Is there any validation process of smart contract's write operations ?
All smart contracts are replicated across all nodes. And their code is part of a block, and therefore part of the block's hash value. So if you somehow 'hack' a smart contract in one node all other nodes would ignore the changes as they have the right version which produces the right hash value for the block.
Smart contract interactions are validated also in the same way: all transactions are replayed by all nodes so any strangeness in the interaction would be detected and ignored by all other nodes.