ima: provide double buffering for hash calculation

The asynchronous hash API allows initiating a hash calculation and
then performing other tasks, while waiting for the hash calculation
to complete.

This patch introduces usage of double buffering for simultaneous
hashing and reading of the next chunk of data from storage.

Changes in v3:
- better comments
Signed-off-by: Dmitry Kasatkin <d.kasatkin@samsung.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>

security/integrity/ima/ima_crypto.c

@@ -253,12 +253,12 @@ static int ima_calc_file_hash_atfm(struct file *file,
 				   struct crypto_ahash *tfm)
 {
 	loff_t i_size, offset;
-	char *rbuf;
-	int rc, read = 0, rbuf_len;
+	char *rbuf[2] = { NULL, };
+	int rc, read = 0, rbuf_len, active = 0, ahash_rc = 0;
 	struct ahash_request *req;
 	struct scatterlist sg[1];
 	struct ahash_completion res;
-	size_t rbuf_size;
+	size_t rbuf_size[2];
 
 	hash->length = crypto_ahash_digestsize(tfm);
 
@@ -284,36 +284,69 @@ static int ima_calc_file_hash_atfm(struct file *file,
 	 * Try to allocate maximum size of memory.
 	 * Fail if even a single page cannot be allocated.
 	 */
-	rbuf = ima_alloc_pages(i_size, &rbuf_size, 1);
-	if (!rbuf) {
+	rbuf[0] = ima_alloc_pages(i_size, &rbuf_size[0], 1);
+	if (!rbuf[0]) {
 		rc = -ENOMEM;
 		goto out1;
 	}
 
+	/* Only allocate one buffer if that is enough. */
+	if (i_size > rbuf_size[0]) {
+		/*
+		 * Try to allocate secondary buffer. If that fails fallback to
+		 * using single buffering. Use previous memory allocation size
+		 * as baseline for possible allocation size.
+		 */
+		rbuf[1] = ima_alloc_pages(i_size - rbuf_size[0],
+					  &rbuf_size[1], 0);
+	}
+
 	if (!(file->f_mode & FMODE_READ)) {
 		file->f_mode |= FMODE_READ;
 		read = 1;
 	}
 
 	for (offset = 0; offset < i_size; offset += rbuf_len) {
-		rbuf_len = ima_kernel_read(file, offset, rbuf, PAGE_SIZE);
-		if (rbuf_len < 0) {
-			rc = rbuf_len;
-			break;
+		if (!rbuf[1] && offset) {
+			/* Not using two buffers, and it is not the first
+			 * read/request, wait for the completion of the
+			 * previous ahash_update() request.
+			 */
+			rc = ahash_wait(ahash_rc, &res);
+			if (rc)
+				goto out3;
+		}
+		/* read buffer */
+		rbuf_len = min_t(loff_t, i_size - offset, rbuf_size[active]);
+		rc = ima_kernel_read(file, offset, rbuf[active], rbuf_len);
+		if (rc != rbuf_len)
+			goto out3;
+
+		if (rbuf[1] && offset) {
+			/* Using two buffers, and it is not the first
+			 * read/request, wait for the completion of the
+			 * previous ahash_update() request.
+			 */
+			rc = ahash_wait(ahash_rc, &res);
+			if (rc)
+				goto out3;
 		}
-		if (rbuf_len == 0)
-			break;
 
-		sg_init_one(&sg[0], rbuf, rbuf_len);
+		sg_init_one(&sg[0], rbuf[active], rbuf_len);
 		ahash_request_set_crypt(req, sg, NULL, rbuf_len);
 
-		rc = ahash_wait(crypto_ahash_update(req), &res);
-		if (rc)
-			break;
+		ahash_rc = crypto_ahash_update(req);
+
+		if (rbuf[1])
+			active = !active; /* swap buffers, if we use two */
 	}
+	/* wait for the last update request to complete */
+	rc = ahash_wait(ahash_rc, &res);
+out3:
 	if (read)
 		file->f_mode &= ~FMODE_READ;
-	ima_free_pages(rbuf, rbuf_size);
+	ima_free_pages(rbuf[0], rbuf_size[0]);
+	ima_free_pages(rbuf[1], rbuf_size[1]);
 out2:
 	if (!rc) {
 		ahash_request_set_crypt(req, NULL, hash->digest, 0);