linux-kernel - Re: [PATCH Part1 RFC v3 21/22] x86/sev: Register SNP guest request platform device

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Message-ID: <YMeQd6z1iwYyj6JK@work-vm>
Date:   Mon, 14 Jun 2021 18:23:03 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>, tony.luck@...el.com,
        npmccallum@...hat.com
Subject: Re: [PATCH Part1 RFC v3 21/22] x86/sev: Register SNP guest request
 platform device

* Brijesh Singh (brijesh.singh@....com) wrote:
> I see that Tom answered few comments. I will cover others.
> 
> 
> On 6/9/21 2:24 PM, Dr. David Alan Gilbert wrote:
> + /*
> >> +	 * The message sequence counter for the SNP guest request is a 64-bit value
> >> +	 * but the version 2 of GHCB specification defines the 32-bit storage for the
> >> +	 * it.
> >> +	 */
> >> +	if ((count + 1) >= INT_MAX)
> >> +		return 0;
> > Is that UINT_MAX?
> 
> Good catch. It should be UINT_MAX.

OK, but I'm also confused by two things:
  a) Why +1 given that Tom's reply says this gets incremented by 2 each
time (once for the message, once for the reply)
  b) Why >= ? I think here is count was INT_MAX-1 you'd skip to 0,
skipping INT_MAX - is that what you want?

> 
> > +	/*
> > +	 * The secret page contains the VM encryption key used for encrypting the
> > +	 * messages between the guest and the PSP. The secrets page location is
> > +	 * available either through the setup_data or EFI configuration table.
> > +	 */
> > +	if (hdr->cc_blob_address) {
> > +		paddr = hdr->cc_blob_address;
> > Can you trust the paddr the host has given you or do you need to do some
> > form of validation?
> The paddr is mapped encrypted. That means that data  in the paddr must
> be encrypted either through the guest or PSP. After locating the paddr,
> we perform a simply sanity check (32-bit magic string "AMDE"). See the
> verify header check below. Unfortunately the secrets page itself does
> not contain any magic key which we can use to ensure that
> hdr->secret_paddr is actually pointing to the secrets pages but all of
> these memory is accessed encrypted so its safe to access it. If VMM
> lying to us that basically means guest will not be able to communicate
> with the PSP and can't do the attestation etc.

OK; that nails pretty much anything bad that can happen - I was just
thinking if the host did something odd like give you an address in the
middle of some other useful structure.

Dave

> >
> > Dave
> > +	} else if (efi_enabled(EFI_CONFIG_TABLES)) {
> > +#ifdef CONFIG_EFI
> > +		paddr = cc_blob_phys;
> > +#else
> > +		return -ENODEV;
> > +#endif
> > +	} else {
> > +		return -ENODEV;
> > +	}
> > +
> > +	info = memremap(paddr, sizeof(*info), MEMREMAP_WB);
> > +	if (!info)
> > +		return -ENOMEM;
> > +
> > +	/* Verify the header that its a valid SEV_SNP CC header */
> > +	if ((info->magic == CC_BLOB_SEV_HDR_MAGIC) &&
> > +	    info->secrets_phys &&
> > +	    (info->secrets_len == PAGE_SIZE)) {
> > +		res->start = info->secrets_phys;
> > +		res->end = info->secrets_phys + info->secrets_len;
> > +		res->flags = IORESOURCE_MEM;
> > +		snp_secrets_phys = info->secrets_phys;
> > +		ret = 0;
> > +	}
> > +
> > +	memunmap(info);
> > +	return ret;
> > +}
> > +
> 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK