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>

2 files changed, 13 insertions, 5 deletions

diff --git a/crypto/cipher-gcrypt.c b/crypto/cipher-gcrypt.c
index ede2f70df8..3652aa1e1b 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);
     }
 
diff --git a/crypto/cipher-nettle.c b/crypto/cipher-nettle.c
index 70909fb7fe..0267da5ba6 100644
--- a/crypto/cipher-nettle.c
+++ b/crypto/cipher-nettle.c
@@ -361,6 +361,13 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
         goto error;
     }
 
+    if (mode == QCRYPTO_CIPHER_MODE_XTS &&
+        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);
     cipher->opaque = ctx;
 
@@ -456,11 +463,6 @@ int qcrypto_cipher_decrypt(QCryptoCipher *cipher,
         break;
 
     case QCRYPTO_CIPHER_MODE_XTS:
-        if (ctx->blocksize != XTS_BLOCK_SIZE) {
-            error_setg(errp, "Block size must be %d not %zu",
-                       XTS_BLOCK_SIZE, ctx->blocksize);
-            return -1;
-        }
         xts_decrypt(ctx->ctx, ctx->ctx_tweak,
                     ctx->alg_encrypt_wrapper, ctx->alg_decrypt_wrapper,
                     ctx->iv, len, out, in);