linux-kernel - Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

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Date:   Tue, 8 Feb 2022 15:43:27 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, samitolvanen@...gle.com
Subject: Re: [RFC][PATCH 6/6] objtool: Add IBT validation / fixups

On Wed, Nov 24, 2021 at 11:30:37AM -0800, Josh Poimboeuf wrote:
> On Mon, Nov 22, 2021 at 06:03:07PM +0100, Peter Zijlstra wrote:
> > +static int validate_ibt_reloc(struct objtool_file *file, struct reloc *reloc, char **name)
> > +{
> > +	struct instruction *dest;
> > +	struct section *sec;
> > +	unsigned long off;
> > +
> > +	sec = reloc->sym->sec;
> > +	off = reloc->sym->offset + reloc->addend;
> > +
> > +	dest = find_insn(file, sec, off);
> > +	if (!dest)
> > +		return 0;
> > +
> > +	if (name && dest->func)
> > +		*name = dest->func->name;
> 
> I think these checks can be further narrowed down by only looking for
> X86_64_64 relocs.
> 
> > +	list_for_each_entry(insn, &file->endbr_list, call_node) {
> > +		if (ibt_seal) {
> > +			elf_write_insn(file->elf, insn->sec,
> > +				       insn->offset, insn->len,
> > +				       arch_nop_insn(insn->len));
> > +		}
> 
> Like the retpoline rewriting, I'd much rather have objtool create
> annotations which the kernel can then read and patch itself.
> 
> e.g. have '.ibt.direct_call_sites' and '.ibt.unused_endbr_insns'
> sections.

Why have the kernel do that work at every boot when it can be known at
link time?

-- 
Kees Cook