x86/power/hibernate_64: Remove VLA usage

In the quest to remove all stack VLA usage from the kernel [1], this
removes the discouraged use of AHASH_REQUEST_ON_STACK by switching to
shash directly and allocating the descriptor in heap memory (which should
be fine: the tfm has already been allocated there too).

Link: https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com # [1]
Signed-off-by: Kees Cook <keescook@chromium.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/x86/power/hibernate_64.c

@@ -233,29 +233,35 @@ struct restore_data_record {
  */
 static int get_e820_md5(struct e820_table *table, void *buf)
 {
-	struct scatterlist sg;
-	struct crypto_ahash *tfm;
+	struct crypto_shash *tfm;
+	struct shash_desc *desc;
 	int size;
 	int ret = 0;
 
-	tfm = crypto_alloc_ahash("md5", 0, CRYPTO_ALG_ASYNC);
+	tfm = crypto_alloc_shash("md5", 0, 0);
 	if (IS_ERR(tfm))
 		return -ENOMEM;
 
-	{
-		AHASH_REQUEST_ON_STACK(req, tfm);
-		size = offsetof(struct e820_table, entries) + sizeof(struct e820_entry) * table->nr_entries;
-		ahash_request_set_tfm(req, tfm);
-		sg_init_one(&sg, (u8 *)table, size);
-		ahash_request_set_callback(req, 0, NULL, NULL);
-		ahash_request_set_crypt(req, &sg, buf, size);
-
-		if (crypto_ahash_digest(req))
-			ret = -EINVAL;
-		ahash_request_zero(req);
+	desc = kmalloc(sizeof(struct shash_desc) + crypto_shash_descsize(tfm),
+		       GFP_KERNEL);
+	if (!desc) {
+		ret = -ENOMEM;
+		goto free_tfm;
 	}
-	crypto_free_ahash(tfm);
 
+	desc->tfm = tfm;
+	desc->flags = 0;
+
+	size = offsetof(struct e820_table, entries) +
+		sizeof(struct e820_entry) * table->nr_entries;
+
+	if (crypto_shash_digest(desc, (u8 *)table, size, buf))
+		ret = -EINVAL;
+
+	kzfree(desc);
+
+free_tfm:
+	crypto_free_shash(tfm);
 	return ret;
 }