ukify: Support multiple PCR signing keys to sign phases separately

To be able to bind a secret against a signed PCR policy so that it only
unlocks in the initrd but not the final system one has to sign the
policies with separate keys because that's what one actually binds to.
This is supported in ukify as "phases" and ukify accepts the PCR
signing keys specified multiple times together with the phase they are
for. We can't extend the existing config setting for this scheme, so
instead we have to add a new setting which can hold the configuration
entries. Also, we need a new setting to specify which PCR pub key goes
into the .pcrpkey UKI section because when we have many, ukify won't
add any by default.
Add SignExpectedPcrWithPhases= as alternative to SignExpectedPcrKey=/
SignExpectedPcrCertificate=/*Source= where we can now specify the
phase signing configuration one line each. A line contains
KEY CERT PHASES [KEYSRC [CERTSRC]] where KEYSRC/CERTSRC defaults to
"file". One can also set "-" for PHASES to omit --phases for ukify so
that it chooses the defaults. Also add SignExpectedPcrUKIPublicKey= to
be able to specify the key for the .pcrpkey UKI section.

Closes #4109

Author: pothos

mkosi/__init__.py

@@ -63,6 +63,7 @@
     ManifestFormat,
     Network,
     OutputFormat,
+    PcrSigner,
     SecureBootSignTool,
     ShimBootloader,
     Ssh,
@@ -1583,8 +1584,10 @@ def want_signed_pcrs(config: Config) -> bool:
     return config.sign_expected_pcr == ConfigFeature.enabled or (
         config.sign_expected_pcr == ConfigFeature.auto
         and config.find_binary("systemd-measure", "/usr/lib/systemd/systemd-measure") is not None
-        and bool(config.sign_expected_pcr_key)
-        and bool(config.sign_expected_pcr_certificate)
+        and (
+            (bool(config.sign_expected_pcr_key) and bool(config.sign_expected_pcr_certificate))
+            or bool(config.sign_expected_pcr_with_phases)
+        )
     )
 
 
@@ -1755,73 +1758,98 @@ def build_uki(
 
         arguments += ["--sign-kernel"]
 
-    if want_signed_pcrs(context.config):
-        assert context.config.sign_expected_pcr_key
-        assert context.config.sign_expected_pcr_certificate
+    seen_bind_paths: set[Path] = set()
 
+    if want_signed_pcrs(context.config):
         arguments += [
             # SHA1 might be disabled in OpenSSL depending on the distro so we opt to not sign
             # for SHA1 to avoid having to manage a bunch of configuration to re-enable SHA1.
             "--pcr-banks", "sha256",
         ]  # fmt: skip
 
-        if (
-            systemd_tool_version(
-                python_binary(context.config),
-                ukify,
-                sandbox=context.sandbox,
-            )
-            >= "258"
-        ):
+        ukify_version = systemd_tool_version(
+            python_binary(context.config),
+            ukify,
+            sandbox=context.sandbox,
+        )
+
+        if ukify_version >= "258":
             cert_parameter = "--pcr-certificate"
         else:
             cert_parameter = "--pcr-public-key"
 
-        # If we're providing the private key via an engine or provider, we have to pass in a X.509
-        # certificate via --pcr-certificate as well.
-        if context.config.sign_expected_pcr_key_source.type != KeySourceType.file:
-            if context.config.sign_expected_pcr_certificate_source.type == CertificateSourceType.provider:
-                arguments += [
-                    "--certificate-provider",
-                    f"provider:{context.config.sign_expected_pcr_certificate_source.source}",
-                ]
+        if context.config.sign_expected_pcr_with_phases:
+            signers = context.config.sign_expected_pcr_with_phases
+            if ukify_version < "253":
+                die(f"ukify {ukify_version} does not support --phases (need >= 253)")
+        else:
+            assert context.config.sign_expected_pcr_key
+            assert context.config.sign_expected_pcr_certificate
+            signers = [
+                PcrSigner(
+                    key=context.config.sign_expected_pcr_key,
+                    certificate=context.config.sign_expected_pcr_certificate,
+                    phases=[],
+                    key_source=context.config.sign_expected_pcr_key_source,
+                    certificate_source=context.config.sign_expected_pcr_certificate_source,
+                )
+            ]
 
-            options += ["--bind", "/run", "/run"]
+        need_run_bind = False
 
-            if context.config.sign_expected_pcr_certificate.exists():
-                arguments += [
-                    cert_parameter, workdir(context.config.sign_expected_pcr_certificate),
-                ]  # fmt: skip
-                options += [
-                    "--ro-bind", context.config.sign_expected_pcr_certificate, workdir(context.config.sign_expected_pcr_certificate),  # noqa: E501
-                ]  # fmt: skip
+        for signer in signers:
+            # If we're providing the private key via an engine or provider, we have to pass in
+            # a X.509 certificate via --pcr-certificate as well.
+            if signer.key_source.type != KeySourceType.file:
+                if signer.certificate_source.type == CertificateSourceType.provider:
+                    arguments += [
+                        "--certificate-provider", f"provider:{signer.certificate_source.source}",
+                    ]  # fmt: skip
+
+                need_run_bind = True
+
+                if signer.certificate.exists():
+                    arguments += [cert_parameter, workdir(signer.certificate)]
+                    seen_bind_paths.add(signer.certificate)
+                else:
+                    arguments += [cert_parameter, signer.certificate]
+
+            if signer.key_source.type == KeySourceType.engine:
+                arguments += ["--signing-engine", signer.key_source.source]
+            elif signer.key_source.type == KeySourceType.provider:
+                arguments += ["--signing-provider", signer.key_source.source]
+
+            if signer.key.exists():
+                arguments += ["--pcr-private-key", workdir(signer.key)]
+                seen_bind_paths.add(signer.key)
             else:
-                arguments += [cert_parameter, context.config.sign_expected_pcr_certificate]
+                arguments += ["--pcr-private-key", signer.key]
 
-        if context.config.sign_expected_pcr_key_source.type == KeySourceType.engine:
-            arguments += ["--signing-engine", context.config.sign_expected_pcr_key_source.source]
-        elif context.config.sign_expected_pcr_key_source.type == KeySourceType.provider:
-            arguments += ["--signing-provider", context.config.sign_expected_pcr_key_source.source]
+            if signer.phases:
+                arguments += ["--phases", " ".join(signer.phases)]
 
-        if context.config.sign_expected_pcr_key.exists():
-            arguments += ["--pcr-private-key", workdir(context.config.sign_expected_pcr_key)]
-            options += [
-                "--ro-bind", context.config.sign_expected_pcr_key, workdir(context.config.sign_expected_pcr_key),  # noqa: E501
-            ]  # fmt: skip
-        else:
-            arguments += ["--pcr-private-key", context.config.sign_expected_pcr_key]
+        if need_run_bind:
+            options += ["--bind", "/run", "/run"]
     elif ArtifactOutput.pcrs in context.config.split_artifacts:
         assert context.config.sign_expected_pcr_certificate
 
         json_out = True
+        cert = context.config.sign_expected_pcr_certificate
         arguments += [
             "--policy-digest",
             "--pcr-banks", "sha256",
-            "--pcr-certificate", workdir(context.config.sign_expected_pcr_certificate),
-        ]  # fmt: skip
-        options += [
-            "--ro-bind", context.config.sign_expected_pcr_certificate, workdir(context.config.sign_expected_pcr_certificate),  # noqa: E501
+            "--pcr-certificate", workdir(cert),
         ]  # fmt: skip
+        seen_bind_paths.add(cert)
+
+    if (pcrpkey := context.config.sign_expected_pcr_uki_public_key) is not None:
+        if not pcrpkey.exists():
+            die(f"SignExpectedPcrUKIPublicKey={pcrpkey} does not exist")
+        arguments += ["--pcrpkey", workdir(pcrpkey)]
+        seen_bind_paths.add(pcrpkey)
+
+    for file in seen_bind_paths:
+        options += ["--ro-bind", file, workdir(file)]
 
     if microcodes:
         # new .ucode section support?
@@ -2723,25 +2751,39 @@ def check_inputs(config: Config) -> None:
             hint="Run mkosi genkey to generate a key/certificate pair",
         )
 
-    if config.sign_expected_pcr == ConfigFeature.enabled and not config.sign_expected_pcr_key:
+    if (
+        config.sign_expected_pcr == ConfigFeature.enabled
+        and not config.sign_expected_pcr_with_phases
+        and not config.sign_expected_pcr_key
+    ):
         die(
             "SignExpectedPcr= is enabled but no private key is configured",
             hint="Run mkosi genkey to generate a key/certificate pair",
         )
 
-    if config.sign_expected_pcr == ConfigFeature.enabled and not config.sign_expected_pcr_certificate:
+    if (
+        config.sign_expected_pcr == ConfigFeature.enabled
+        and not config.sign_expected_pcr_with_phases
+        and not config.sign_expected_pcr_certificate
+    ):
         die(
             "SignExpectedPcr= is enabled but no certificate is configured",
             hint="Run mkosi genkey to generate a key/certificate pair",
         )
 
-    if config.secure_boot_key_source != config.sign_expected_pcr_key_source:
-        die("Secure boot key source and expected PCR signatures key source have to be the same")
+    # When SignExpectedPcrWithPhases= is used the key/certificate sources live per-entry, so the
+    # scalar sign_expected_pcr_{key,certificate}_source fields default to "file" regardless of
+    # what the per-signer sources are. Comparing those scalars against secure boot's sources
+    # would be meaningless, so skip the check in that case and rely on ukify to reject
+    # inconsistent sources at invocation time.
+    if not config.sign_expected_pcr_with_phases:
+        if config.secure_boot_key_source != config.sign_expected_pcr_key_source:
+            die("Secure boot key source and expected PCR signatures key source have to be the same")
 
-    if config.secure_boot_certificate_source != config.sign_expected_pcr_certificate_source:
-        die(
-            "Secure boot certificate source and expected PCR signatures certificate source have to be the same"  # noqa: E501
-        )  # fmt: skip
+        if config.secure_boot_certificate_source != config.sign_expected_pcr_certificate_source:
+            die(
+                "Secure boot certificate source and expected PCR signatures certificate source have to be the same"  # noqa: E501
+            )  # fmt: skip
 
     if config.verity == Verity.signed and not config.verity_key:
         die(