[POC] Linux Managed Identity key provider (KMPP/OP-TEE non-exportable RSA-PSS)

[Gladwin Johnson (gladjohn)]

Summary (POC / draft)

Adds a Linux Managed Identity key provider that sources a non-exportable RSA-PSS key from the KMPP (KeyIso / OP-TEE / TrustZone) enclave via libkmpp.so — the Linux equivalent of a Windows CredentialGuard / KeyGuard key.

This is a proof of concept for the MSI v2 mTLS-PoP story on Linux. It gets the flow to where MSAL itself creates the non-exportable key and builds the CSR; attestation is the remaining gap (below).

Changes

• ManagedIdentity/KeyProviders/KmppNativeMethods.cs — P/Invoke into libkmpp.so (keygen, open, in-enclave sign, public cert).
• ManagedIdentity/KeyProviders/KmppRsa.cs — RSA whose private key stays in the enclave. Public key read from the enclave self-signed cert; RSA-PSS/SHA-256 signing runs in-enclave; private-key export throws (non-exportable).
• ManagedIdentity/KeyProviders/LinuxManagedIdentityKeyProvider.cs — creates the KMPP key (KeyIso_create_self_sign_pfx_to_key_id), reports it as KeyGuard, falls back to in-memory RSA when the enclave is unavailable.
• ManagedIdentityKeyProviderFactory.cs — selects the Linux provider on net8.0+.
• ManagedIdentityPopExtensions.cs — allows MI mTLS PoP on Linux (net8.0+), previously Windows-only.
• Microsoft.Identity.Client.csproj — excludes the net8-only KMPP files from net462/net472/netstandard2.0.

Validation

Built (all TFMs) and run E2E on an Azure Linux 3 (LVBS "shielded secrets") Trusted Launch VM:

[MI][KeyProviderFactory] Linux detected - using KMPP/OP-TEE managed identity key provider.
GET .../metadata/identity/getplatformmetadata?cred-api-version=2.0
[MI][LinuxKeyProvider] KMPP enclave key created (2048-bit, non-exportable).
MsalClientException: credential_guard_requires_cng -
  [ImdsV2] Credential Guard attestation currently supports only RSA CNG keys on Windows.

A standalone probe confirmed the native path: keygen -> public key -> in-enclave RSA-PSS sign -> verify = VALID; private key non-exportable (only the public cert is extractable).

Known gap / not in scope

• Attestation on Linux — fails at ImdsV2ManagedIdentitySource.GetAttestationJwtAsync (MSAL only attests Windows CNG/CredentialGuard keys). A Linux attestation path (e.g. KMPP KeyIso_create_key_claim -> MAA) is the follow-up.
• No unit tests yet (POC). KMPP requires OP-TEE initialized on the host.

Draft for discussion — feedback welcome on the provider seam, the net8.0+ gating, and the attestation follow-up.

[Copilot]

Pull request overview

Adds a proof-of-concept Linux Managed Identity mTLS-PoP key provider backed by KMPP/OP-TEE (non-exportable RSA-PSS), enables MI mTLS-PoP on Linux for net8.0+, and changes KeyGuard/Credential Guard attestation to fail closed (surface MsalServiceException("attestation_failed")) while bridging native MAA logs into MSAL logging.

Changes:

• Introduces Linux KMPP-backed key provider (KmppNativeMethods, KmppRsa, LinuxManagedIdentityKeyProvider) and selects it on Linux for net8.0+ with TFM gating in the main csproj.
• Changes IMDSv2 attestation behavior to throw attestation_failed when a configured provider yields no token / errors (and preserves cancellation).
• Adds/updates unit tests to validate the new attestation failure semantics and native-log-to-MSAL logger bridging.

Reviewed changes

Copilot reviewed 13 out of 13 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/Microsoft.Identity.Test.Unit/ManagedIdentityTests/ImdsV2Tests.cs	Expands test coverage for attestation failure/cancellation semantics and logger bridging.
src/client/Microsoft.Identity.Client/Microsoft.Identity.Client.csproj	Excludes net8+ KMPP provider files from older TFMs.
src/client/Microsoft.Identity.Client/ManagedIdentity/V2/ImdsV2ManagedIdentitySource.cs	Fails closed on missing/empty attestation token and surfaces attestation_failed.
src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityPopExtensions.cs	Enables MI mTLS PoP on Linux for net8.0+.
src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityKeyProviderFactory.cs	Selects Linux KMPP provider on net8.0+ Linux.
src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/LinuxManagedIdentityKeyProvider.cs	New Linux provider that creates a KMPP enclave key and falls back to in-memory RSA.
src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/KmppRsa.cs	New RSA implementation that signs in-enclave and prevents private key export.
src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/KmppNativeMethods.cs	New P/Invoke surface to libkmpp.so functions used by the Linux provider.
src/client/Microsoft.Identity.Client.KeyAttestation/PopKeyAttestor.cs	Passes MSAL logger into native attestation client for log bridging.
src/client/Microsoft.Identity.Client.KeyAttestation/ManagedIdentityAttestationExtensions.cs	Makes WithAttestationSupport() throw attestation_failed when attestation does not produce a usable JWT.
src/client/Microsoft.Identity.Client.KeyAttestation/Attestation/AttestationLogger.cs	Adds mapping/formatting + bridge of native attestation logs into MSAL ILoggerAdapter.
src/client/Microsoft.Identity.Client.KeyAttestation/Attestation/AttestationClient.cs	Keeps a strong ref to native log callback and translates native error codes to readable reasons.
CHANGELOG.md	Documents the attestation behavior change and logging bridge addition.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (3)

src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityPopExtensions.cs:59

• On net8.0+ this enables mTLS PoP on Linux, but the IMDSv2 attestation path currently hard-requires a Windows RSACng key handle (see ImdsV2ManagedIdentitySource.GetAttestationJwtAsync). Since KeyGuard scenarios are documented to require .WithAttestationSupport(), Linux callers will end up with an unavoidable runtime failure instead of a clear "not supported" error. Until a Linux attestation/token-provider contract exists, keep Linux excluded here (or gate behind an explicit experimental switch).

            bool isSupportedOs = DesktopOsHelper.IsWindows();
#if NET8_0_OR_GREATER
            isSupportedOs = isSupportedOs || DesktopOsHelper.IsLinux();
#endif
            if (!isSupportedOs)

src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/KmppRsa.cs:33

• _publicLoaded is used as a publication flag for _publicParameters, but it's not volatile / not accessed via Volatile.Read/Write. Under concurrent use of the cached RSA instance, another thread can observe _publicLoaded == true while still seeing a stale/default _publicParameters. Marking the flag volatile provides the required memory barriers for safe publication.

        private readonly string _keyId;
        private RSAParameters _publicParameters;
        private bool _publicLoaded;

src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/KmppRsa.cs:165

• KeyIso_CLIENT_pkey_rsa_sign returns the signature length. Even if RSA-2048 is expected to be 256 bytes, returning the entire buffer unconditionally can return trailing zeros if the native method ever returns a shorter length (or if key size changes). Return exactly the number of bytes reported by rc.

                    throw new CryptographicException($"KMPP enclave RSA-PSS sign failed (rc={rc}).");
                }

                // RSA-2048 PSS signatures are always 256 bytes; the enclave fills the full buffer.
                return signature;

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (3)

src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityPopExtensions.cs:60

• On NET8+, Linux is treated as a supported OS for managed-identity mTLS PoP, but the IMDSv2 attestation path currently only supports Windows RSA CNG keys (see ImdsV2ManagedIdentitySource.GetAttestationJwtAsync). As a result, calling .WithAttestationSupport() on Linux will deterministically fail at runtime with credential_guard_requires_cng. Consider gating Linux/KMPP support behind an explicit opt-in until a Linux attestation implementation exists, so the public API doesn’t advertise a broken path by default.

            bool isSupportedOs = DesktopOsHelper.IsWindows();
#if NET8_0_OR_GREATER
            isSupportedOs = isSupportedOs || DesktopOsHelper.IsLinux();
#endif
            if (!isSupportedOs)

src/client/Microsoft.Identity.Client/ManagedIdentity/ManagedIdentityKeyProviderFactory.cs:118

• The Linux provider is selected unconditionally on NET8_0_OR_GREATER. Given that IMDSv2 attestation currently enforces RSACng (Windows-only), selecting this provider by default can cause Linux hosts to be advertised as KeyGuard-capable and later fail in the attestation path. Consider making Linux/KMPP selection explicitly opt-in until Linux attestation is implemented (e.g., via an environment variable) to avoid changing default runtime behavior on Linux net8.

#if NET8_0_OR_GREATER
            if (DesktopOsHelper.IsLinux())
            {
                // Linux: source a non-exportable RSA-PSS key from the KMPP/OP-TEE enclave
                // (KeyGuard-equivalent), falling back to in-memory inside the provider when the

src/client/Microsoft.Identity.Client/ManagedIdentity/KeyProviders/LinuxManagedIdentityKeyProvider.cs:125

• If KeyIso_create_self_sign_pfx_to_key_id returns rc==0 but still allocates a non-null keyIdPtr, the current early-return path leaks that native allocation because KeyIso_clear_free_string is only called on the success path. Free the pointer before returning false whenever keyIdPtr is non-zero.

            if (rc == 0 || keyIdPtr == IntPtr.Zero)
            {
                logger?.Info(() => $"[MI][LinuxKeyProvider] KMPP keygen (KeyIso_create_self_sign_pfx_to_key_id) returned rc={rc}.");
                return false;
            }