ima: define a new hook to measure and appraise a file already in memory

This patch defines a new IMA hook ima_post_read_file() for measuring
and appraising files read by the kernel. The caller loads the file into
memory before calling this function, which calculates the hash followed by
the normal IMA policy based processing.

Changelog v5:
- fail ima_post_read_file() if either file or buf is NULL
v3:
- rename ima_hash_and_process_file() to ima_post_read_file()

v1:
- split patch

Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>
Acked-by: Dmitry Kasatkin <dmitry.kasatkin@huawei.com>

security/security.c

@@ -913,7 +913,12 @@ int security_kernel_module_from_file(struct file *file)
 int security_kernel_post_read_file(struct file *file, char *buf, loff_t size,
 				   enum kernel_read_file_id id)
 {
-	return call_int_hook(kernel_post_read_file, 0, file, buf, size, id);
+	int ret;
+
+	ret = call_int_hook(kernel_post_read_file, 0, file, buf, size, id);
+	if (ret)
+		return ret;
+	return ima_post_read_file(file, buf, size, id);
 }
 EXPORT_SYMBOL_GPL(security_kernel_post_read_file);