ima: Always return a file measurement in ima_file_hash()

__ima_inode_hash() checks if a digest has been already calculated by
looking for the integrity_iint_cache structure associated to the passed
inode.

Users of ima_file_hash() (e.g. eBPF) might be interested in obtaining the
information without having to setup an IMA policy so that the digest is
always available at the time they call this function.

In addition, they likely expect the digest to be fresh, e.g. recalculated
by IMA after a file write. Although getting the digest from the
bprm_committed_creds hook (as in the eBPF test) ensures that the digest is
fresh, as the IMA hook is executed before that hook, this is not always the
case (e.g. for the mmap_file hook).

Call ima_collect_measurement() in __ima_inode_hash(), if the file
descriptor is available (passed by ima_file_hash()) and the digest is not
available/not fresh, and store the file measurement in a temporary
integrity_iint_cache structure.

This change does not cause memory usage increase, due to using the
temporary integrity_iint_cache structure, and due to freeing the
ima_digest_data structure inside integrity_iint_cache before exiting from
__ima_inode_hash().

For compatibility reasons, the behavior of ima_inode_hash() remains
unchanged.
Signed-off-by: Roberto Sassu <roberto.sassu@huawei.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Link: https://lore.kernel.org/bpf/20220302111404.193900-3-roberto.sassu@huawei.com

security/integrity/ima/ima_main.c

@@ -520,20 +520,38 @@ int ima_file_check(struct file *file, int mask)
 }
 EXPORT_SYMBOL_GPL(ima_file_check);
 
-static int __ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
+static int __ima_inode_hash(struct inode *inode, struct file *file, char *buf,
+			    size_t buf_size)
 {
-	struct integrity_iint_cache *iint;
-	int hash_algo;
+	struct integrity_iint_cache *iint = NULL, tmp_iint;
+	int rc, hash_algo;
 
-	if (!ima_policy_flag)
-		return -EOPNOTSUPP;
+	if (ima_policy_flag) {
+		iint = integrity_iint_find(inode);
+		if (iint)
+			mutex_lock(&iint->mutex);
+	}
+
+	if ((!iint || !(iint->flags & IMA_COLLECTED)) && file) {
+		if (iint)
+			mutex_unlock(&iint->mutex);
+
+		memset(&tmp_iint, 0, sizeof(tmp_iint));
+		tmp_iint.inode = inode;
+		mutex_init(&tmp_iint.mutex);
+
+		rc = ima_collect_measurement(&tmp_iint, file, NULL, 0,
+					     ima_hash_algo, NULL);
+		if (rc < 0)
+			return -EOPNOTSUPP;
+
+		iint = &tmp_iint;
+		mutex_lock(&iint->mutex);
+	}
 
-	iint = integrity_iint_find(inode);
 	if (!iint)
 		return -EOPNOTSUPP;
 
-	mutex_lock(&iint->mutex);
-
 	/*
 	 * ima_file_hash can be called when ima_collect_measurement has still
 	 * not been called, we might not always have a hash.
@@ -552,12 +570,14 @@ static int __ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
 	hash_algo = iint->ima_hash->algo;
 	mutex_unlock(&iint->mutex);
 
+	if (iint == &tmp_iint)
+		kfree(iint->ima_hash);
+
 	return hash_algo;
 }
 
 /**
- * ima_file_hash - return the stored measurement if a file has been hashed and
- * is in the iint cache.
+ * ima_file_hash - return a measurement of the file
  * @file: pointer to the file
  * @buf: buffer in which to store the hash
  * @buf_size: length of the buffer
@@ -570,7 +590,7 @@ static int __ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
  * The file hash returned is based on the entire file, including the appended
  * signature.
  *
- * If IMA is disabled or if no measurement is available, return -EOPNOTSUPP.
+ * If the measurement cannot be performed, return -EOPNOTSUPP.
  * If the parameters are incorrect, return -EINVAL.
  */
 int ima_file_hash(struct file *file, char *buf, size_t buf_size)
@@ -578,7 +598,7 @@ int ima_file_hash(struct file *file, char *buf, size_t buf_size)
 	if (!file)
 		return -EINVAL;
 
-	return __ima_inode_hash(file_inode(file), buf, buf_size);
+	return __ima_inode_hash(file_inode(file), file, buf, buf_size);
 }
 EXPORT_SYMBOL_GPL(ima_file_hash);
 
@@ -605,7 +625,7 @@ int ima_inode_hash(struct inode *inode, char *buf, size_t buf_size)
 	if (!inode)
 		return -EINVAL;
 
-	return __ima_inode_hash(inode, buf, buf_size);
+	return __ima_inode_hash(inode, NULL, buf, buf_size);
 }
 EXPORT_SYMBOL_GPL(ima_inode_hash);