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 <stefanb@linux.ibm.com>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

1 files changed, 38 insertions, 8 deletions

diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
index 390a8faa77f9..73917413365b 100644
--- a/security/integrity/ima/ima_policy.c
+++ b/security/integrity/ima/ima_policy.c
@@ -1310,6 +1310,18 @@ static bool ima_validate_rule(struct ima_rule_entry *entry)
 	    !(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;
 }
 
@@ -1727,21 +1739,37 @@ static int ima_parse_rule(char *rule, struct ima_rule_entry *entry)
 			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);
@@ -2183,7 +2211,9 @@ int ima_policy_show(struct seq_file *m, void *v)
 	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 ");