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

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Message-ID: <20220209050900.vb676ocdrt6gsomf@treble>
Date:   Tue, 8 Feb 2022 21:09:00 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Kees Cook <keescook@...omium.org>
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 Tue, Feb 08, 2022 at 03:43:27PM -0800, Kees Cook wrote:
> 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?

True, but:

- The kernel is already doing several other flavors of boot-time
  self-patching.  IMO it's better to consolidate such craziness in one
  place (or a limited number of places) if we can.  It seems more
  robust, and limits confusion about who's patching what and when.

- Patching text at link time has pitfalls and I'd like to avoid (as much
  as reasonably possible) objtool having the ability to brick the
  kernel.

-- 
Josh