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Date:   Thu, 20 May 2021 00:00:53 +0000
From:   Sean Christopherson <seanjc@google.com>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@suse.de>,
        X86 ML <x86@kernel.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "Williams, Dan J" <dan.j.williams@intel.com>,
        "Hansen, Dave" <dave.hansen@intel.com>,
        "Shankar, Ravi V" <ravi.v.shankar@intel.com>,
        Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH v2 00/11] x86: Support Intel Key Locker
Message-ID: <YKWmtUFHK1uZt4TL@google.com>
References: <20210514201508.27967-1-chang.seok.bae@intel.com>
 <9f556d3b-49d3-5b0b-0d92-126294ea082d@kernel.org>
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 <247d9a25-f32f-d01b-61ff-b1966e382907@kernel.org>
 <YKP+1cjRWN/IOEpd@google.com>
 <112f7ceb-d699-fc1e-ea5f-89d505e0d6d8@kernel.org>
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On Wed, May 19, 2021, Sean Christopherson wrote:
> On Wed, May 19, 2021, Andy Lutomirski wrote:
> > On 5/18/21 10:52 AM, Sean Christopherson wrote:
> > > On Tue, May 18, 2021, Andy Lutomirski wrote:
> > >> On 5/17/21 11:21 AM, Bae, Chang Seok wrote:
> > >>> First of all, there is an RFC series for KVM [2].
> > >>>
> > >>> Each CPU has one internal key state so it needs to reload it between guest and
> > >>> host if both are enabled. The proposed approach enables it exclusively; expose
> > >>> it to guests only when disabled in a host. Then, I guess a guest may enable it.
> > >>
> > >> I read that series.  This is not a good solution.
> > >>
> > >> I can think of at least a few reasonable ways that a host and a guest
> > >> can cooperate to, potentially, make KL useful.
> > >>
> > >> a) Host knows that the guest will never migrate, and guest delegates
> > >> IWKEY management to the host.  The host generates a random key and does
> > >> not permit the guest to use LOADIWKEY.  The guest shares the random key
> > >> with the host.  Of course, this means that a host key handle that leaks
> > >> to a guest can be used within the guest.
> > > 
> > > If the guest and host share a random key, then they also share the key handle.
> > > And that handle+key would also need to be shared across all guests.  I doubt this
> > > option is acceptable on the security front.
> > > 
> > 
> > Indeed.  Oddly, SGX has the exact same problem for any scenario in which
> > SGX is used for HSM-like functionality, and people still use SGX.
> 
> The entire PRM/EPC shares a single key, but SGX doesn't rely on encryption to
> isolate enclaves from other software, including other enclaves.  E.g. Intel could
> ship a CPU with the EPC backed entirely by on-die cache and avoid hardware
> encryption entirely.

Ha!  I belatedly see your point: in the end, virtualized KL would also rely on a
trusted entity to isolate its sensitive data via paging-like mechanisms.

The difference in my mind is that encryption is a means to an end for SGX,
whereas hiding the key is the entire point of KL.  E.g. the guest is already
relying on the VMM to isolate its code and data, adding KL doesn't change that.
Sharing an IWKEY across multiple guests would add intra-VM protection, at the
cost of making cross-VM attacks easier to some degree.