block: fix data type casting for crypto payload offset

The crypto APIs report the offset of the data payload as an uint64_t
type, but the block driver is casting to size_t or ssize_t which will
potentially truncate.

Most of the block APIs use int64_t for offsets meanwhile, so even if
using uint64_t in the crypto block driver we are still at risk of
truncation.

Change the block crypto driver to use uint64_t, but add asserts that
the value is less than INT64_MAX.

Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 20170927125340.12360-4-berrange@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>

block/crypto.c

@@ -364,8 +364,9 @@ static int block_crypto_truncate(BlockDriverState *bs, int64_t offset,
                                  PreallocMode prealloc, Error **errp)
 {
     BlockCrypto *crypto = bs->opaque;
-    size_t payload_offset =
+    uint64_t payload_offset =
         qcrypto_block_get_payload_offset(crypto->block);
+    assert(payload_offset < (INT64_MAX - offset));
 
     offset += payload_offset;
 
@@ -395,8 +396,9 @@ block_crypto_co_readv(BlockDriverState *bs, int64_t sector_num,
     uint8_t *cipher_data = NULL;
     QEMUIOVector hd_qiov;
     int ret = 0;
-    size_t payload_offset =
+    uint64_t payload_offset =
         qcrypto_block_get_payload_offset(crypto->block) / 512;
+    assert(payload_offset < (INT64_MAX / 512));
 
     qemu_iovec_init(&hd_qiov, qiov->niov);
 
@@ -462,8 +464,9 @@ block_crypto_co_writev(BlockDriverState *bs, int64_t sector_num,
     uint8_t *cipher_data = NULL;
     QEMUIOVector hd_qiov;
     int ret = 0;
-    size_t payload_offset =
+    uint64_t payload_offset =
         qcrypto_block_get_payload_offset(crypto->block) / 512;
+    assert(payload_offset < (INT64_MAX / 512));
 
     qemu_iovec_init(&hd_qiov, qiov->niov);
 
@@ -524,7 +527,9 @@ static int64_t block_crypto_getlength(BlockDriverState *bs)
     BlockCrypto *crypto = bs->opaque;
     int64_t len = bdrv_getlength(bs->file->bs);
 
-    ssize_t offset = qcrypto_block_get_payload_offset(crypto->block);
+    uint64_t offset = qcrypto_block_get_payload_offset(crypto->block);
+    assert(offset < INT64_MAX);
+    assert(offset < len);
 
     len -= offset;