linux-kernel - Re: [PATCH v8 0/4] Allow guest access to EFI confidential computing secret area

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Message-ID: <7696ba46-91c7-7119-bd68-b3521459cf37@linux.ibm.com>
Date:   Tue, 29 Mar 2022 23:28:45 +0300
From:   Dov Murik <dovmurik@...ux.ibm.com>
To:     Borislav Petkov <bp@...e.de>
Cc:     linux-efi@...r.kernel.org, Ashish Kalra <ashish.kalra@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrew Scull <ascull@...gle.com>,
        Dave Hansen <dave.hansen@...el.com>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Lenny Szubowicz <lszubowi@...hat.com>,
        Peter Gonda <pgonda@...gle.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        James Bottomley <jejb@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@...ux.ibm.com>,
        Jim Cadden <jcadden@....com>,
        Daniele Buono <dbuono@...ux.vnet.ibm.com>,
        linux-coco@...ts.linux.dev, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, Dov Murik <dovmurik@...ux.ibm.com>
Subject: Re: [PATCH v8 0/4] Allow guest access to EFI confidential computing
 secret area



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.

Is efi_config_parse_tables() early enough?  That's where we learn for
the first time that the firmware has a launch-secrets area that we can
look at.

We can add there (say, next to the call to efi_tpm_eventlog_init()) a
code to:

1. map the secret area (ioremap_encrypted())
2. parse the table, look for the "sev debug enabled" GUID.
3. set the value of the kernel variable that we can later use anywhere.


Of course Ard might know about a better mechanism or place to do that.


-Dov