ima: add the ability to query the cached hash of a given file

This allows other parts of the kernel (perhaps a stacked LSM allowing
system monitoring, eg. the proposed KRSI LSM [1]) to retrieve the hash
of a given file from IMA if it's present in the iint cache.

It's true that the existence of the hash means that it's also in the
audit logs or in /sys/kernel/security/ima/ascii_runtime_measurements,
but it can be difficult to pull that information out for every
subsequent exec. This is especially true if a given host has been up
for a long time and the file was first measured a long time ago.

It should be kept in mind that this function gives access to cached
entries which can be removed, for instance on security_inode_free().

This is based on Peter Moody's patch:
 https://sourceforge.net/p/linux-ima/mailman/message/33036180/

[1] https://lkml.org/lkml/2019/9/10/393

Signed-off-by: Florent Revest <revest@google.com>
Reviewed-by: KP Singh <kpsingh@chromium.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

include/linux/ima.h

@@ -23,6 +23,7 @@ extern int ima_read_file(struct file *file, enum kernel_read_file_id id);
 extern int ima_post_read_file(struct file *file, void *buf, loff_t size,
 			      enum kernel_read_file_id id);
 extern void ima_post_path_mknod(struct dentry *dentry);
+extern int ima_file_hash(struct file *file, char *buf, size_t buf_size);
 extern void ima_kexec_cmdline(const void *buf, int size);
 
 #ifdef CONFIG_IMA_KEXEC
@@ -91,6 +92,11 @@ static inline void ima_post_path_mknod(struct dentry *dentry)
 	return;
 }
 
+static inline int ima_file_hash(struct file *file, char *buf, size_t buf_size)
+{
+	return -EOPNOTSUPP;
+}
+
 static inline void ima_kexec_cmdline(const void *buf, int size) {}
 #endif /* CONFIG_IMA */
 

security/integrity/ima/ima_main.c

@@ -445,6 +445,55 @@ int ima_file_check(struct file *file, int mask)
 }
 EXPORT_SYMBOL_GPL(ima_file_check);
 
+/**
+ * ima_file_hash - return the stored measurement if a file has been hashed and
+ * is in the iint cache.
+ * @file: pointer to the file
+ * @buf: buffer in which to store the hash
+ * @buf_size: length of the buffer
+ *
+ * On success, return the hash algorithm (as defined in the enum hash_algo).
+ * If buf is not NULL, this function also outputs the hash into buf.
+ * If the hash is larger than buf_size, then only buf_size bytes will be copied.
+ * It generally just makes sense to pass a buffer capable of holding the largest
+ * possible hash: IMA_MAX_DIGEST_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 parameters are incorrect, return -EINVAL.
+ */
+int ima_file_hash(struct file *file, char *buf, size_t buf_size)
+{
+	struct inode *inode;
+	struct integrity_iint_cache *iint;
+	int hash_algo;
+
+	if (!file)
+		return -EINVAL;
+
+	if (!ima_policy_flag)
+		return -EOPNOTSUPP;
+
+	inode = file_inode(file);
+	iint = integrity_iint_find(inode);
+	if (!iint)
+		return -EOPNOTSUPP;
+
+	mutex_lock(&iint->mutex);
+	if (buf) {
+		size_t copied_size;
+
+		copied_size = min_t(size_t, iint->ima_hash->length, buf_size);
+		memcpy(buf, iint->ima_hash->digest, copied_size);
+	}
+	hash_algo = iint->ima_hash->algo;
+	mutex_unlock(&iint->mutex);
+
+	return hash_algo;
+}
+EXPORT_SYMBOL_GPL(ima_file_hash);
+
 /**
  * ima_post_create_tmpfile - mark newly created tmpfile as new
  * @file : newly created tmpfile