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Date:   Wed, 30 Mar 2022 09:11:54 +0300
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Subject: Re: [PATCH v8 0/4] Allow guest access to EFI confidential computing
 secret area
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To:     Borislav Petkov <bp@suse.de>
Cc:     linux-efi@vger.kernel.org, Ashish Kalra <ashish.kalra@amd.com>,
        Brijesh Singh <brijesh.singh@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        James Morris <jmorris@namei.org>,
        "Serge E. Hallyn" <serge@hallyn.com>,
        Andi Kleen <ak@linux.intel.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Andrew Scull <ascull@google.com>,
        Dave Hansen <dave.hansen@intel.com>,
        "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
        Gerd Hoffmann <kraxel@redhat.com>,
        Lenny Szubowicz <lszubowi@redhat.com>,
        Peter Gonda <pgonda@google.com>,
        Matthew Garrett <mjg59@srcf.ucam.org>,
        James Bottomley <jejb@linux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@linux.ibm.com>,
        Jim Cadden <jcadden@ibm.com>,
        Daniele Buono <dbuono@linux.vnet.ibm.com>,
        linux-coco@lists.linux.dev, linux-security-module@vger.kernel.org,
        linux-kernel@vger.kernel.org, Dov Murik <dovmurik@linux.ibm.com>
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From:   Dov Murik <dovmurik@linux.ibm.com>
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On 29/03/2022 23:28, Dov Murik wrote:
> 
> 
> On 29/03/2022 21:30, Borislav Petkov wrote:
> 
>>
>> So now that I think of it, it would be even nicer if the fact whether
>> guest debugging is allowed, were available to the guest *very early*
>> during boot. Because I think the most important cases where you'd want
>> to singlestep a SEV* guest with the qemu gdbstub is early guest kernel
>> boot code. So it would be cool if we'd have access to the debugging
>> setting that early.
>>
>> Lemme have a look at your patches in detail to get an idea what's
>> happening there.
> 

After a night's sleep I figured out that an SEV guest cannot tell if a
value it's reading was (a) encrypted by the host using
KVM_SEV_LAUNCH_UPDATE_DATA, or (b) added using secret injection using
KVM_SEV_LAUNCH_SECRET.

The only difference is that if the host is using
KVM_SEV_LAUNCH_UPDATE_DATA, then it changes the measurement.  But maybe
for debugging scenarios we (= Guest Owner) don't care about the
measurement being correct.

If that's the case, we don't need a secure channel and secret injection.
You can use a simple "sev=debug" (or whatever) in the kernel
command-line to indicate your needs.


Did I miss something?


-Dov