libvirt/tools/virt-qemu-sev-validate

#!/usr/bin/python3
#
# SPDX-License-Identifier: LGPL-2.1-or-later
#
# Validates a guest AMD SEV launch measurement
#
# A general principle in writing this tool is that it must calculate the
# expected measurement based entirely on information it receives on the CLI
# from the guest owner.
#
# It cannot generally trust information obtained from the guest XML or from the
# virtualization host OS. The main exceptions are:
#
#  - The guest measurement
#
#    This is a result of cryptographic operation using a shared secret known
#    only to the guest owner and SEV platform, not the host OS.
#
#  - The guest policy
#
#    This is encoded in the launch session blob that is encrypted with a shared
#    secret known only to the guest owner and SEV platform, not the host OS. It
#    is impossible for the host OS to maliciously launch a guest with different
#    policy and the user provided launch session blob.
#
#    CAVEAT: the user must ALWAYS create a launch blob with freshly generated
#    TIK/TEK for every new VM. Re-use of the same TIK/TEK for multiple VMs
#    is insecure.
#
#  - The SEV API version / build ID
#
#    This does not have an impact on the security of the measurement, unless
#    the guest owner needs a guarantee that the host is not using specific
#    firmware versions with known flaws.
#

import abc
import argparse
from base64 import b64decode
from hashlib import sha256
import hmac
import logging
import re
import socket
import sys
import traceback
from uuid import UUID

from lxml import etree
import libvirt

log = logging.getLogger()


class AttestationFailedException(Exception):
    pass


class UnsupportedUsageException(Exception):
    pass


class InsecureUsageException(Exception):
    pass


class IncorrectConfigException(Exception):
    pass


class InvalidStateException(Exception):
    pass


class GUIDTable(abc.ABC):
    GUID_LEN = 16

    def __init__(self, guid, lenlen=2):
        self.guid = guid
        self.lenlen = lenlen

    @abc.abstractmethod
    def entries(self):
        pass

    def build_entry(self, guid, payload, lenlen):
        dummylen = int(0).to_bytes(lenlen, 'little')
        entry = bytearray(guid + dummylen + payload)

        lenle = len(entry).to_bytes(lenlen, 'little')
        entry[self.GUID_LEN:(self.GUID_LEN + lenlen)] = lenle

        return bytes(entry)

    def build(self):
        payload = self.entries()

        if len(payload) == 0:
            return bytes([])

        dummylen = int(0).to_bytes(self.lenlen, 'little')
        table = bytearray(self.guid + dummylen + payload)

        guidlen = len(table).to_bytes(self.lenlen, 'little')
        table[self.GUID_LEN:(self.GUID_LEN + self.lenlen)] = guidlen

        pad = 16 - (len(table) % 16)
        table += bytes([0]) * pad

        log.debug("Table(hex): %s", bytes(table).hex())
        return bytes(table)


class KernelTable(GUIDTable):

    TABLE_GUID = UUID('{9438d606-4f22-4cc9-b479-a793-d411fd21}').bytes_le
    KERNEL_GUID = UUID('{4de79437-abd2-427f-b835-d5b1-72d2045b}').bytes_le
    INITRD_GUID = UUID('{44baf731-3a2f-4bd7-9af1-41e2-9169781d}').bytes_le
    CMDLINE_GUID = UUID('{97d02dd8-bd20-4c94-aa78-e771-4d36ab2a}').bytes_le

    def __init__(self):
        super().__init__(guid=self.TABLE_GUID,
                         lenlen=2)

        self.kernel = None
        self.initrd = None
        self.cmdline = None

    def load_kernel(self, path):
        with open(path, "rb") as fh:
            self.kernel = sha256(fh.read()).digest()

    def load_initrd(self, path):
        with open(path, "rb") as fh:
            self.initrd = sha256(fh.read()).digest()

    def load_cmdline(self, val):
        self.cmdline = sha256(val.encode("utf8") + bytes([0])).digest()

    def entries(self):
        entries = bytes([])
        if self.kernel is None:
            return entries

        if self.initrd is None:
            self.initrd = sha256(bytes([])).digest()
        if self.cmdline is None:
            self.cmdline = sha256(bytes([0])).digest()

        log.debug("Kernel(sha256): %s", self.kernel.hex())
        log.debug("Initrd(sha256): %s", self.initrd.hex())
        log.debug("Cmdline(sha256): %s", self.cmdline.hex())
        entries += self.build_entry(self.CMDLINE_GUID, self.cmdline, 2)
        entries += self.build_entry(self.INITRD_GUID, self.initrd, 2)
        entries += self.build_entry(self.KERNEL_GUID, self.kernel, 2)

        return entries


class ConfidentialVM(object):

    def __init__(self,
                 measurement=None,
                 api_major=None,
                 api_minor=None,
                 build_id=None,
                 policy=None):
        self.measurement = measurement
        self.api_major = api_major
        self.api_minor = api_minor
        self.build_id = build_id
        self.policy = policy

        self.firmware = None
        self.tik = None
        self.tek = None

        self.kernel_table = KernelTable()

    def load_tik_tek(self, tik_path, tek_path):
        with open(tik_path, 'rb') as fh:
            self.tik = fh.read()
        log.debug("TIK(hex): %s", self.tik.hex())

        if len(self.tik) != 16:
            raise UnsupportedUsageException(
                "Expected 16 bytes in TIK file, but got %d" % len(self.tik))

        with open(tek_path, 'rb') as fh:
            self.tek = fh.read()
        log.debug("TEK(hex): %s", self.tek.hex())

        if len(self.tek) != 16:
            raise UnsupportedUsageException(
                "Expected 16 bytes in TEK file, but got %d" % len(self.tek))

    def load_tk(self, tk_path):
        with open(tk_path, 'rb') as fh:
            tk = fh.read()

        if len(tk) != 32:
            raise UnsupportedUsageException(
                "Expected 32 bytes in TIK/TEK file, but got %d" % len(tk))

        self.tek = tk[0:16]
        self.tik = tk[16:32]
        log.debug("TIK(hex): %s", self.tik.hex())
        log.debug("TEK(hex): %s", self.tek.hex())

    def load_firmware(self, firmware_path):
        with open(firmware_path, 'rb') as fh:
            self.firmware = fh.read()
        log.debug("Firmware(sha256): %s", sha256(self.firmware).hexdigest())

    # Get the full set of measured launch data for the domain
    #
    # The measured data that the guest is initialized with is the concatenation
    # of the following:
    #
    #  - The firmware blob
    #  - The kernel GUID table
    def get_measured_data(self):
        measured_data = (self.firmware +
                         self.kernel_table.build())
        log.debug("Measured-data(sha256): %s",
                  sha256(measured_data).hexdigest())
        return measured_data

    # Get the reported and computed launch measurements for the domain
    #
    # AMD Secure Encrypted Virtualization API , section 6.5:
    #
    # measurement = HMAC(0x04 || API_MAJOR || API_MINOR || BUILD ||
    #                    GCTX.POLICY || GCTX.LD || MNONCE; GCTX.TIK)
    #
    # Where GCTX.LD covers all the measured data the guest is initialized with
    # per get_measured_data().
    def get_measurements(self):
        measurement = b64decode(self.measurement)
        reported = measurement[0:32]
        nonce = measurement[32:48]

        measured_data = self.get_measured_data()
        msg = (
            bytes([0x4]) +
            self.api_major.to_bytes(1, 'little') +
            self.api_minor.to_bytes(1, 'little') +
            self.build_id.to_bytes(1, 'little') +
            self.policy.to_bytes(4, 'little') +
            sha256(measured_data).digest() +
            nonce
        )
        log.debug("Measured-msg(hex): %s", msg.hex())

        computed = hmac.new(self.tik, msg, 'sha256').digest()

        log.debug("Measurement reported(hex): %s", reported.hex())
        log.debug("Measurement computed(hex): %s", computed.hex())

        return reported, computed

    def attest(self):
        reported, computed = self.get_measurements()

        if reported != computed:
            raise AttestationFailedException(
                "Measurement does not match, VM is not trustworthy")


class LibvirtConfidentialVM(ConfidentialVM):
    def __init__(self, **kwargs):
        super().__init__(**kwargs)

        self.conn = None
        self.dom = None

    def check_domain(self, doc, secure):
        ls = doc.xpath("/domain/launchSecurity[@type='sev']")
        if len(ls) != 1:
            raise IncorrectConfigException(
                "Domain is not configured with SEV launch security")

        dh = doc.xpath("/domain/launchSecurity[@type='sev']/dhCert")
        if len(dh) != 1:
            raise IncorrectConfigException(
                "Domain must have SEV owner cert to validate measurement")

        session = doc.xpath("/domain/launchSecurity[@type='sev']/session")
        if len(session) != 1:
            raise IncorrectConfigException(
                "Domain must have SEV session data to validate measurement")

        nvramnodes = doc.xpath("/domain/os/nvram")
        if len(nvramnodes) != 0 and secure:
            raise InsecureUsageException(
                "Domain firmware with NVRAM cannot be securely measured")

        loadernodes = doc.xpath("/domain/os/loader")
        if len(loadernodes) != 1:
            raise IncorrectConfigException(
                "Domain must have one firmware path")

        measure_kernel_nodes = doc.xpath(
            "/domain/launchSecurity[@type='sev']/@kernelHashes")
        measure_kernel = False
        if len(measure_kernel_nodes) == 1:
            if measure_kernel_nodes[0] == "yes":
                measure_kernel = True

        xp_kernel = "/domain/os/kernel"
        xp_initrd = "/domain/os/initrd"
        xp_cmdline = "/domain/os/cmdline"
        kern_nodes = (doc.xpath(xp_kernel) +
                      doc.xpath(xp_initrd) +
                      doc.xpath(xp_cmdline))
        if not measure_kernel:
            if len(self.kernel_table.entries()) != 0:
                raise UnsupportedUsageException(
                    "kernel/initrd/cmdline provided but kernel "
                    "measurement not enabled")

            # Check for an insecure scenario
            if len(kern_nodes) != 0 and secure:
                raise InsecureUsageException(
                    "direct kernel boot present without measurement")
        else:
            if len(kern_nodes) == 0:
                raise IncorrectConfigException(
                    "kernel/initrd/cmdline not provided but kernel "
                    "measurement is enabled")

    def load_domain(self, uri, id_name_uuid, secure, ignore_config):
        self.conn = libvirt.open(uri)

        remote = socket.gethostname() != self.conn.getHostname()
        if not remote and secure:
            raise InsecureUsageException(
                "running locally on the hypervisor host is not secure")

        if re.match(r'^\d+$', id_name_uuid):
            self.dom = self.conn.lookupByID(int(id_name_uuid))
        elif re.match(r'^[-a-f0-9]+$', id_name_uuid):
            self.dom = self.conn.lookupByUUIDString(id_name_uuid)
        else:
            self.dom = self.conn.lookupByName(id_name_uuid)

        log.debug("VM: id=%d name=%s uuid=%s",
            self.dom.ID(), self.dom.name(), self.dom.UUIDString())

        if not self.dom.isActive():
            raise InvalidStateException(
                "Domain must be running to validate measurement")

        state = self.dom.info()[0]
        if state != libvirt.VIR_DOMAIN_PAUSED and secure:
            raise InvalidStateException(
                "Domain must be paused to validate measurement")

        xml = self.dom.XMLDesc()

        doc = etree.fromstring(xml)
        if not ignore_config:
            self.check_domain(doc, secure)

        # See comments at top of file wrt why we are OK to trust the
        # sev-api-major, sev-api-minor, sev-build-id and sev-policy data
        # reported by the host
        sevinfo = self.dom.launchSecurityInfo()

        if "sev-api-major" not in sevinfo:
            raise UnsupportedUsageException(
                "'api-major' not reported in domain launch security info")

        if self.measurement is None:
            self.measurement = sevinfo["sev-measurement"]
        if self.api_major is None:
            self.api_major = sevinfo["sev-api-major"]
        if self.api_minor is None:
            self.api_minor = sevinfo["sev-api-minor"]
        if self.build_id is None:
            self.build_id = sevinfo["sev-build-id"]
        if self.policy is None:
            self.policy = sevinfo["sev-policy"]

        if self.firmware is None:
            if remote:
                raise UnsupportedUsageException(
                    "Cannot access firmware path remotely")
            if secure:
                raise InsecureUsageException(
                    "Using firmware path from XML is not secure")

            loadernodes = doc.xpath("/domain/os/loader")
            if len(loadernodes) == 0:
                raise UnsupportedUsageException(
                    "--firmware not specified and no firmware path found")

            self.load_firmware(loadernodes[0].text)

        if self.kernel_table.kernel is None:
            kernelnodes = doc.xpath("/domain/os/kernel")
            if len(kernelnodes) != 0:
                if remote:
                    raise UnsupportedUsageException(
                        "Cannot access kernel path remotely")
                if secure:
                    raise InsecureUsageException(
                        "Using kernel path from XML is not secure")
                self.kernel_table.load_kernel(kernelnodes[0].text)

        if self.kernel_table.initrd is None:
            initrdnodes = doc.xpath("/domain/os/initrd")
            if len(initrdnodes) != 0:
                if remote:
                    raise UnsupportedUsageException(
                        "Cannot access initrd path remotely")
                if secure:
                    raise InsecureUsageException(
                        "Using initrd path from XML is not secure")
                self.kernel_table.load_initrd(initrdnodes[0].text)

        if self.kernel_table.cmdline is None:
            cmdlinenodes = doc.xpath("/domain/os/cmdline")
            if len(cmdlinenodes) != 0:
                if secure:
                    raise InsecureUsageException(
                        "Using cmdline string from XML is not secure")
                self.kernel_table.load_cmdline(cmdlinenodes[0].text)


def parse_command_line():
    parser = argparse.ArgumentParser(
        description='Validate guest AMD SEV launch measurement')
    parser.add_argument('--debug', '-d', action='store_true',
                        help='Show debug information')
    parser.add_argument('--quiet', '-q', action='store_true',
                        help='Do not display status')

    # Arguments related to the state of the launched guest
    vmstate = parser.add_argument_group("Virtual machine launch state")
    vmstate.add_argument('--measurement', '-m',
                         help='Measurement for the running domain')
    vmstate.add_argument('--api-major', type=int,
                         help='SEV API major version for the running domain')
    vmstate.add_argument('--api-minor', type=int,
                         help='SEV API minor version for the running domain')
    vmstate.add_argument('--build-id', type=int,
                         help='SEV build ID for the running domain')
    vmstate.add_argument('--policy', type=int,
                         help='SEV policy for the running domain')

    # Arguments related to calculation of the expected launch measurement
    vmconfig = parser.add_argument_group("Virtual machine config")
    vmconfig.add_argument('--firmware', '-f',
                          help='Path to the firmware binary')
    vmconfig.add_argument('--kernel', '-k',
                          help='Path to the kernel binary')
    vmconfig.add_argument('--initrd', '-r',
                          help='Path to the initrd binary')
    vmconfig.add_argument('--cmdline', '-e',
                          help='Cmdline string booted with')
    vmconfig.add_argument('--tik',
                          help='TIK file for domain')
    vmconfig.add_argument('--tek',
                          help='TEK file for domain')
    vmconfig.add_argument('--tk',
                          help='TEK/TIK combined file for domain')

    # Arguments related to the connection to libvirt
    vmconn = parser.add_argument_group("Libvirt guest connection")
    vmconn.add_argument('--connect', '-c', default="qemu:///system",
                        help='libvirt connection URI')
    vmconn.add_argument('--domain', '-o',
                        help='domain ID / Name / UUID')
    vmconn.add_argument('--insecure', '-i', action='store_true',
                        help='Proceed even if usage scenario is insecure')
    vmconn.add_argument('--ignore-config', '-g', action='store_true',
                        help='Do not attempt to sanity check the guest config')

    return parser.parse_args()


# Sanity check the set of CLI args specified provide enough info for us to do
# the job
def check_usage(args):
    if args.tk is not None:
        if args.tik is not None or args.tek is not None:
            raise UnsupportedUsageException(
                "--tk is mutually exclusive with --tek/--tik")
    else:
        if args.tik is None or args.tek is None:
            raise UnsupportedUsageException(
                "Either --tk or both of --tek/--tik are required")

    if args.domain is None:
        if args.measurement is None:
            raise UnsupportedUsageException(
                "Either --measurement or --domain is required")

        if args.api_major is None:
            raise UnsupportedUsageException(
                "Either --api-major or --domain is required")

        if args.api_minor is None:
            raise UnsupportedUsageException(
                "Either --api-minor or --domain is required")

        if args.build_id is None:
            raise UnsupportedUsageException(
                "Either --build-id or --domain is required")

        if args.policy is None:
            raise UnsupportedUsageException(
                "Either --policy or --domain is required")

        if args.firmware is None:
            raise UnsupportedUsageException(
                "Either --firmware or --domain is required")

    if args.kernel is None:
        if args.initrd is not None or args.cmdline is not None:
            raise UnsupportedUsageException(
                "--initrd/--cmdline require --kernel")


def attest(args):
    if args.domain is None:
        cvm = ConfidentialVM(measurement=args.measurement,
                             api_major=args.api_major,
                             api_minor=args.api_minor,
                             build_id=args.build_id,
                             policy=args.policy)
    else:
        cvm = LibvirtConfidentialVM(measurement=args.measurement,
                                    api_major=args.api_major,
                                    api_minor=args.api_minor,
                                    build_id=args.build_id,
                                    policy=args.policy)

    if args.firmware is not None:
        cvm.load_firmware(args.firmware)

    if args.tk is not None:
        cvm.load_tk(args.tk)
    else:
        cvm.load_tik_tek(args.tik, args.tek)

    if args.kernel is not None:
        cvm.kernel_table.load_kernel(args.kernel)

    if args.initrd is not None:
        cvm.kernel_table.load_initrd(args.initrd)

    if args.cmdline is not None:
        cvm.kernel_table.load_cmdline(args.cmdline)

    if args.domain is not None:
        cvm.load_domain(args.connect,
                        args.domain,
                        not args.insecure,
                        args.ignore_config)

    cvm.attest()

    if not args.quiet:
        print("OK: Looks good to me")

def main():
    args = parse_command_line()
    if args.debug:
        logging.basicConfig(level="DEBUG")
        formatter = logging.Formatter("[%(levelname)s]: %(message)s")
        handler = log.handlers[0]
        handler.setFormatter(formatter)

    try:
        check_usage(args)

        attest(args)

        sys.exit(0)
    except AttestationFailedException as e:
        if args.debug:
            traceback.print_tb(e.__traceback__)
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(1)
    except UnsupportedUsageException as e:
        if args.debug:
            traceback.print_tb(e.__traceback__)
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(2)
    except InsecureUsageException as e:
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(3)
    except IncorrectConfigException as e:
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(4)
    except InvalidStateException as e:
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(5)
    except Exception as e:
        if args.debug:
            traceback.print_tb(e.__traceback__)
        if not args.quiet:
            print("ERROR: %s" % e, file=sys.stderr)
        sys.exit(6)

if __name__ == "__main__":
    main()