linux-kernel - Re: [PATCH v2 13/13] objtool: Validate kCFI calls

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Message-ID: <20250430190328.GO4439@noisy.programming.kicks-ass.net>
Date: Wed, 30 Apr 2025 21:03:28 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, kys@...rosoft.com, haiyangz@...rosoft.com,
	wei.liu@...nel.org, decui@...rosoft.com, tglx@...utronix.de,
	mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
	hpa@...or.com, seanjc@...gle.com, pbonzini@...hat.com,
	ardb@...nel.org, kees@...nel.org, Arnd Bergmann <arnd@...db.de>,
	gregkh@...uxfoundation.org, linux-hyperv@...r.kernel.org,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-efi@...r.kernel.org, samitolvanen@...gle.com,
	ojeda@...nel.org
Subject: Re: [PATCH v2 13/13] objtool: Validate kCFI calls

On Wed, Apr 30, 2025 at 08:59:53AM -0700, Josh Poimboeuf wrote:
> On Wed, Apr 30, 2025 at 01:07:47PM +0200, Peter Zijlstra wrote:
> > +	case ANNOTYPE_NOCFI:
> > +		sym = insn->sym;
> > +		if (!sym) {
> > +			ERROR_INSN(insn, "dodgy NOCFI annotation");
> > +			break;
> 
> return -1;

Oh right.

> > +	/*
> > +	 * 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.
> > +	 */
> > +	list_for_each_entry(insn, &file->retpoline_call_list, call_node) {
> > +		struct symbol *sym = insn->sym;
> > +
> > +		if (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!");
> > +				warnings++;
> 
> Do we not care about indirect calls from !STT_FUNC?

Let me try, see what happens.