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/integrity/ima/ima_api.c

@@ -188,7 +188,8 @@ int ima_get_action(struct inode *inode, int mask, enum ima_hooks func)
  * Return 0 on success, error code otherwise
  */
 int ima_collect_measurement(struct integrity_iint_cache *iint,
-			    struct file *file, enum hash_algo algo)
+			    struct file *file, void *buf, loff_t size,
+			    enum hash_algo algo)
 {
 	const char *audit_cause = "failed";
 	struct inode *inode = file_inode(file);
@@ -210,7 +211,8 @@ int ima_collect_measurement(struct integrity_iint_cache *iint,
 
 		hash.hdr.algo = algo;
 
-		result = ima_calc_file_hash(file, &hash.hdr);
+		result = (!buf) ?  ima_calc_file_hash(file, &hash.hdr) :
+			ima_calc_buffer_hash(buf, size, &hash.hdr);
 		if (!result) {
 			int length = sizeof(hash.hdr) + hash.hdr.length;
 			void *tmpbuf = krealloc(iint->ima_hash, length,