[RFC,v3,10/10] ima: Use digest caches for appraisal

Commit Message

Roberto Sassu Sept. 5, 2024, 3:25 p.m. UTC

From: Roberto Sassu <[e-mail]>

Similarly to measurement, enable the new appraisal style too using digest
caches.

It allows files to be verified by only checking the signature of the
package they belong to (as opposed to checking individual file signatures),
and by doing a digest lookup in the digest cache.

The first benefit is that packages are already signed (e.g. RPM package
headers) and also contain file digests, so it is not necessary to further
add signatures for each file.

The second benefit is performance, which improves due to verifying less
signatures (computationally expensive).

Appraisal with a digest cache is successful only if the allowed usage
(AND of policy usage and verified usage) has the
IMA_DIGEST_CACHE_APPRAISE_DATA bit set, and if EVM is disabled or the file
does not have any protected xattr

The digest cache appraisal method is tried first, for performance reasons.

Signed-off-by: Roberto Sassu <[e-mail]>
---
 security/integrity/ima/ima.h          |  6 ++++--
 security/integrity/ima/ima_appraise.c | 31 +++++++++++++++++++++------
 security/integrity/ima/ima_main.c     |  4 +++-
 3 files changed, 31 insertions(+), 10 deletions(-)

Patch

diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
index c915339dd0d4..8f74783e6174 100644
--- a/security/integrity/ima/ima.h
+++ b/security/integrity/ima/ima.h
@@ -440,7 +440,8 @@  int ima_check_blacklist(struct ima_iint_cache *iint,
 int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 			     struct file *file, const unsigned char *filename,
 			     struct evm_ima_xattr_data *xattr_value,
-			     int xattr_len, const struct modsig *modsig);
+			     int xattr_len, const struct modsig *modsig,
+			     u64 allowed_usage);
 int ima_must_appraise(struct mnt_idmap *idmap, struct inode *inode,
 		      int mask, enum ima_hooks func);
 void ima_update_xattr(struct ima_iint_cache *iint, struct file *file);
@@ -465,7 +466,8 @@  static inline int ima_appraise_measurement(enum ima_hooks func,
 					   const unsigned char *filename,
 					   struct evm_ima_xattr_data *xattr_value,
 					   int xattr_len,
-					   const struct modsig *modsig)
+					   const struct modsig *modsig,
+					   u64 allowed_usage)
 {
 	return INTEGRITY_UNKNOWN;
 }
diff --git a/security/integrity/ima/ima_appraise.c b/security/integrity/ima/ima_appraise.c
index d479cf58d859..d4745d90ae23 100644
--- a/security/integrity/ima/ima_appraise.c
+++ b/security/integrity/ima/ima_appraise.c
@@ -480,7 +480,8 @@  int ima_check_blacklist(struct ima_iint_cache *iint,
 int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 			     struct file *file, const unsigned char *filename,
 			     struct evm_ima_xattr_data *xattr_value,
-			     int xattr_len, const struct modsig *modsig)
+			     int xattr_len, const struct modsig *modsig,
+			     u64 allowed_usage)
 {
 	static const char op[] = "appraise_data";
 	const char *cause = "unknown";
@@ -489,13 +490,20 @@  int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 	enum integrity_status status = INTEGRITY_UNKNOWN;
 	int rc = xattr_len;
 	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
+	bool cache_pass = (allowed_usage & IMA_DIGEST_CACHE_APPRAISE_DATA);
 
-	/* If not appraising a modsig, we need an xattr. */
-	if (!(inode->i_opflags & IOP_XATTR) && !try_modsig)
+	/*
+	 * If not appraising a modsig/there is no digest cache match, we need
+	 * an xattr.
+	 */
+	if (!(inode->i_opflags & IOP_XATTR) && !try_modsig && !cache_pass)
 		return INTEGRITY_UNKNOWN;
 
-	/* If reading the xattr failed and there's no modsig, error out. */
-	if (rc <= 0 && !try_modsig) {
+	/*
+	 * If reading the xattr failed and there's no modsig/digest cache match,
+	 * error out.
+	 */
+	if (rc <= 0 && !try_modsig && !cache_pass) {
 		if (rc && rc != -ENODATA)
 			goto out;
 
@@ -526,8 +534,11 @@  int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 	case INTEGRITY_UNKNOWN:
 		break;
 	case INTEGRITY_NOXATTRS:	/* No EVM protected xattrs. */
-		/* It's fine not to have xattrs when using a modsig. */
-		if (try_modsig)
+		/*
+		 * It's fine not to have xattrs when using a modsig or there
+		 * is a digest cache match.
+		 */
+		if (try_modsig || cache_pass)
 			break;
 		fallthrough;
 	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
@@ -544,6 +555,12 @@  int ima_appraise_measurement(enum ima_hooks func, struct ima_iint_cache *iint,
 		WARN_ONCE(true, "Unexpected integrity status %d\n", status);
 	}
 
+	if (cache_pass) {
+		status = INTEGRITY_PASS;
+		rc = 0;
+		goto out;
+	}
+
 	if (xattr_value)
 		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
 				  &cause);
diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index 3b34bfa5c891..aa2f117ae0ba 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -399,7 +399,9 @@  static int process_measurement(struct file *file, const struct cred *cred,
 			inode_lock(inode);
 			rc = ima_appraise_measurement(func, iint, file,
 						      pathname, xattr_value,
-						      xattr_len, modsig);
+						      xattr_len, modsig,
+						      (policy_usage &
+						       verified_usage));
 			inode_unlock(inode);
 		}
 		if (!rc)