linux-kernel - Re: [PATCH 0/2] Support userspace hypercalls for TDX

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Message-ID: <20240705160404.GA15452@prme-hs2-i1009>
Date: Fri, 5 Jul 2024 09:04:17 -0700
From: Tim Merrifield <tim.merrifield@...adcom.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
	Xin Li <xin3.li@...el.com>, Ard Biesheuvel <ardb@...nel.org>,
	Kai Huang <kai.huang@...el.com>,
	Kevin Loughlin <kevinloughlin@...gle.com>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Kees Cook <kees@...nel.org>, Mike Rapoport <rppt@...nel.org>,
	Brian Gerst <brgerst@...il.com>, linux-coco@...ts.linux.dev,
	linux-kernel@...r.kernel.org, Ajay Kaher <ajay.kaher@...adcom.com>,
	Alexey Makhalov <alexey.amakhalov@...adcom.com>,
	Broadcom internal kernel review list <bcm-kernel-feedback-list@...adcom.com>,
	virtualization@...ts.linux.dev, alex.james@...adcom.com,
	doug.covelli@...adcom.com, jeffrey.sheldon@...adcom.com
Subject: Re: [PATCH 0/2] Support userspace hypercalls for TDX


Thanks for the response, Dave.

On Wed, Jul 03, 2024 at 05:18:22PM -0700, Dave Hansen wrote:
> 
> Could we please be frank and transparent about what you actually want
> here and how you expect this mechanism to be used?
>

Sorry for being unclear. open-vm-tools is currently broken on TDX and
the intent here is to fix that. The idea is that versions of open-vm-tools
that have been audited and restricted to certain hypercalls, would execute
prctl to mark the process as capable of executing hypercalls.
    
> This inheritance model seems more suited to wrapping a tiny helper app
> around an existing binary, a la:
> 
> 	prctl(ARCH_SET_COCO_USER_HCALL);
> 	execve("/existing/binary/that/i/surely/did/not/audit", ...);
> 
> ... as opposed to something that you set in new versions of
> open-vm-tools after an extensive audit and a bug fixing campaign to
> clean up everything that the audit found.

I understand the concern about inheritance. I chose prctl primarily
because of some existing options that seemed similar, mainly speculation
control. Is there an alternative approach that doesn't suffer from the
inheritance issue?