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Thu, 16 Dec 2021 21:41:32 +0000 (GMT) Received: from b03ledav006.gho.boulder.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 8D900C6063; Thu, 16 Dec 2021 21:41:29 +0000 (GMT) Received: from [9.160.32.17] (unknown [9.160.32.17]) by b03ledav006.gho.boulder.ibm.com (Postfix) with ESMTP; Thu, 16 Dec 2021 21:41:29 +0000 (GMT) Message-ID: <1c972953-a7b0-f06c-7d78-0e5fbf13b00e@linux.ibm.com> Date: Thu, 16 Dec 2021 23:41:27 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH] docs: Add measurement calculation details to amd-memory-encryption.txt Content-Language: en-US To: =?UTF-8?Q?Daniel_P=2e_Berrang=c3=a9?= References: <20211214135910.2732101-1-dovmurik@linux.ibm.com> <336cbad3-06da-f11c-8cd1-ca058dd9c6b0@linux.ibm.com> From: Dov Murik In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: bWX6Sb6VdSP1YhElQnSzTuhOWju-FQMs X-Proofpoint-GUID: BJbXbflXOSCU2I9yXNeueRNAQT-pIgvV X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.790,Hydra:6.0.425,FMLib:17.11.62.513 definitions=2021-12-16_08,2021-12-16_01,2021-12-02_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 mlxlogscore=999 lowpriorityscore=0 priorityscore=1501 impostorscore=0 suspectscore=0 clxscore=1015 malwarescore=0 spamscore=0 mlxscore=0 adultscore=0 bulkscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2110150000 definitions=main-2112160115 Received-SPF: pass client-ip=148.163.156.1; envelope-from=dovmurik@linux.ibm.com; helo=mx0a-001b2d01.pphosted.com X-Spam_score_int: -19 X-Spam_score: -2.0 X-Spam_bar: -- X-Spam_report: (-2.0 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.034, RCVD_IN_MSPIKE_H2=-0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Tom Lendacky , Ashish Kalra , Brijesh Singh , James Bottomley , Marcelo Tosatti , qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Dov Murik , Tobin Feldman-Fitzthum , Paolo Bonzini , =?UTF-8?Q?Philippe_Mathieu-Daud=c3=a9?= Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 16/12/2021 18:09, Daniel P. Berrangé wrote: > On Thu, Dec 16, 2021 at 12:38:34PM +0200, Dov Murik wrote: >> >> >> On 14/12/2021 20:39, Daniel P. Berrangé wrote: >>> On Tue, Dec 14, 2021 at 01:59:10PM +0000, Dov Murik wrote: >>>> Add a section explaining how the Guest Owner should calculate the >>>> expected guest launch measurement for SEV and SEV-ES. >>>> >>>> Also update the name and link to the SEV API Spec document. >>>> >>>> Signed-off-by: Dov Murik >>>> Suggested-by: Daniel P. Berrangé >>>> --- >>>> docs/amd-memory-encryption.txt | 50 +++++++++++++++++++++++++++++++--- >>>> 1 file changed, 46 insertions(+), 4 deletions(-) >>>> >>>> diff --git a/docs/amd-memory-encryption.txt b/docs/amd-memory-encryption.txt >>>> index ffca382b5f..f97727482f 100644 >>>> --- a/docs/amd-memory-encryption.txt >>>> +++ b/docs/amd-memory-encryption.txt >>>> @@ -43,7 +43,7 @@ The guest policy is passed as plaintext. A hypervisor may choose to read it, >>>> but should not modify it (any modification of the policy bits will result >>>> in bad measurement). The guest policy is a 4-byte data structure containing >>>> several flags that restricts what can be done on a running SEV guest. >>>> -See KM Spec section 3 and 6.2 for more details. >>>> +See SEV API Spec [1] section 3 and 6.2 for more details. >>>> >>>> The guest policy can be provided via the 'policy' property (see below) >>>> >>>> @@ -88,7 +88,7 @@ expects. >>>> LAUNCH_FINISH finalizes the guest launch and destroys the cryptographic >>>> context. >>>> >>>> -See SEV KM API Spec [1] 'Launching a guest' usage flow (Appendix A) for the >>>> +See SEV API Spec [1] 'Launching a guest' usage flow (Appendix A) for the >>>> complete flow chart. >>>> >>>> To launch a SEV guest >>>> @@ -113,6 +113,45 @@ a SEV-ES guest: >>>> - Requires in-kernel irqchip - the burden is placed on the hypervisor to >>>> manage booting APs. >>>> >>>> +Calculating expected guest launch measurement >>>> +--------------------------------------------- >>>> +In order to verify the guest launch measurement, The Guest Owner must compute >>>> +it in the exact same way as it is calculated by the AMD-SP. SEV API Spec [1] >>>> +section 6.5.1 describes the AMD-SP operations: >>>> + >>>> + GCTX.LD is finalized, producing the hash digest of all plaintext data >>>> + imported into the guest. >>>> + >>>> + The launch measurement is calculated as: >>>> + >>>> + HMAC(0x04 || API_MAJOR || API_MINOR || BUILD || GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK) >>>> + >>>> + where "||" represents concatenation. >>>> + >>>> +The values of API_MAJOR, API_MINOR, BUILD, and GCTX.POLICY can be obtained >>>> +from the 'query-sev' qmp command. >>>> + >>>> +The value of MNONCE is part of the response of 'query-sev-launch-measure': it >>>> +is the last 16 bytes of the base64-decoded data field (see SEV API Spec [1] >>>> +section 6.5.2 Table 52: LAUNCH_MEASURE Measurement Buffer). >>>> + >>>> +The value of GCTX.LD is SHA256(firmware_blob || kernel_hashes_blob || vmsas_blob), >>>> +where: >>>> + >>>> +* firmware_blob is the content of the entire firmware flash file (for example, >>>> + OVMF.fd). >>> >>> Lets add a caveat that the firmware flash should be built to be stateless >>> ie that it is not secure to attempt to measure a guest where the firmware >>> uses an NVRAM store. >>> >> >> * firmware_blob is the content of the entire firmware flash file (for >> example, OVMF.fd). Note that you must build a stateless firmware file >> which doesn't use an NVRAM store, because the NVRAM area is not >> measured, and therefore it is not secure to use a firmware which uses >> state from an NVRAM store. > > Looks good to me. > >>>> +* if kernel is used, and kernel-hashes=on, then kernel_hashes_blob is the >>>> + content of PaddedSevHashTable (including the zero padding), which itself >>>> + includes the hashes of kernel, initrd, and cmdline that are passed to the >>>> + guest. The PaddedSevHashTable struct is defined in target/i386/sev.c . >>>> +* if SEV-ES is enabled (policy & 0x4 != 0), vmsas_blob is the concatenation of >>>> + all VMSAs of the guest vcpus. Each VMSA is 4096 bytes long; its content is >>>> + defined inside Linux kernel code as struct vmcb_save_area, or in AMD APM >>>> + Volume 2 [2] Table B-2: VMCB Layout, State Save Area. >>> >>> Is there any practical guidance we can give apps on the way the VMSAs >>> can be expected to be initialized ? eg can they assume essentially >>> all fields in vmcb_save_area are 0 initialized except for certain >>> ones ? Is initialization likely to vary at all across KVM or EDK2 >>> vesions or something ? >> >> From my own experience, the VMSA of vcpu0 doesn't change; it is basically what QEMU >> sets up in x86_cpu_reset() (which is mostly zeros but not all). I don't know if it >> may change in newer QEMU (machine types?) or kvm. As for vcpu1+, in SEV-ES the >> CS:EIP for the APs is taken from a GUIDed table at the end of the OVMF image, and has >> actually changed a few months ago when the memory layout changed to support both TDX >> and SEV. > > That is an unplesantly large number of moving parts that could > potentially impact the expected state :-( I think we need to > be careful to avoid gratuitous changes, to avoid creating a > combinatorial expansion in the number of possibly valid VMSA > blocks. > > It makes me wonder if we need to think about defining some > standard approach for distro vendors (and/or cloud vendors) > to publish the expected contents for various combinations > of their software pieces. > >> >> >> Here are the VMSAs for my 2-vcpu SEV-ES VM: >> >> >> $ hd vmsa/vmsa_cpu0.bin > > ...snipp... > > was there a nice approach / tool you used to capture > this initial state ? > I wouldn't qualify this as nice: I ended up modifying my host kernel's kvm (see patch below). Later I wrote a script to parse that hex dump from the kernel log into proper 4096-byte binary VMSA files. diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c index 7fbce342eec4..4e45fe37b93d 100644 --- a/arch/x86/kvm/svm/sev.c +++ b/arch/x86/kvm/svm/sev.c @@ -624,6 +624,12 @@ static int sev_launch_update_vmsa(struct kvm *kvm, struct kvm_sev_cmd *argp) */ clflush_cache_range(svm->vmsa, PAGE_SIZE); + /* dubek */ + pr_info("DEBUG_VMSA - cpu %d START ---------------\n", i); + print_hex_dump(KERN_INFO, "DEBUG_VMSA", DUMP_PREFIX_OFFSET, 16, 1, svm->vmsa, PAGE_SIZE, true); + pr_info("DEBUG_VMSA - cpu %d END ---------------\n", i); + /* ----- */ + vmsa.handle = sev->handle; vmsa.address = __sme_pa(svm->vmsa); vmsa.len = PAGE_SIZE;