linux-kernel - Re: [PATCH v2 00/21] Runtime TDX Module update support

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Message-ID: <CAGtprH8-UGFkh4NmuY1ETPYmg7Uk+bm24Er2PPxf8tUOSR_byQ@mail.gmail.com>
Date: Sat, 25 Oct 2025 04:55:46 -0700
From: Vishal Annapurve <vannapurve@...gle.com>
To: dan.j.williams@...el.com
Cc: Dave Hansen <dave.hansen@...el.com>, Chao Gao <chao.gao@...el.com>, 
	"Reshetova, Elena" <elena.reshetova@...el.com>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, 
	"Chatre, Reinette" <reinette.chatre@...el.com>, "Weiny, Ira" <ira.weiny@...el.com>, 
	"Huang, Kai" <kai.huang@...el.com>, "yilun.xu@...ux.intel.com" <yilun.xu@...ux.intel.com>, 
	"sagis@...gle.com" <sagis@...gle.com>, "paulmck@...nel.org" <paulmck@...nel.org>, 
	"nik.borisov@...e.com" <nik.borisov@...e.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, "Kirill A. Shutemov" <kas@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, 
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 00/21] Runtime TDX Module update support

On Fri, Oct 24, 2025 at 6:42 PM <dan.j.williams@...el.com> wrote:
>
> Vishal Annapurve wrote:
> > On Fri, Oct 24, 2025 at 2:19 PM Dave Hansen <dave.hansen@...el.com> wrote:
> > >
> > > On 10/24/25 14:12, dan.j.williams@...el.com wrote:
> > > >> The SGX solution, btw, was to at least ensure forward progress (CPUSVN
> > > >> update) when the last enclave goes away. So new enclaves aren't
> > > >> *prevented* from starting but the window when the first one starts
> > > >> (enclave count going from 0->1) is leveraged to do the update.
> > > > The status quo does ensure forward progress. The TD does get built and
> > > > the update does complete, just the small matter of TD attestation
> > > > failures, right?
> >
> > I would think that it's not a "small" problem if confidential
> > workloads on the hosts are not able to pass attestation.
>
> "Small" as in "not the kernel's problem". Userspace asked for the
> update, update is documented to clobber build sometimes, userspace ran
> an update anyway. Userspace asked for the clobber.
>
> It would be lovely if this clobbering does not happen at all and the
> update mechanism did not come with this misfeature. Otherwise, the kernel
> has no interface to solve that problem. The best it can do is document
> that this new update facility has this side effect.

In this case, host kernel has a way to ensure that userspace can't
trigger such clobbering at all. That IIUC is "Avoid updates during
update sensitive times". Best kernel can do is prevent userspace from
screwing up the state of TDs.

>
> Userspace always has the choice to not update, coordinate update with
> build, or do nothing and let tenants try to launch again.  Userspace
> could even retry the build and hide the tenant failure if it knew about
> the clobber,

IIUC host userspace has no way to know if the TD state got clobbered.

> but be clear that the problem is the clobber not the kernel
> doing what userspace asked.
>
> The clobber, as I understand, is also limited to cases where the update
> includes crypto library changes. I am not sure how often that happens in
> practice. Suffice to say, the fact that the clobber is conditioned on
> the contents of the update also puts it further away from being a kernel

The knowledge of things getting clobbered are well much further away
from userspace.

> problem. The clobber does not corrupt kernel state.
>
> > > Oh, yeah, for sure.
> > >
> > > If we do _nothing_ in the kernel (no build vs. module update
> > > synchronization), then the downside is being exposed to attestation
> > > failures if userspace either also does nothing or has bugs.
> > >
> > > That's actually, by far, my preferred solution to this whole mess:
> > > Userspace plays stupid games, userspace wins stupid prizes.
> > >
> >
> > IIUC, enforcing "Avoid updates during update sensitive times" is not
> > that complex and will ensure to avoid any issues with user space
> > logic.
>
> Userspace logic avoids issues by honoring the documentation that these
> ABIs sequences need synchronization. Otherwise, kernel blocking update
> during build just trades one error for another.

Kernel blocking update during build makes the production systems much
safer and prevents userspace from screwing up the state that it has no
way to detect after the fact.

>
> Treat this like any other userspace solution for requiring "atomic"
> semantics when the kernel mechanisms are not themselves designed to be
> atomic, wrap it in userspace synchronization.

In general if this is something userspace detectable I would agree,
TDX module is the closest entity that can detect the problematic
sequence and the host kernel has a very simple way to ensure that such
a problematic sequence is not at all allowed to happen by toggling
some seamcall controls. It would be very helpful IMO to ensure that
userspace is not able to screw up production workloads especially if
the mess is not all visible to userspace.