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

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Date:   Wed, 9 Feb 2022 12:41:41 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, 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?

I have patches that write a 4 byte #UD there instead of a nop. That
would make !IBT hardware splat as well when it hits a sealed function
(and in that case actually having those extra ENDBR generated is a
bonus).

Anyway, I have some newer patches and some hardware, except it's a NUC
and working with those things is a royal pain in the arse since they
don't have serial. I finally did get XHCI debug port working, but
there's no XDBC grub support, so now I managed to boot a dead kernel and
the thing is a brick until I can be arsed to connect a keybaord and
screen to it again :-(

KVM/qemu has no IBT support merged yet, so I can't use that either.