Add support for OVMF hashes #11

agraf · 2023-03-24T21:58:25Z

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, users no longer need to copy full OVMF binaries around. Instead, for almost all SEV-SNP capable OVMF binaries in existence today, we can merely share their hash values and are still able to validate VM measurements.

dubek

Thanks @agraf for your contribution! I think it's a good idea to allow easily generating launch digests for several OVMFs without holding the full OVMF binaries.

See few comments below:

dubek · 2023-03-27T09:59:26Z

README.md

@@ -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


maybe this should be --snp-ovmf-hash because it's only working for SNP guests?

That's a great point, will change here and in mode.

dubek · 2023-03-27T09:59:57Z

README.md

+## 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:


you mention here --gen-ovmf-hash but I don't see it anywhere else.

I originally had --gen-ovmf-hash as a special argument instead of --mode=ovmf-hash. But while writing the README and seeing how clumsy everything became, I figured I'd fold it in with --mode.

This is a leftover from the original version. I'll change it :)

dubek · 2023-03-27T10:01:39Z

sevsnpmeasure/cli.py

@@ -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')


Suggested change

parser.add_argument('--ovmf-hash', metavar='HASH', help='Precalculated hash of the OVMF binary')

parser.add_argument('--ovmf-hash', metavar='HASH', help='Precalculated hash of the OVMF binary (hex string)')

dubek · 2023-03-27T10:35:39Z

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)


It's a bit strange to add this to the --mode argument. What do you think about introducing a new argument --only-produce-ovmf-hash , and in that case, verify that mode is snp ?

I had that originally and found it a lot more clumsy, because now you have a mandatory mode parameter which isn't actually mandatory because you're really only generating half the digest.

What if we change this to snp-ovmf-hash and change the description of the feature in a way that explains SNP digest generation as a 2 step process. This mode only executes the first step. The snp mode can ingest the first step as input.

Even better: While changing it accordingly, I realized we can call the mode snp:ovmf-hash. That makes it obvious that we're executing part of the snp hash calculation.

sevsnpmeasure/guest.py

dubek · 2023-03-27T10:42:15Z

Also, please run make typecheck - I see some type hints errors:

  make typecheck
  shell: /usr/bin/bash -e {0}
  env:
    pythonLocation: /opt/hostedtoolcache/Python/3.8.16/x64
    LD_LIBRARY_PATH: /opt/hostedtoolcache/Python/3.8.16/x64/lib
mypy .
sevsnpmeasure/guest.py:18: error: Incompatible default for argument "ovmf_hash_str" (default has type "None", argument has type "str")  [assignment]
sevsnpmeasure/guest.py:18: note: PEP 484 prohibits implicit Optional. Accordingly, mypy has changed its default to no_implicit_optional=True
sevsnpmeasure/guest.py:18: note: Use https://github.com/hauntsaninja/no_implicit_optional to automatically upgrade your codebase
sevsnpmeasure/cli.py:49: error: Return value expected  [return-value]
Found 2 errors in 2 files (checked 12 source files)
make: *** [Makefile:10: typecheck] Error 1
Error: Process completed with exit code 2.

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 snp: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 --snp-ovmf-hash cab7e[...] d5269[...] which is identical to the full calculation with the new OVMF.fd: $ sev-snp-measure --mode snp --vcpus=1 --vcpu-type=EPYC-v4 --ovmf=OVMF.fd 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>

agraf · 2023-03-27T16:51:09Z

I posted v2 with comments above addressed :)

dubek

Great. Thanks for fixing.

dubek · 2023-04-13T07:38:30Z

Thanks again @agraf . This is now part of release sev-snp-measure 0.0.4 on pypi.

dubek added the enhancement New feature or request label Mar 26, 2023

dubek reviewed Mar 27, 2023

View reviewed changes

agraf force-pushed the ovmf-hash branch from 47e03d4 to bd04510 Compare March 27, 2023 12:12

dubek approved these changes Mar 28, 2023

View reviewed changes

dubek merged commit b6db121 into virtee:main Mar 28, 2023

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Add support for OVMF hashes #11

Add support for OVMF hashes #11

agraf commented Mar 24, 2023

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dubek commented Apr 13, 2023

	parser.add_argument('--ovmf-hash', metavar='HASH', help='Precalculated hash of the OVMF binary')
	parser.add_argument('--ovmf-hash', metavar='HASH', help='Precalculated hash of the OVMF binary (hex string)')

Add support for OVMF hashes #11

Add support for OVMF hashes #11

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