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Date:   Wed, 27 Oct 2021 15:04:55 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Ard Biesheuvel <ardb@kernel.org>
Cc:     Mark Rutland <mark.rutland@arm.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        X86 ML <x86@kernel.org>, Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Sedat Dilek <sedat.dilek@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        linux-hardening@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        llvm@lists.linux.dev
Subject: Re: [PATCH v5 00/15] x86: Add support for Clang CFI
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On Wed, Oct 27, 2021 at 02:48:52PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 27, 2021 at 02:22:27PM +0200, Ard Biesheuvel wrote:
> > On Wed, 27 Oct 2021 at 14:05, Mark Rutland <mark.rutland@arm.com> wrote:
> 
> > > > Should not this jump-table thingy get converted to an actual function
> > > > address somewhere around arch_static_call_transform() ? This also seems
> > > > relevant for arm64 (which already has CLANG_CFI supported) given:
> > > >
> > > >   https://lkml.kernel.org/r/20211025122102.46089-3-frederic@kernel.org
> > >
> > > Ugh, yeah, we'll need to do the function_nocfi() dance somewhere...
> > >
> > 
> > Sadly, that only works on symbol names, so we cannot use it to strip
> > CFI-ness from void *func arguments passed into the static call API,
> > unfortunately.
> 
> Right, and while mostly static_call_update() is used, whcih is a macro
> and could possibly be used to wrap this, we very much rely on
> __static_call_update() also working without that wrapper and then we're
> up a creek without no paddles.

Specifically, we support code like:

struct foo {
	void (*func1)(args1);
	void (*func2)(args2);
}

struct foo global_foo;

...

DEFINE_STATIC_CALL_NULL(func1, *global_foo.func1);

...

__init foo_init()
{
	// whatever code that fills out foo

	static_call_update(func1, global_foo.func1);
}

...

hot_function()
{
	...
	static_cal(func1)(args1);
	...
}

cold_function()
{
	...
	global_foo->func1(args1);
	...
}

And there is no way I can see this working sanely with CFI as presented.

Even though the above uses static_call_update(), we can't no longer use
function_nocfi() on the @func argument, because it's not a symbol, it's
already a function pointer.

Nor can we fill global_foo.func1 with function_nocfi() because then
cold_function() goes sideways.