linux-kernel - Re: [RFC 6/8] KEYS: PGP data parser

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Message-ID: <Zc-s-42WoZhW_2c8@casper.infradead.org>
Date: Fri, 16 Feb 2024 18:44:11 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Roberto Sassu <roberto.sassu@...weicloud.com>
Cc: Petr Tesarik <petrtesarik@...weicloud.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Petr Tesařík <petr@...arici.cz>,
	Jonathan Corbet <corbet@....net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Andy Lutomirski <luto@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, Xin Li <xin3.li@...el.com>,
	Arnd Bergmann <arnd@...db.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Kees Cook <keescook@...omium.org>,
	"Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
	Pengfei Xu <pengfei.xu@...el.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>, Ze Gao <zegao2021@...il.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Kai Huang <kai.huang@...el.com>,
	David Woodhouse <dwmw@...zon.co.uk>,
	Brian Gerst <brgerst@...il.com>, Jason Gunthorpe <jgg@...pe.ca>,
	Joerg Roedel <jroedel@...e.de>,
	"Mike Rapoport (IBM)" <rppt@...nel.org>,
	Tina Zhang <tina.zhang@...el.com>,
	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>,
	David Howells <dhowells@...hat.com>,
	Petr Tesarik <petr.tesarik1@...wei-partners.com>
Subject: Re: [RFC 6/8] KEYS: PGP data parser

On Fri, Feb 16, 2024 at 05:53:01PM +0100, Roberto Sassu wrote:
> On Fri, 2024-02-16 at 16:44 +0000, Matthew Wilcox wrote:
> > On Fri, Feb 16, 2024 at 04:24:33PM +0100, Petr Tesarik wrote:
> > > From: David Howells <dhowells@...hat.com>
> > > 
> > > Implement a PGP data parser for the crypto key type to use when
> > > instantiating a key.
> > > 
> > > This parser attempts to parse the instantiation data as a PGP packet
> > > sequence (RFC 4880) and if it parses okay, attempts to extract a public-key
> > > algorithm key or subkey from it.
> > 
> > I don't understand why we want to do this in-kernel instead of in
> > userspace and then pass in the actual key.
> 
> Sigh, this is a long discussion.

Well, yes.  When you don't lay out why this is of value, it turns into a
long discussion.  This isn't fun for me either.

> PGP keys would be used as a system-wide trust anchor to verify RPM
> package headers, which already contain file digests that can be used as
> reference values for kernel-enforced integrity appraisal.

The one example we have of usage comes in patch 7 of this series and is:

gpg --dearmor < <PGP key> | keyctl padd asymmetric "" @u

And you're already using two userspace programs there.  Why not a third?

gpg --dearmor < <PGP key> | ./scripts/parse-pgp-packets | keyctl padd asymmetric "" @u

> With the assumptions that:
> 
> - In a locked-down system the kernel has more privileges than root
> - The kernel cannot offload this task to an user space process due to
>   insufficient isolation
> 
> the only available option is to do it in the kernel (that is what I got
> as suggestion).

This sounds like there's some other way of getting the key into the
kernel which doesn't rely on userspace.  Or are you assuming that nobody
bothered to trojan 'cat'?