linux-kernel - Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type information to relocate

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Message-ID: <orpk67p2yolcb4gi462wqwphaaio6ivny2gcfcw4jna3uyaak5@oty7arlxxykl>
Date: Tue, 18 Mar 2025 15:41:43 -0700
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: David Woodhouse <dwmw2@...radead.org>
Cc: kexec@...ts.infradead.org, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>, 
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, Kai Huang <kai.huang@...el.com>, 
	Nikolay Borisov <nik.borisov@...e.com>, linux-kernel@...r.kernel.org, Simon Horman <horms@...nel.org>, 
	Dave Young <dyoung@...hat.com>, Peter Zijlstra <peterz@...radead.org>, bsz@...zon.de
Subject: Re: [PATCH v7 8/8] [DO NOT MERGE] x86/kexec: Add CFI type
 information to relocate_kernel()

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