ima: Add support for appraisal with digest lists

hulk inclusion
category: feature
feature: IMA Digest Lists extension
bugzilla: 46797

-------------------------------------------------

IMA-Appraise grants access to files with a valid signature or with actual
file digest equal to the digest included in security.ima.

This patch adds support for appraisal based on digest lists. Instead of
using the reference value from security.ima, this patch checks if the
calculated file digest is included in the uploaded digest lists.

This functionality must be explicitly enabled by providing one of the
following values for the ima_appraise_digest_list= kernel option:

- digest: this mode enables appraisal verification with digest lists until
  EVM is initialized; after that, EVM verification must be successful even
  if the file digest is found in a digest list;

- digest-nometadata: this mode enables appraisal verification with digest
  lists even after EVM has been initialized; files without security.evm are
  allowed if the digest of the content is found in the digest list, and
  security.evm is created with current values of xattrs (trust at first
  use); all files created in this way will have the new security.ima type
  EVM_IMA_XATTR_DIGEST_LIST; they can be accessed later only if this mode
  has been selected.
Signed-off-by: NRoberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: NTianxing Zhang <zhangtianxing3@huawei.com>
Reviewed-by: NJason Yan <yanaijie@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

Documentation/admin-guide/kernel-parameters.txt

@@ -1737,6 +1737,15 @@
 				  "enforce-evm" | "log-evm" }
 			default: "enforce"
 
+	ima_appraise_digest_list= [IMA]
+			Format: { "digest" | "digest-nometadata" }
+
+			digest: enables appraisal of files with digest lists
+			until EVM is initialized.
+
+			digest-nometadata: enables appraisal of files with
+			digest lists even after EVM is initialized.
+
 	ima_appraise_tcb [IMA] Deprecated.  Use ima_policy= instead.
 			The builtin appraise policy appraises all files
 			owned by uid=0.

security/integrity/ima/ima.h

@@ -313,7 +313,8 @@ int ima_appraise_measurement(enum ima_hooks func,
 			     struct integrity_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,
+			     struct ima_digest *found_digest);
 int ima_must_appraise(struct inode *inode, int mask, enum ima_hooks func);
 void ima_update_xattr(struct integrity_iint_cache *iint, struct file *file);
 enum integrity_status ima_get_cache_status(struct integrity_iint_cache *iint,
@@ -331,7 +332,8 @@ 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,
+					   struct ima_digest *found_digest)
 {
 	return INTEGRITY_UNKNOWN;
 }

security/integrity/ima/ima_appraise.c

@@ -15,6 +15,7 @@
 #include <keys/system_keyring.h>
 
 #include "ima.h"
+#include "ima_digest_list.h"
 
 static bool ima_appraise_req_evm __ro_after_init;
 static int __init default_appraise_setup(char *str)
@@ -52,6 +53,22 @@ static int __init default_appraise_setup(char *str)
 
 __setup("ima_appraise=", default_appraise_setup);
 
+static bool ima_appraise_no_metadata __ro_after_init;
+#ifdef CONFIG_IMA_DIGEST_LIST
+static int __init appraise_digest_list_setup(char *str)
+{
+	if (!strncmp(str, "digest", 6)) {
+		ima_digest_list_actions |= IMA_APPRAISE;
+
+		if (!strcmp(str + 6, "-nometadata"))
+			ima_appraise_no_metadata = true;
+	}
+
+	return 1;
+}
+__setup("ima_appraise_digest_list=", appraise_digest_list_setup);
+#endif
+
 /*
  * is_ima_appraise_enabled - return appraise status
  *
@@ -91,6 +108,9 @@ static int ima_fix_xattr(struct dentry *dentry,
 	} else {
 		offset = 0;
 		iint->ima_hash->xattr.ng.type = IMA_XATTR_DIGEST_NG;
+		if (test_bit(IMA_DIGEST_LIST, &iint->atomic_flags))
+			iint->ima_hash->xattr.ng.type =
+						EVM_IMA_XATTR_DIGEST_LIST;
 		iint->ima_hash->xattr.ng.algo = algo;
 	}
 	rc = __vfs_setxattr_noperm(dentry, XATTR_NAME_IMA,
@@ -178,17 +198,32 @@ static void ima_cache_flags(struct integrity_iint_cache *iint,
  */
 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)
+			enum integrity_status *status, const char **cause,
+			struct ima_digest *found_digest)
 {
 	int rc = -EINVAL, hash_start = 0;
 
+	if (found_digest && *status != INTEGRITY_PASS &&
+	    *status != INTEGRITY_PASS_IMMUTABLE)
+		set_bit(IMA_DIGEST_LIST, &iint->atomic_flags);
+
 	switch (xattr_value->type) {
+	case EVM_IMA_XATTR_DIGEST_LIST:
+		set_bit(IMA_DIGEST_LIST, &iint->atomic_flags);
+
+		if (!ima_appraise_no_metadata) {
+			*cause = "IMA-xattr-untrusted";
+			*status = INTEGRITY_FAIL;
+			break;
+		}
+		fallthrough;
 	case IMA_XATTR_DIGEST_NG:
 		/* first byte contains algorithm id */
 		hash_start = 1;
 		fallthrough;
 	case IMA_XATTR_DIGEST:
-		if (*status != INTEGRITY_PASS_IMMUTABLE) {
+		if (*status != INTEGRITY_PASS_IMMUTABLE &&
+		    (!found_digest || !ima_digest_is_immutable(found_digest))) {
 			if (iint->flags & IMA_DIGSIG_REQUIRED) {
 				*cause = "IMA-signature-required";
 				*status = INTEGRITY_FAIL;
@@ -321,14 +356,16 @@ int ima_appraise_measurement(enum ima_hooks func,
 			     struct integrity_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,
+			     struct ima_digest *found_digest)
 {
 	static const char op[] = "appraise_data";
 	const char *cause = "unknown";
 	struct dentry *dentry = file_dentry(file);
 	struct inode *inode = d_backing_inode(dentry);
 	enum integrity_status status = INTEGRITY_UNKNOWN;
-	int rc = xattr_len;
+	int rc = xattr_len, rc_evm;
+	char _buf[sizeof(struct evm_ima_xattr_data) + 1 + SHA512_DIGEST_SIZE];
 	bool try_modsig = iint->flags & IMA_MODSIG_ALLOWED && modsig;
 
 	/* If not appraising a modsig, we need an xattr. */
@@ -339,6 +376,25 @@ int ima_appraise_measurement(enum ima_hooks func,
 	    xattr_len == sizeof(struct signature_v2_hdr))
 		rc = -ENODATA;
 
+	if (rc == -ENODATA && found_digest &&
+	    !(file->f_mode & FMODE_CREATED)) {
+		struct evm_ima_xattr_data *xattr_data = NULL;
+
+		rc_evm = vfs_getxattr_alloc(dentry, XATTR_NAME_EVM,
+					(char **)&xattr_data, 0, GFP_NOFS);
+		if (rc_evm > 0) {
+			kfree(xattr_data);
+		} else {
+			xattr_value = (struct evm_ima_xattr_data *)_buf;
+			xattr_value->type = IMA_XATTR_DIGEST_NG;
+			xattr_value->data[0] = found_digest->algo;
+			memcpy(&xattr_value->data[1], found_digest->digest,
+			       hash_digest_size[found_digest->algo]);
+			xattr_len = hash_digest_size[found_digest->algo] + 2;
+			rc = xattr_len;
+		}
+	}
+
 	/* If reading the xattr failed and there's no modsig, error out. */
 	if (rc <= 0 && !try_modsig) {
 		if (rc && rc != -ENODATA)
@@ -363,7 +419,7 @@ int ima_appraise_measurement(enum ima_hooks func,
 		break;
 	case INTEGRITY_UNKNOWN:
 		if (ima_appraise_req_evm &&
-		    xattr_value->type != EVM_IMA_XATTR_DIGSIG)
+		    xattr_value->type != EVM_IMA_XATTR_DIGSIG && !found_digest)
 			goto out;
 		break;
 	case INTEGRITY_NOXATTRS:	/* No EVM protected xattrs. */
@@ -372,6 +428,23 @@ int ima_appraise_measurement(enum ima_hooks func,
 			break;
 		fallthrough;
 	case INTEGRITY_NOLABEL:		/* No security.evm xattr. */
+		/*
+		 * If the digest-nometadata mode is selected, allow access
+		 * without metadata check. EVM will eventually create an HMAC
+		 * based on current xattr values.
+		 */
+		if (ima_appraise_no_metadata && found_digest)
+			break;
+		/* Allow access to digest lists without metadata, only if they
+		 * are signed or found in a digest list (immutable)
+		 */
+		if (func == DIGEST_LIST_CHECK) {
+			if (xattr_value->type == EVM_IMA_XATTR_DIGSIG)
+				break;
+			if (found_digest &&
+			    ima_digest_is_immutable(found_digest))
+				break;
+		}
 		cause = "missing-HMAC";
 		goto out;
 	case INTEGRITY_FAIL_IMMUTABLE:
@@ -386,7 +459,7 @@ int ima_appraise_measurement(enum ima_hooks func,
 
 	if (xattr_value)
 		rc = xattr_verify(func, iint, xattr_value, xattr_len, &status,
-				  &cause);
+				  &cause, found_digest);
 
 	/*
 	 * If we have a modsig and either no imasig or the imasig's key isn't

security/integrity/ima/ima_main.c

@@ -413,8 +413,10 @@ static int process_measurement(struct file *file, const struct cred *cred,
 		if (rc != -EPERM) {
 			inode_lock(inode);
 			rc = ima_appraise_measurement(func, iint, file,
-						      pathname, xattr_value,
-						      xattr_len, modsig);
+					      pathname, xattr_value,
+					      xattr_len, modsig,
+					      ima_digest_allow(found_digest,
+							       IMA_APPRAISE));
 			inode_unlock(inode);
 		}
 		if (!rc)

security/integrity/integrity.h

@@ -71,6 +71,7 @@
 #define IMA_CHANGE_ATTR		2
 #define IMA_DIGSIG		3
 #define IMA_MUST_MEASURE	4
+#define IMA_DIGEST_LIST		5
 
 enum evm_ima_xattr_type {
 	IMA_XATTR_DIGEST = 0x01,