[net processing] Don't process mutated blocks

We preemptively perform a block mutation check before further processing
a block message (similar to early sanity checks on other messsage
types). The main reasons for this change are as follows:

- `CBlock::GetHash()` is a foot-gun without a prior mutation check, as
  the hash returned only commits to the header but not to the actual
  transactions (`CBlock::vtx`) contained in the block.
- We have observed attacks that abused mutated blocks in the past, which
  could have been prevented by simply not processing mutated blocks
  (e.g. bitcoin#27608).

src/net_processing.cpp

@@ -4719,6 +4719,16 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
 
         LogPrint(BCLog::NET, "received block %s peer=%d\n", pblock->GetHash().ToString(), pfrom.GetId());
 
+        const CBlockIndex* prev_block{WITH_LOCK(m_chainman.GetMutex(), return m_chainman.m_blockman.LookupBlockIndex(pblock->hashPrevBlock))};
+
+        if (IsBlockMutated(/*block=*/*pblock,
+                           /*check_witness_root=*/DeploymentActiveAfter(prev_block, m_chainman, Consensus::DEPLOYMENT_SEGWIT))) {
+            LogDebug(BCLog::NET, "Received mutated block from peer=%d\n", peer->m_id);
+            Misbehaving(*peer, 100, "mutated block");
+            WITH_LOCK(cs_main, RemoveBlockRequest(pblock->GetHash(), peer->m_id));
+            return;
+        }
+
         bool forceProcessing = false;
         const uint256 hash(pblock->GetHash());
         bool min_pow_checked = false;
@@ -4734,7 +4744,6 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
             mapBlockSource.emplace(hash, std::make_pair(pfrom.GetId(), true));
 
             // Check work on this block against our anti-dos thresholds.
-            const CBlockIndex* prev_block = m_chainman.m_blockman.LookupBlockIndex(pblock->hashPrevBlock);
             if (prev_block && prev_block->nChainWork + CalculateHeadersWork({pblock->GetBlockHeader()}) >= GetAntiDoSWorkThreshold()) {
                 min_pow_checked = true;
             }