crypto: ensure XTS is only used with ciphers with 16 byte blocks

The XTS cipher mode needs to be used with a cipher which has
a block size of 16 bytes. If a mis-matching block size is used,
the code will either corrupt memory beyond the IV array, or
not fully encrypt/decrypt the IV.

This fixes a memory corruption crash when attempting to use
cast5-128 with xts, since the former has an 8 byte block size.

A test case is added to ensure the cipher creation fails with
such an invalid combination.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/crypto/cipher-gcrypt.c b/crypto/cipher-gcrypt.c
index ede2f70..3652aa1 100644
--- a/crypto/cipher-gcrypt.c
+++ b/crypto/cipher-gcrypt.c
@@ -192,6 +192,12 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
     }
 
     if (cipher->mode == QCRYPTO_CIPHER_MODE_XTS) {
+        if (ctx->blocksize != XTS_BLOCK_SIZE) {
+            error_setg(errp,
+                       "Cipher block size %zu must equal XTS block size %d",
+                       ctx->blocksize, XTS_BLOCK_SIZE);
+            goto error;
+        }
         ctx->iv = g_new0(uint8_t, ctx->blocksize);
     }