linux-kernel - Re: [PATCH v2 0/8] x86/coco: Mark CoCo VM pages not present when changing encrypted state

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20231129151012.4un33hvk4nrsicou@box>
Date:   Wed, 29 Nov 2023 18:10:12 +0300
From:   "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
To:     Michael Kelley <mhklinux@...look.com>
Cc:     "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>,
        "kys@...rosoft.com" <kys@...rosoft.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>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.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 v2 0/8] x86/coco: Mark CoCo VM pages not present when
 changing encrypted state

On Tue, Nov 28, 2023 at 07:12:33PM +0000, Michael Kelley wrote:
> From: kirill.shutemov@...ux.intel.com <kirill.shutemov@...ux.intel.com> Sent: Friday, November 24, 2023 2:06 AM
> > 
> > On Tue, Nov 21, 2023 at 01:20:08PM -0800, mhkelley58@...il.com wrote:
> > > From: Michael Kelley <mhklinux@...look.com>
> > >
> > > In a CoCo VM when a page transitions from encrypted to decrypted, or vice
> > > versa, attributes in the PTE must be updated *and* the hypervisor must
> > > be notified of the change.
> > 
> > Strictly speaking it is not true for TDX. Conversion to shared can be
> > implicit: set shared bit and touch the page will do the conversion. MapGPA
> > is optional.
> 
> Interesting.  Given that, is there a reason to use the explicit
> hypervisor callbacks in for private->shared transitions in 
> __set_mem_enc_pgtable()?   It probably doesn't have direct relevance
> to this patch series, but I'm just trying to understand the tradeoffs of
> the implicit vs. explicit approach.  And am I correct that
> shared->private transitions must use the explicit approach?

It must be explicit in sense, that the memory has to be accepted before
use. MapGPA() is still optional.

I don't like this implicit tricks. I spent a lot of time debugging an
issue that was obscured by this semantics.

But I think it is going to say :/

> > > Because there are two separate steps, there's
> > > a window where the settings are inconsistent.  Normally the code that
> > > initiates the transition (via set_memory_decrypted() or
> > > set_memory_encrypted()) ensures that the memory is not being accessed
> > > during a transition, so the window of inconsistency is not a problem.
> > > However, the load_unaligned_zeropad() function can read arbitrary memory
> > > pages at arbitrary times, which could read a transitioning page during
> > > the window.  In such a case, CoCo VM specific exceptions are taken
> > > (depending on the CoCo architecture in use).  Current code in those
> > > exception handlers recovers and does "fixup" on the result returned by
> > > load_unaligned_zeropad().  Unfortunately, this exception handling can't
> > > work in paravisor scenarios (TDX Paritioning and SEV-SNP in vTOM mode)
> > > if the exceptions are routed to the paravisor.  The paravisor can't
> > > do load_unaligned_zeropad() fixup, so the exceptions would need to
> > > be forwarded from the paravisor to the Linux guest, but there are
> > > no architectural specs for how to do that.
> > 
> > Hm. Can't we inject #PF (or #GP) into L2 if #VE/#VC handler in L1 sees
> > cross-page access to shared memory while no fixup entry for the page in
> > L1. It would give L2 chance to handle the situation in a transparent way.
> > 
> > Maybe I miss something, I donno.
> 
> I'm recounting what the Hyper-V paravisor folks say without knowing all the
> details. :-(   But it seems like any kind of forwarding scheme needs to be a
> well-defined contract that would work for both TDX and SEV-SNP.   The
> paravisor in L1 might or might not be Linux-based, so the contract must be OS
> independent.  And the L2 guest might or might not be Linux, so there's
> potential for some other kind of error to be confused with a Linux
> load_unaligned_zeropad() reference.

Okay, fair enough. I have hard time reasoning if it is okay for L2 which
is not Linux.


-- 
  Kiryl Shutsemau / Kirill A. Shutemov