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

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Date:   Thu, 28 Oct 2021 10:12:32 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ard Biesheuvel <ardb@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        X86 ML <x86@...nel.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 Kernel Mailing List <linux-kernel@...r.kernel.org>,
        llvm@...ts.linux.dev
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI

On Thu, Oct 28, 2021 at 01:09:39PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 27, 2021 at 03:27:59PM -0700, Kees Cook wrote:
> 
> > Right -- though wouldn't just adding __ro_after_init do the same?
> > 
> > DEFINE_STATIC_CALL(static_call_name, func_a) __ro_after_init;
> 
> That breaks modules (and your jump_label patch doing the same is
> similarly broken).

Well that's no fun. :) I'd like to understand this better so I can fix
it!

> 
> When a module is loaded that uses the static_call(), it needs to
> register it's .static_call_sites range with the static_call_key which
> requires modifying it.

Reading static_call_add_module() leaves me with even more questions. ;)

It looks like module static calls need to write to kernel text? I don't
understand. Is this when a module is using an non-module key for a call
site? And in that case, this happens:

                key |= s_key & STATIC_CALL_SITE_FLAGS;

Where "key" is not in the module?

And the flags can be:

#define STATIC_CALL_SITE_TAIL 1UL       /* tail call */
#define STATIC_CALL_SITE_INIT 2UL       /* init section */

But aren't these per-site attributes? Why are they stored per-key?

                        if (!init && static_call_is_init(site))
                                continue;

			...

                        arch_static_call_transform(site_addr, NULL, func,
                                                   static_call_is_tail(site));


-- 
Kees Cook