linux-kernel - RE: [RFC RFT PATCH 1/4] hv: Leak pages if set_memory

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Date: Fri, 1 Mar 2024 20:21:45 +0000
From: Michael Kelley <mhklinux@...look.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
	"davem@...emloft.net" <davem@...emloft.net>, "haiyangz@...rosoft.com"
	<haiyangz@...rosoft.com>, "kirill.shutemov@...ux.intel.com"
	<kirill.shutemov@...ux.intel.com>, "pabeni@...hat.com" <pabeni@...hat.com>,
	"edumazet@...gle.com" <edumazet@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "kys@...rosoft.com" <kys@...rosoft.com>,
	"Cui, Dexuan" <decui@...rosoft.com>, "kuba@...nel.org" <kuba@...nel.org>,
	"linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
	"wei.liu@...nel.org" <wei.liu@...nel.org>, "gregkh@...uxfoundation.org"
	<gregkh@...uxfoundation.org>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>
CC: "sathyanarayanan.kuppuswamy@...ux.intel.com"
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, "Reshetova, Elena"
	<elena.reshetova@...el.com>
Subject: RE: [RFC RFT PATCH 1/4] hv: Leak pages if set_memory_encrypted()
 fails

From: Edgecombe, Rick P <rick.p.edgecombe@...el.com> Sent: Friday, March 1, 2024 11:13 AM
> >
> > > On TDX it is possible for the untrusted host to cause
> >
> > I'd argue that this is for CoCo VMs in general, not just TDX.  I don't know
> > all the failure modes for SEV-SNP, but the code paths you are changing
> > are run in both TDX and SEV-SNP CoCo VMs.
> 
> On SEV-SNP the host can cause the call to fail too was my
> understanding. But in Linux, that side panics and never gets to the
> point of being able to free the shared memory. So it's not TDX
> architecture specific, it's just how Linux handles it on the different
> sids. For TDX the suggestion was to avoid panicing because it is
> possible to handle in SW, as Linux usually tries it's best to do.
> 

The Hyper-V case can actually be a third path when a paravisor
is being used.  In that case, for both TDX and SEV-SNP, the
hypervisor callbacks in __set_memory_enc_pgtable() go
to Hyper-V specific functions that talk to the paravisor. Those
callbacks never panic. After a failure, either at the paravisor
level or in the paravisor talking to the hypervisor/VMM, the
decrypted/encrypted state of the memory isn't known.  So
leaking the memory is still the right thing to do, and your
patch set is good. But in the Hyper-V with paravisor case,
the leaking is applicable more broadly than just TDX.

The text in the commit message isn't something that I'll
go to the mat over.  But I wanted to offer the slightly broader
perspective. 

Michael