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Date: Mon, 8 Jan 2024 19:13:24 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>, "mingo@...hat.com"
	<mingo@...hat.com>, "bp@...en8.de" <bp@...en8.de>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "x86@...nel.org"
	<x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
	"haiyangz@...rosoft.com" <haiyangz@...rosoft.com>, "wei.liu@...nel.org"
	<wei.liu@...nel.org>, "decui@...rosoft.com" <decui@...rosoft.com>,
	"luto@...nel.org" <luto@...nel.org>, "peterz@...radead.org"
	<peterz@...radead.org>, "akpm@...ux-foundation.org"
	<akpm@...ux-foundation.org>, "urezki@...il.com" <urezki@...il.com>,
	"hch@...radead.org" <hch@...radead.org>, "lstoakes@...il.com"
	<lstoakes@...il.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>,
	"ardb@...nel.org" <ardb@...nel.org>, "jroedel@...e.de" <jroedel@...e.de>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "rick.p.edgecombe@...el.com"
	<rick.p.edgecombe@...el.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "linux-coco@...ts.linux.dev"
	<linux-coco@...ts.linux.dev>, "linux-hyperv@...r.kernel.org"
	<linux-hyperv@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: RE: [PATCH v3 3/3] x86/hyperv: Make encrypted/decrypted changes safe
 for load_unaligned_zeropad()

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
Sent: Monday, January 8, 2024 10:37 AM
> 
> On 1/5/2024 10:30 AM, mhkelley58@...il.com wrote:
> > From: Michael Kelley <mhklinux@...look.com>
> >
> > In a CoCo VM, when transitioning memory from encrypted to decrypted, or
> > vice versa, the caller of set_memory_encrypted() or set_memory_decrypted()
> > is responsible for ensuring the memory isn't in use and isn't referenced
> > while the transition is in progress.  The transition has multiple steps,
> > and the memory is in an inconsistent state until all steps are complete.
> > A reference while the state is inconsistent could result in an exception
> > that can't be cleanly fixed up.
> >
> > However, the kernel load_unaligned_zeropad() mechanism could cause a stray
> > reference that can't be prevented by the caller of set_memory_encrypted()
> > or set_memory_decrypted(), so there's specific code to handle this case.
> > But a CoCo VM running on Hyper-V may be configured to run with a paravisor,
> > with the #VC or #VE exception routed to the paravisor. There's no
> > architectural way to forward the exceptions back to the guest kernel, and
> > in such a case, the load_unaligned_zeropad() specific code doesn't work.
> >
> > To avoid this problem, mark pages as "not present" while a transition
> > is in progress. If load_unaligned_zeropad() causes a stray reference, a
> > normal page fault is generated instead of #VC or #VE, and the
> > page-fault-based fixup handlers for load_unaligned_zeropad() resolve the
> > reference. When the encrypted/decrypted transition is complete, mark the
> > pages as "present" again.
> 
> Change looks good to me. But I am wondering why are adding it part of
> prepare and finish callbacks instead of directly in set_memory_encrypted() function.
> 

The prepare/finish callbacks are different for TDX, SEV-SNP, and
Hyper-V CoCo guests running with a paravisor -- so there are three sets
of callbacks.  As described in the cover letter, I've given up on using this
scheme for the TDX and SEV-SNP cases, because of the difficulty with
the SEV-SNP callbacks needing a valid virtual address (whereas TDX and
Hyper-V paravisor need only a physical address).  So it seems like the
callbacks specific to the Hyper-V paravisor are the natural place for the
code.  That leaves the TDX and SEV-SNP code paths unchanged, which
was my intent.

Or maybe I'm not understanding your comment?  If that's the case,
please elaborate.

Michael

> Reviewed-by: Kuppuswamy Sathyanarayanan
> <sathyanarayanan.kuppuswamy@...ux.intel.com>
> 

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