crypto: cts - fix crash on short inputs

BugLink: https://bugs.launchpad.net/bugs/1818237

[It's a minimal fix for a bug that was fixed incidentally by a large
refactoring in v4.8.]

In the CTS template, when the input length is <= one block cipher block
(e.g. <= 16 bytes for AES) pass the correct length to the underlying CBC
transform rather than one block.  This matches the upstream behavior and
makes the encryption/decryption operation correctly return -EINVAL when
1 <= nbytes < bsize or succeed when nbytes == 0, rather than crashing.

This was fixed upstream incidentally by a large refactoring,
commit 0605c41c ("crypto: cts - Convert to skcipher").  But
syzkaller easily trips over this when running on older kernels, as it's
easily reachable via AF_ALG.  Therefore, this patch makes the minimal
fix for older kernels.

Cc: linux-crypto@vger.kernel.org
Fixes: 76cb9521 ("[CRYPTO] cts: Add CTS mode required for Kerberos AES support")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Juerg Haefliger <juergh@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

crypto/cts.c

@@ -137,8 +137,8 @@ static int crypto_cts_encrypt(struct blkcipher_desc *desc,
 	lcldesc.info = desc->info;
 	lcldesc.flags = desc->flags;
 
-	if (tot_blocks == 1) {
-		err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src, bsize);
+	if (tot_blocks <= 1) {
+		err = crypto_blkcipher_encrypt_iv(&lcldesc, dst, src, nbytes);
 	} else if (nbytes <= bsize * 2) {
 		err = cts_cbc_encrypt(ctx, desc, dst, src, 0, nbytes);
 	} else {
@@ -232,8 +232,8 @@ static int crypto_cts_decrypt(struct blkcipher_desc *desc,
 	lcldesc.info = desc->info;
 	lcldesc.flags = desc->flags;
 
-	if (tot_blocks == 1) {
-		err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src, bsize);
+	if (tot_blocks <= 1) {
+		err = crypto_blkcipher_decrypt_iv(&lcldesc, dst, src, nbytes);
 	} else if (nbytes <= bsize * 2) {
 		err = cts_cbc_decrypt(ctx, desc, dst, src, 0, nbytes);
 	} else {