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

Josh Poimboeuf
2022-02-09T05:27:22Z

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