate the check right now, but I agree,
> it's not quite ideal.

Oohh, indeed. Looking at that, I think .kcfi_traps would be better as
relative offsets; eg. 'addr = (void*)s + *s' like. Halfs the amount of
storage needed for it.

Also, that code can use a few {} extra.