From 3c41d9a9724d0539d99791e4f1560dd7a5285bc6 Mon Sep 17 00:00:00 2001
From: Chris Coulson <chris.coulson@canonical.com>
Date: Wed, 10 Apr 2024 14:55:12 +0100
Subject: [PATCH] cryptutil,policyutil: avoid some panics in various places

---
 cryptutil/secret.go     | 3 +++
 cryptutil/signatures.go | 8 ++++++--
 policyutil/auth.go      | 7 +++----
 policyutil/builder.go   | 3 ++-
 4 files changed, 14 insertions(+), 7 deletions(-)

diff --git a/cryptutil/secret.go b/cryptutil/secret.go
index 258e5ec..1ce83cb 100644
--- a/cryptutil/secret.go
+++ b/cryptutil/secret.go
@@ -28,6 +28,9 @@ import (
 // The specified digest algorithm must match the name algorithm of the public area associated with
 // the supplied private key.
 func SecretDecrypt(priv crypto.PrivateKey, hashAlg tpm2.HashAlgorithmId, label, secret []byte) (seed []byte, err error) {
+	if !hashAlg.Available() {
+		return nil, errors.New("digest algorithm is not available")
+	}
 	return internal_crypt.SecretDecrypt(priv, hashAlg.GetHash(), label, secret)
 }
 
diff --git a/cryptutil/signatures.go b/cryptutil/signatures.go
index 7970c2e..6e0a68e 100644
--- a/cryptutil/signatures.go
+++ b/cryptutil/signatures.go
@@ -62,8 +62,6 @@ func digestFromSignerOpts(opts crypto.SignerOpts) (tpm2.HashAlgorithmId, error)
 // Sign creates a signature of the supplied digest using the supplied signer and options.
 // Note that only RSA-SSA, RSA-PSS, ECDSA and HMAC signatures can be created. The returned
 // signature can be verified on a TPM using the associated public key.
-//
-// This may panic if the requested digest algorithm is not available.
 func Sign(rand io.Reader, signer crypto.Signer, digest []byte, opts crypto.SignerOpts) (*tpm2.Signature, error) {
 	hashAlg, err := digestFromSignerOpts(opts)
 	if err != nil {
@@ -75,10 +73,16 @@ func Sign(rand io.Reader, signer crypto.Signer, digest []byte, opts crypto.Signe
 	switch k := signer.Public().(type) {
 	case *rsa.PublicKey:
 		_ = k
+		if _, pss := opts.(*rsa.PSSOptions); pss && !hashAlg.Available() {
+			return nil, errors.New("digest algorithm is not available")
+		}
 	case *ecdsa.PublicKey:
 		_ = k
 	case HMACKey:
 		_ = k
+		if !hashAlg.Available() {
+			return nil, errors.New("digest algorithm is not available")
+		}
 	default:
 		return nil, errors.New("unsupported key type")
 	}
diff --git a/policyutil/auth.go b/policyutil/auth.go
index 696975b..c6e09f7 100644
--- a/policyutil/auth.go
+++ b/policyutil/auth.go
@@ -43,9 +43,10 @@ type PolicyAuthorization struct {
 // approved policy digest. This can sign authorizations for TPM2_PolicySigned as well, but
 // [SignPolicySignedAuthorization] is preferred for that because it constructs the message
 // appropriately.
-//
-// This will panic if the specified digest algorithm is not available.
 func SignPolicyAuthorization(rand io.Reader, message []byte, authKey *tpm2.Public, policyRef tpm2.Nonce, signer crypto.Signer, opts crypto.SignerOpts) (*PolicyAuthorization, error) {
+	if !opts.HashFunc().Available() {
+		return nil, errors.New("digest algorithm is not available")
+	}
 	digest := ComputePolicyAuthorizationTBSDigest(opts.HashFunc(), message, policyRef)
 	sig, err := cryptutil.Sign(rand, signer, digest, opts)
 	if err != nil {
@@ -127,8 +128,6 @@ type PolicySignedParams struct {
 // sessions, and its validity period and scope are restricted by the expiration and cpHashA
 // arguments. If the authorization is not bound to a specific session, the ticket will expire on
 // the next TPM reset if this occurs before the calculated expiration time
-//
-// This will panic if the requested digest algorithm is not available.
 func SignPolicySignedAuthorization(rand io.Reader, params *PolicySignedParams, authKey *tpm2.Public, policyRef tpm2.Nonce, signer crypto.Signer, opts crypto.SignerOpts) (*PolicySignedAuthorization, error) {
 	if params == nil {
 		params = new(PolicySignedParams)
diff --git a/policyutil/builder.go b/policyutil/builder.go
index b7e4e39..cad6e10 100644
--- a/policyutil/builder.go
+++ b/policyutil/builder.go
@@ -716,7 +716,8 @@ type PolicyBuilder struct {
 	err  error
 }
 
-// NewPolicyBuilder returns a new PolicyBuilder.
+// NewPolicyBuilder returns a new PolicyBuilder. It will panic if the supplied algorithm
+// is not available.
 func NewPolicyBuilder(alg tpm2.HashAlgorithmId) *PolicyBuilder {
 	if !alg.Available() {
 		panic("invalid algorithm")