ima: support fs-verity file digest based version 3 signatures

IMA may verify a file's integrity against a "good" value stored in the
'security.ima' xattr or as an appended signature, based on policy. When
the "good value" is stored in the xattr, the xattr may contain a file
hash or signature. In either case, the "good" value is preceded by a
header. The first byte of the xattr header indicates the type of data
- hash, signature - stored in the xattr. To support storing fs-verity
signatures in the 'security.ima' xattr requires further differentiating
the fs-verity signature from the existing IMA signature.

In addition the signatures stored in 'security.ima' xattr, need to be
disambiguated. Instead of directly signing the fs-verity digest, a new
signature format version 3 is defined as the hash of the ima_file_id
structure, which identifies the type of signature and the digest.

The IMA policy defines "which" files are to be measured, verified, and/or
audited. For those files being verified, the policy rules indicate "how"
the file should be verified. For example to require a file be signed,
the appraise policy rule must include the 'appraise_type' option.

appraise_type:= [imasig] | [imasig|modsig] | [sigv3]
where 'imasig' is the original or signature format v2 (default),
where 'modsig' is an appended signature,
where 'sigv3' is the signature format v3.

The policy rule must also indicate the type of digest, if not the IMA
default, by first specifying the digest type:

digest_type:= [verity]

The following policy rule requires fsverity signatures. The rule may be
constrained, for example based on a fsuuid or LSM label.

appraise func=BPRM_CHECK digest_type=verity appraise_type=sigv3

Acked-by: Stefan Berger <[e-mail]>
Signed-off-by: Mimi Zohar <[e-mail]>

Documentation/ABI/testing/ima_policy

···
 			fgroup:= decimal value
 		  lsm:  are LSM specific
 		  option:
-			appraise_type:= [imasig] [imasig|modsig]
+			appraise_type:= [imasig] | [imasig|modsig] | [sigv3]
+			    where 'imasig' is the original or the signature
+				format v2.
+			    where 'modsig' is an appended signature,
+			    where 'sigv3' is the signature format v3. (Currently
+				limited to fsverity digest based signatures
+				stored in security.ima xattr. Requires
+				specifying "digest_type=verity" first.)
+
 			appraise_flag:= [check_blacklist]
 			Currently, blacklist check is only for files signed with appended
 			signature.
···
 
 			measure func=FILE_CHECK digest_type=verity \
 				template=ima-ngv2
+
+		Example of 'measure' and 'appraise' rules requiring fs-verity
+		signatures (format version 3) stored in security.ima xattr.
+
+		The 'measure' rule specifies the 'ima-sigv3' template option,
+		which includes the indication of type of digest and the file
+		signature in the measurement list.
+
+			measure func=BPRM_CHECK digest_type=verity \
+				template=ima-sigv3
+
+
+		The 'appraise' rule specifies the type and signature format
+		version (sigv3) required.
+
+			appraise func=BPRM_CHECK digest_type=verity \
+				appraise_type=sigv3
+
+		All of these policy rules could, for example, be constrained
+		either based on a filesystem's UUID (fsuuid) or based on LSM
+		labels.

Documentation/security/IMA-templates.rst

···
    (field format: <digest type>:<hash algo>:digest);
  - 'd-modsig': the digest of the event without the appended modsig;
  - 'n-ng': the name of the event, without size limitations;
- - 'sig': the file signature, or the EVM portable signature if the file
-   signature is not found;
+ - 'sig': the file signature, based on either the file's/fsverity's digest[1],
+   or the EVM portable signature, if 'security.ima' contains a file hash.
  - 'modsig' the appended file signature;
  - 'buf': the buffer data that was used to generate the hash without size limitations;
  - 'evmsig': the EVM portable signature;

security/integrity/digsig.c

···
 		/* 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 signature */
+	case 3: /* struct ima_file_id data based signature */
 		return asymmetric_verify(keyring, sig, siglen, digest,
 					 digestlen);
 	}

security/integrity/ima/ima_appraise.c

···
 #include <linux/magic.h>
 #include <linux/ima.h>
 #include <linux/evm.h>
+#include <linux/fsverity.h>
 #include <keys/system_keyring.h>
+#include <uapi/linux/fsverity.h>
 
 #include "ima.h"
 
···
 		return ima_hash_algo;
 
 	switch (xattr_value->type) {
+	case IMA_VERITY_DIGSIG:
+		sig = (typeof(sig))xattr_value;
+		if (sig->version != 3 || xattr_len <= sizeof(*sig) ||
+		    sig->hash_algo >= HASH_ALGO__LAST)
+			return ima_hash_algo;
+		return sig->hash_algo;
 	case EVM_IMA_XATTR_DIGSIG:
 		sig = (typeof(sig))xattr_value;
 		if (sig->version != 2 || xattr_len <= sizeof(*sig)
 		    || sig->hash_algo >= HASH_ALGO__LAST)
 			return ima_hash_algo;
 		return sig->hash_algo;
-		break;
 	case IMA_XATTR_DIGEST_NG:
 		/* first byte contains algorithm id */
 		ret = xattr_value->data[0];
···
 }
 
 /*
+ * 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.
+ *
+ * Signing the ima_file_id struct is currently only supported for
+ * IMA_VERITY_DIGSIG type xattrs.
+ *
+ * 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_digest_data *hash)
+{
+	struct ima_file_id file_id = {
+		.hash_type = IMA_VERITY_DIGSIG, .hash_algorithm = algo};
+	unsigned int unused = HASH_MAX_DIGESTSIZE - hash_digest_size[algo];
+
+	if (type != IMA_VERITY_DIGSIG)
+		return -EINVAL;
+
+	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, hash);
+}
+
+/*
  * xattr_verify - verify xattr digest or signature
  *
  * Verify whether the hash or signature matches the file contents.
···
 			struct evm_ima_xattr_data *xattr_value, int xattr_len,
 			enum integrity_status *status, const char **cause)
 {
+	struct ima_max_digest_data hash;
+	struct signature_v2_hdr *sig;
 	int rc = -EINVAL, hash_start = 0;
+	int mask;
 
 	switch (xattr_value->type) {
 	case IMA_XATTR_DIGEST_NG:
···
 	case IMA_XATTR_DIGEST:
 		if (*status != INTEGRITY_PASS_IMMUTABLE) {
 			if (iint->flags & IMA_DIGSIG_REQUIRED) {
-				*cause = "IMA-signature-required";
+				if (iint->flags & IMA_VERITY_REQUIRED)
+					*cause = "verity-signature-required";
+				else
+					*cause = "IMA-signature-required";
 				*status = INTEGRITY_FAIL;
 				break;
 			}
···
 		break;
 	case EVM_IMA_XATTR_DIGSIG:
 		set_bit(IMA_DIGSIG, &iint->atomic_flags);
+
+		mask = IMA_DIGSIG_REQUIRED | IMA_VERITY_REQUIRED;
+		if ((iint->flags & mask) == mask) {
+			*cause = "verity-signature-required";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+
+		sig = (typeof(sig))xattr_value;
+		if (sig->version >= 3) {
+			*cause = "invalid-signature-version";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
 		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
 					     (const char *)xattr_value,
 					     xattr_len,
···
 		} else {
 			*status = INTEGRITY_PASS;
 		}
+		break;
+	case IMA_VERITY_DIGSIG:
+		set_bit(IMA_DIGSIG, &iint->atomic_flags);
+
+		if (iint->flags & IMA_DIGSIG_REQUIRED) {
+			if (!(iint->flags & IMA_VERITY_REQUIRED)) {
+				*cause = "IMA-signature-required";
+				*status = INTEGRITY_FAIL;
+				break;
+			}
+		}
+
+		sig = (typeof(sig))xattr_value;
+		if (sig->version != 3) {
+			*cause = "invalid-signature-version";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+
+		rc = calc_file_id_hash(IMA_VERITY_DIGSIG, iint->ima_hash->algo,
+				       iint->ima_hash->digest, &hash.hdr);
+		if (rc) {
+			*cause = "sigv3-hashing-error";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+
+		rc = integrity_digsig_verify(INTEGRITY_KEYRING_IMA,
+					     (const char *)xattr_value,
+					     xattr_len, hash.digest,
+					     hash.hdr.length);
+		if (rc) {
+			*cause = "invalid-verity-signature";
+			*status = INTEGRITY_FAIL;
+		} else {
+			*status = INTEGRITY_PASS;
+		}
+
 		break;
 	default:
 		*status = INTEGRITY_UNKNOWN;
···
 		if (rc && rc != -ENODATA)
 			goto out;
 
-		cause = iint->flags & IMA_DIGSIG_REQUIRED ?
-				"IMA-signature-required" : "missing-hash";
+		if (iint->flags & IMA_DIGSIG_REQUIRED) {
+			if (iint->flags & IMA_VERITY_REQUIRED)
+				cause = "verity-signature-required";
+			else
+				cause = "IMA-signature-required";
+		} else {
+			cause = "missing-hash";
+		}
+
 		status = INTEGRITY_NOLABEL;
 		if (file->f_mode & FMODE_CREATED)
 			iint->flags |= IMA_NEW_FILE;

security/integrity/ima/ima_policy.c

···
 	    !(entry->flags & IMA_MODSIG_ALLOWED))
 		return false;
 
+	/*
+	 * Unlike for regular IMA 'appraise' policy rules where security.ima
+	 * xattr may contain either a file hash or signature, the security.ima
+	 * xattr for fsverity must contain a file signature (sigv3).  Ensure
+	 * that 'appraise' rules for fsverity require file signatures by
+	 * checking the IMA_DIGSIG_REQUIRED flag is set.
+	 */
+	if (entry->action == APPRAISE &&
+	    (entry->flags & IMA_VERITY_REQUIRED) &&
+	    !(entry->flags & IMA_DIGSIG_REQUIRED))
+		return false;
+
 	return true;
 }
 
···
 			break;
 		case Opt_digest_type:
 			ima_log_string(ab, "digest_type", args[0].from);
-			if ((strcmp(args[0].from, "verity")) == 0)
+			if (entry->flags & IMA_DIGSIG_REQUIRED)
+				result = -EINVAL;
+			else if ((strcmp(args[0].from, "verity")) == 0)
 				entry->flags |= IMA_VERITY_REQUIRED;
 			else
 				result = -EINVAL;
 			break;
 		case Opt_appraise_type:
 			ima_log_string(ab, "appraise_type", args[0].from);
-			if ((strcmp(args[0].from, "imasig")) == 0)
-				entry->flags |= IMA_DIGSIG_REQUIRED;
-			else if (IS_ENABLED(CONFIG_IMA_APPRAISE_MODSIG) &&
-				 strcmp(args[0].from, "imasig|modsig") == 0)
-				entry->flags |= IMA_DIGSIG_REQUIRED |
+
+			if ((strcmp(args[0].from, "imasig")) == 0) {
+				if (entry->flags & IMA_VERITY_REQUIRED)
+					result = -EINVAL;
+				else
+					entry->flags |= IMA_DIGSIG_REQUIRED;
+			} else if (strcmp(args[0].from, "sigv3") == 0) {
+				/* Only fsverity supports sigv3 for now */
+				if (entry->flags & IMA_VERITY_REQUIRED)
+					entry->flags |= IMA_DIGSIG_REQUIRED;
+				else
+					result = -EINVAL;
+			} else if (IS_ENABLED(CONFIG_IMA_APPRAISE_MODSIG) &&
+				 strcmp(args[0].from, "imasig|modsig") == 0) {
+				if (entry->flags & IMA_VERITY_REQUIRED)
+					result = -EINVAL;
+				else
+					entry->flags |= IMA_DIGSIG_REQUIRED |
 						IMA_MODSIG_ALLOWED;
-			else
+			} else {
 				result = -EINVAL;
+			}
 			break;
 		case Opt_appraise_flag:
 			ima_log_string(ab, "appraise_flag", args[0].from);
···
 	if (entry->template)
 		seq_printf(m, "template=%s ", entry->template->name);
 	if (entry->flags & IMA_DIGSIG_REQUIRED) {
-		if (entry->flags & IMA_MODSIG_ALLOWED)
+		if (entry->flags & IMA_VERITY_REQUIRED)
+			seq_puts(m, "appraise_type=sigv3 ");
+		else if (entry->flags & IMA_MODSIG_ALLOWED)
 			seq_puts(m, "appraise_type=imasig|modsig ");
 		else
 			seq_puts(m, "appraise_type=imasig ");

security/integrity/ima/ima_template_lib.c

···
 {
 	struct evm_ima_xattr_data *xattr_value = event_data->xattr_value;
 
-	if ((!xattr_value) || (xattr_value->type != EVM_IMA_XATTR_DIGSIG))
+	if (!xattr_value ||
+	    (xattr_value->type != EVM_IMA_XATTR_DIGSIG &&
+	     xattr_value->type != IMA_VERITY_DIGSIG))
 		return ima_eventevmsig_init(event_data, field_data);
 
 	return ima_write_template_field_data(xattr_value, event_data->xattr_len,

security/integrity/integrity.h

···
 	EVM_IMA_XATTR_DIGSIG,
 	IMA_XATTR_DIGEST_NG,
 	EVM_XATTR_PORTABLE_DIGSIG,
+	IMA_VERITY_DIGSIG,
 	IMA_XATTR_LAST
 };
 
···
 	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;
···
 } __packed;
 
 /*
- * signature format v2 - for using with asymmetric keys
+ * signature header format v2 - for using with asymmetric keys
+ *
+ * The signature_v2_hdr struct includes a signature format version
+ * to simplify defining new signature formats.
+ *
+ * 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 */
···
 	__be32 keyid;		/* IMA key identifier - not X509/PGP specific */
 	__be16 sig_size;	/* signature size */
 	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,
+ * containing either the fsverity_descriptor struct digest or, in the
+ * future, the regular IMA file hash.
+ *
+ * (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 */