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

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Message-ID: <8BD39200-583E-4835-BA0B-90409E0E55C4@infradead.org>
Date: Wed, 19 Mar 2025 13:04:20 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Josh Poimboeuf <jpoimboe@...nel.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 18 March 2025 22:41:43 GMT, Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>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?

Not sure of the details. The machine_kexec() function which is the *caller* is currently marked with the __nocfi tag which stops any software checks. I guess any hardware feature which requires an endbr to be the target of an indirect branch has to already disabled on the way down? What specifically am I looking for, to check that? Or the hardware support has just never worked with kexec, perhaps?

>> > > — 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?

In an executable page, yes.