linux-kernel - Re: [PATCH] objtool: Detect _

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Message-ID: <x5ltxw6z45gty6hylw2flde5w7qnmztg5mzaq7ffh4tl5ssm6r@h2xnxzqvnljb>
Date: Thu, 10 Apr 2025 12:09:02 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Paweł Anikiel <panikiel@...gle.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Kees Cook <kees@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, 
	Ingo Molnar <mingo@...hat.com>, Masahiro Yamada <masahiroy@...nel.org>, 
	Miguel Ojeda <ojeda@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Alice Ryhl <aliceryhl@...gle.com>, Nathan Chancellor <nathan@...nel.org>, x86@...nel.org, 
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] objtool: Detect __nocfi calls

On Thu, Apr 10, 2025 at 05:45:56PM +0200, Peter Zijlstra wrote:
> On Thu, Apr 10, 2025 at 03:25:22PM +0200, Peter Zijlstra wrote:
> 
> > I should get objtool to warn about those. They undermine the point of
> > CFI.
> 
> ---
> Subject: objtool: Detect __nocfi calls

"Warn on indirect calls in __nocfi functions" ?

>  static int add_retpoline_call(struct objtool_file *file, struct instruction *insn)
>  {
> +	struct symbol *sym = insn->sym;
> +
> +	/*
> +	 * kCFI call sites look like:
> +	 *
> +	 *     movl $(-0x12345678), %r10d
> +	 *     addl -4(%r11), %r10d
> +	 *     jz 1f
> +	 *     ud2
> +	 *  1: cs call __x86_indirect_thunk_r11
> +	 *
> +	 * Verify all indirect calls are kCFI adorned by checking for the UD2.
> +	 * Notably, doing __nocfi calls to regular (cfi) functions is broken.
> +	 */
> +	if (opts.cfi && sym && sym->type == STT_FUNC && !sym->nocfi) {
> +		struct instruction *prev = prev_insn_same_sym(file, insn);
> +		if (!prev || prev->type != INSN_BUG)
> +			WARN_INSN(insn, "no-cfi indirect call!");

Since this can break things pretty badly at runtime, this should
actually fail the build on CONFIG_OBJTOOL_WERROR.

The warning counts aren't plumbed in this early, so can this check be
done later?  validate_retpoline() or validate_call()?

-- 
Josh