[PATCH v4 6/8] ima: define signature version 3

[Mimi Zohar]

To disambiguate the signed data stored in the 'security.ima' xattr,
define signature version 3 as the hash of the ima_file_id structure.

Signed-off-by: Mimi Zohar <zohar@xxxxxxxxxxxxx>
---
 security/integrity/digsig.c           |  3 ++-
 security/integrity/ima/ima_appraise.c | 36 +++++++++++++++++++++++++++
 security/integrity/integrity.h        | 20 +++++++++++++--
 3 files changed, 56 insertions(+), 3 deletions(-)

diff --git a/security/integrity/digsig.c b/security/integrity/digsig.c
index 3b06a01bd0fd..fd8f77d92a62 100644
--- a/security/integrity/digsig.c
+++ b/security/integrity/digsig.c
@@ -74,7 +74,8 @@ int integrity_digsig_verify(const unsigned int id, const char *sig, int siglen,
 		/* v1 API expect signature without xattr type */
 		return digsig_verify(keyring, sig + 1, siglen - 1, digest,
 				     digestlen);
-	case 2:
+	case 2: /* regular file data hash based sginature */
+	case 3: /* struct ima_file_id data base signature */
 		return asymmetric_verify(keyring, sig, siglen, digest,
 					 digestlen);
 	}
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index 17232bbfb9f9..7bc180bd808e 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -225,6 +225,34 @@ int ima_read_xattr(struct dentry *dentry,
 	return ret;
 }
 
+/*
+ * calc_file_id_hash - calculate the hash of the ima_file_id struct data
+ * @type: xattr type [enum evm_ima_xattr_type]
+ * @algo: hash algorithm [enum hash_algo]
+ * @digest: pointer to the digest to be hashed
+ * @hash: (out) pointer to the hash
+ *
+ * IMA signature version 3 disambiguates the data that is signed by
+ * indirectly signing the hash of the ima_file_id structure data.
+ *
+ * Return 0 on success, error code otherwise.
+ */
+static int calc_file_id_hash(enum evm_ima_xattr_type type,
+			     enum hash_algo algo, const u8 *digest,
+			     struct ima_max_digest_data *hash)
+{
+	struct ima_file_id file_id = {.hash_algorithm = algo};
+	uint unused = HASH_MAX_DIGESTSIZE - hash_digest_size[algo];
+
+	memcpy(file_id.hash, digest, hash_digest_size[algo]);
+
+	hash->algo = algo;
+	hash->length = hash_digest_size[algo];
+
+	return ima_calc_buffer_hash(&file_id, sizeof(file_id) - unused,
+				    (struct ima_digest_data *)hash);
+}
+
 /*
  * xattr_verify - verify xattr digest or signature
  *
@@ -236,6 +264,7 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 			struct evm_ima_xattr_data *xattr_value, int xattr_len,
 			enum integrity_status *status, const char **cause)
 {
+	struct signature_v2_hdr *sig;
 	int rc = -EINVAL, hash_start = 0;
 
 	switch (xattr_value->type) {
@@ -274,6 +303,13 @@ static int xattr_verify(enum ima_hooks func, struct integrity_iint_cache *iint,
 		break;
 	case EVM_IMA_XATTR_DIGSIG:
 		set_bit(IMA_DIGSIG, &iint->atomic_flags);
+
+		sig = (typeof(sig))xattr_value;
+		if (sig->version != 2) {
+			*cause = "invalid-signature-version";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
 		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
 					     (const char *)xattr_value,
 					     xattr_len,
diff --git a/security/integrity/integrity.h b/security/integrity/integrity.h
index cbc41d4288ed..f59f83527c0c 100644
--- a/security/integrity/integrity.h
+++ b/security/integrity/integrity.h
@@ -94,7 +94,7 @@ struct evm_xattr {
 	u8 digest[SHA1_DIGEST_SIZE];
 } __packed;
 
-#define IMA_MAX_DIGEST_SIZE	64
+#define IMA_MAX_DIGEST_SIZE	HASH_MAX_DIGESTSIZE
 
 struct ima_digest_data {
 	u8 algo;
@@ -137,7 +137,11 @@ struct ima_max_digest_data {
 } __packed;
 
 /*
- * signature format v2 - for using with asymmetric keys
+ * signature header format v2 - for using with asymmetric keys
+ *
+ * signature format:
+ * version 2: regular file data hash based signature
+ * version 3: struct ima_file_id data based signature
  */
 struct signature_v2_hdr {
 	uint8_t type;		/* xattr type */
@@ -148,6 +152,18 @@ struct signature_v2_hdr {
 	uint8_t sig[];		/* signature payload */
 } __packed;
 
+/*
+ * IMA signature version 3 disambiguates the data that is signed, by
+ * indirectly signing the hash of the ima_file_id structure data.
+ *
+ * (The hash of the ima_file_id structure is only of the portion used.)
+ */
+struct ima_file_id {
+	__u8 hash_type;		/* xattr type [enum evm_ima_xattr_type] */
+	__u8 hash_algorithm;	/* Digest algorithm [enum hash_algo] */
+	__u8 hash[HASH_MAX_DIGESTSIZE];
+} __packed;
+
 /* integrity data associated with an inode */
 struct integrity_iint_cache {
 	struct rb_node rb_node;	/* rooted in integrity_iint_tree */
-- 
2.27.0