Add support for OVMF hashes

Some times we may not want to have to keep the full OVMF binary handy
when we try to validate a measurement. Most of the data we extract out
of the OVMF binary is static, except for its actual hashes.

So let's introduce a new mode that generates a precalculated launch
digest that contains the SNP ld hash at the point where we fully
ingested OVMF's contents:

  $ sev-snp-measure --mode ovmf-hash --ovmf OVMF.fd
  cab7e[...]

In addition, add a new optional parameter that a user can then use to
consume the hash instead of recalculating it from their OVMF binary at
hand:

  $ sev-snp-measure --mode snp --vcpus=1 --vcpu-type=EPYC-v4 \
                    --ovmf=OVMF.fd.old --ovmf-hash cab7e[...]
  d5269[...]

With this in place, we no longer need to copy full OVMF binaries around.
Instead, for almost all OVMF binaries in existence today, we can merely
share their hash values and are still able to validate the measurement
correctness.

Signed-off-by: Alexander Graf <graf@amazon.com>

README.md

@@ -28,10 +28,11 @@ Clone the Github repo and run the script directly from the local directory:
 
 ```
 $ sev-snp-measure --help
-usage: sev-snp-measure [-h] [--version] [-v] --mode {sev,seves,snp} [--vcpus N]
+usage: sev-snp-measure [-h] [--version] [-v] --mode {sev,seves,snp,ovmf-hash} [--vcpus N]
                        [--vcpu-type CPUTYPE] [--vcpu-sig VALUE] [--vcpu-family FAMILY]
                        [--vcpu-model MODEL] [--vcpu-stepping STEPPING] --ovmf PATH [--kernel PATH]
                        [--initrd PATH] [--append CMDLINE] [--output-format {hex,base64}]
+                       [--ovmf-hash HASH]
 
 Calculate AMD SEV/SEV-ES/SEV-SNP guest launch measurement
 
@@ -56,6 +57,7 @@ optional arguments:
   --append CMDLINE      Kernel command line to calculate hash from (use with --kernel)
   --output-format {hex,base64}
                         Measurement output format
+  --ovmf-hash HASH      Precalculated hash of the OVMF binary
 ```
 
 For example:
@@ -93,6 +95,19 @@ example, the following 3 invocations are identical:
 2. `sev-snp-measure --vcpu-sig=0x800f12 ...`
 3. `sev-snp-measure --vcpu-family=23 --vcpu-model=1 --vcpu-stepping=2 ...`
 
+## Precalculated OVMF hashes
+
+In situations where only minor OVMF changes happen, you may not want to copy the full OVMF binary to the validation system. In these situations, you can use an OVMF hash.
+To generate a hash, use the `--gen-ovmf-hash` parameter:
+
+    $ sev-snp-measure --mode ovmf-hash --ovmf OVMF.fd
+    cab7e085874b3acfdbe2d96dcaa3125111f00c35c6fc9708464c2ae74bfdb048a198cb9a9ccae0b3e5e1a33f5f249819
+
+On a different machine that only has access to an older but compatible OVMF binary, you can then ingest the hash again to generate a full measurement:
+
+    $ sev-snp-measure --mode snp --vcpus=1 --vcpu-type=EPYC-v4 --ovmf=OVMF.fd.old --ovmf-hash cab7e[...]
+    d52697c3e056fb8d698d19cc29adfbed5a8ec9170cb9eb63c2ac957d22b4eb647e25780162036d063a0cf418b8830acc
+
 ## Related projects
 
 * libvirt tools: [virt-dom-sev-validate](https://gitlab.com/berrange/libvirt/-/blob/lgtm/tools/virt-dom-sev-validate.py),

sevsnpmeasure/cli.py

@@ -23,7 +23,7 @@ def main() -> int:
                                      description='Calculate AMD SEV/SEV-ES/SEV-SNP guest launch measurement')
     parser.add_argument('--version', action='version', version=f'%(prog)s {VERSION}')
     parser.add_argument('-v', '--verbose', action='store_true')
-    parser.add_argument('--mode', choices=['sev', 'seves', 'snp'], help='Guest mode', required=True)
+    parser.add_argument('--mode', choices=['sev', 'seves', 'snp', 'ovmf-hash'], help='Guest mode', required=True)
     parser.add_argument('--vcpus', metavar='N', type=int, help='Number of guest vcpus', default=None)
     parser.add_argument('--vcpu-type', metavar='CPUTYPE', choices=list(vcpu_types.CPU_SIGS.keys()),
                         help=f"Type of guest vcpu ({', '.join(vcpu_types.CPU_SIGS.keys())})",
@@ -41,8 +41,13 @@ def main() -> int:
     parser.add_argument('--append', metavar='CMDLINE',
                         help='Kernel command line to calculate hash from (use with --kernel)')
     parser.add_argument('--output-format', choices=['hex', 'base64'], help='Measurement output format', default='hex')
+    parser.add_argument('--ovmf-hash', metavar='HASH', help='Precalculated hash of the OVMF binary')
     args = parser.parse_args()
 
+    if args.mode == 'ovmf-hash':
+        print(guest.calc_ovmf_hash(args.ovmf).hex())
+        return
+
     if args.mode != 'sev' and args.vcpus is None:
         parser.error(f"missing --vcpus N in guest mode '{args.mode}'")
 
@@ -58,7 +63,7 @@ def main() -> int:
             parser.error(f"missing --vcpu-type or --vcpu-sig or --vcpu-family in guest mode '{args.mode}'")
 
     sev_mode = SevMode.from_str(args.mode)
-    ld = guest.calc_launch_digest(sev_mode, args.vcpus, vcpu_sig, args.ovmf, args.kernel, args.initrd, args.append)
+    ld = guest.calc_launch_digest(sev_mode, args.vcpus, vcpu_sig, args.ovmf, args.kernel, args.initrd, args.append, args.ovmf_hash)
 
     if args.output_format == "hex":
         measurement = ld.hex()

sevsnpmeasure/gctx.py

@@ -39,8 +39,8 @@ class GCTX(object):
     # 51 are cleared.
     VMSA_GPA = 0xFFFFFFFFF000
 
-    def __init__(self):
-        self._ld = ZEROS
+    def __init__(self, seed: bytes = ZEROS):
+        self._ld = seed
 
     def ld(self) -> bytes:
         return self._ld

sevsnpmeasure/guest.py

@@ -15,9 +15,9 @@
 
 
 def calc_launch_digest(mode: SevMode, vcpus: int, vcpu_sig: int, ovmf_file: str,
-                       kernel: str, initrd: str, append: str) -> bytes:
+                       kernel: str, initrd: str, append: str, ovmf_hash_str: str = None) -> bytes:
     if mode == SevMode.SEV_SNP:
-        return snp_calc_launch_digest(vcpus, vcpu_sig, ovmf_file, kernel, initrd, append)
+        return snp_calc_launch_digest(vcpus, vcpu_sig, ovmf_file, kernel, initrd, append, ovmf_hash_str)
     elif mode == SevMode.SEV_ES:
         return seves_calc_launch_digest(vcpus, vcpu_sig, ovmf_file, kernel, initrd, append)
     elif mode == SevMode.SEV:
@@ -38,11 +38,26 @@ def snp_update_metadata_pages(gctx, ovmf) -> None:
             raise ValueError("unknown OVMF metadata section type")
 
 
-def snp_calc_launch_digest(vcpus: int, vcpu_sig: int, ovmf_file: str, kernel: str, initrd: str, append: str) -> bytes:
+def calc_ovmf_hash(ovmf_file: str) -> bytes:
     ovmf = OVMF(ovmf_file)
 
     gctx = GCTX()
     gctx.update_normal_pages(ovmf.gpa(), ovmf.data())
+    return gctx.ld()
+
+
+def snp_calc_launch_digest(vcpus: int, vcpu_sig: int, ovmf_file: str, kernel: str, initrd: str, append: str, ovmf_hash_str: str) -> bytes:
+
+    gctx = GCTX()
+    ovmf = OVMF(ovmf_file)
+
+    # Allow users to provide a precalculated OVMF hash.
+    # Ignores the contents of the OVMF file in front of us.
+    if ovmf_hash_str:
+        ovmf_hash = bytearray.fromhex(ovmf_hash_str)
+        gctx = GCTX(seed = ovmf_hash)
+    else:
+        gctx.update_normal_pages(ovmf.gpa(), ovmf.data())
 
     if kernel:
         sev_hashes_table_gpa = ovmf.sev_hashes_table_gpa()