fix: check bytes remaining on monero blocks (tari-project#5610)

Description
---
Checks the bytes left after deserializing monero pow_data to ensure all
bytes have been used.

Motivation and Context
---
Because the struct is stored as raw vec<u8> in headers, we need to
ensure that all the bytes are used after deserializing to ensure that
malicious nodes dont attcked bytes to change the hash or enlarge the
headers. See TARI-010

How Has This Been Tested?
---
unit tests

Audit Finding Number
---
TARI-010

---------

Co-authored-by: Stan Bondi <sdbondi@users.noreply.github.com>

base_layer/core/src/proof_of_work/monero_rx/pow_data.rs

@@ -94,7 +94,15 @@ impl MoneroPowData {
     /// Create a new MoneroPowData struct from the given header
     pub fn from_header(tari_header: &BlockHeader) -> Result<MoneroPowData, MergeMineError> {
         let mut v = tari_header.pow.pow_data.as_slice();
-        BorshDeserialize::deserialize(&mut v).map_err(|e| MergeMineError::DeserializeError(format!("{:?}", e)))
+        let pow_data =
+            BorshDeserialize::deserialize(&mut v).map_err(|e| MergeMineError::DeserializeError(format!("{:?}", e)))?;
+        if !v.is_empty() {
+            return Err(MergeMineError::DeserializeError(format!(
+                "{} bytes leftover after deserialize",
+                v.len()
+            )));
+        }
+        Ok(pow_data)
     }
 
     /// Returns true if the coinbase merkle proof produces the `merkle_root` hash, otherwise false

base_layer/core/tests/tests/block_validation.rs

@@ -150,6 +150,20 @@ async fn test_monero_blocks() {
         },
     };
 
+    // lets try add some bad data to the block
+    let mut extra_bytes_block_3 = block_3.clone();
+    add_bad_monero_data(&mut extra_bytes_block_3, seed2);
+    match db.add_block(Arc::new(extra_bytes_block_3)) {
+        Err(ChainStorageError::ValidationError {
+            source: ValidationError::CustomError(_),
+        }) => (),
+        Err(e) => {
+            panic!("Failed due to other error:{:?}", e);
+        },
+        Ok(res) => {
+            panic!("Block add unexpectedly succeeded with result: {:?}", res);
+        },
+    };
     // now lets fix the seed, and try again
     add_monero_data(&mut block_3, seed2);
     // lets break the nonce count
@@ -200,6 +214,12 @@ fn add_monero_data(tblock: &mut Block, seed_key: &str) {
     tblock.header.pow.pow_data = serialized;
 }
 
+fn add_bad_monero_data(tblock: &mut Block, seed_key: &str) {
+    add_monero_data(tblock, seed_key);
+    // Add some "garbage" bytes to the end of the pow_data
+    tblock.header.pow.pow_data.extend([1u8; 100]);
+}
+
 #[tokio::test]
 async fn inputs_are_not_malleable() {
     let _ = env_logger::try_init();