Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type information to relocate_kernel()

[Josh Poimboeuf]

On Tue, Mar 18, 2025 at 09:06:58PM +0000, David Woodhouse wrote:
> On Tue, 2025-03-18 at 10:14 -0700, Josh Poimboeuf wrote:
> > On Tue, Mar 18, 2025 at 03:56:36PM +0000, David Woodhouse wrote:
> > > For the relocate_kernel() case I don't think we care much about the
> > > first. Without a CFI prologue, no *other* code can be tricked into
> > > calling relocate_kernel()
> > 
> > But for FineIBT the hash is checked on the callee side.  So it loses
> > FineIBT protection.
> 
> Right now the relocate_kernel() code doesn't even have an endbr, does
> it? So it isn't a useful gadget?

In that case wouldn't IBT explode when you indirect call it?  Or is IBT
getting disabled beforehand?

> > > — and besides, it's in the kernel's data
> > > section and isn't executable anyway until the kexec code copies it to a
> > > page that *is*.
> > 
> > Does the code get copied immediately before getting called, or can it be
> > initialized earlier during boot when kdump does its initial setup?
> 
> It's initialized earlier, in machine_kexec_prepare(), and then the page
> is set ROX.

If that happens during boot (like for kdump init) then it'll be in text
the whole time after boot, right?

-- 
Josh