Require CBlockIndex::GetUndoPos() to hold mutex cs_main

src/chain.h

@@ -237,8 +237,9 @@ public:
         return ret;
     }
 
-    FlatFilePos GetUndoPos() const
+    FlatFilePos GetUndoPos() const EXCLUSIVE_LOCKS_REQUIRED(::cs_main)
     {
+        AssertLockHeld(::cs_main);
         FlatFilePos ret;
         if (nStatus & BLOCK_HAVE_UNDO) {
             ret.nFile = nFile;

src/node/blockstorage.cpp

@@ -513,7 +513,8 @@ static bool UndoWriteToDisk(const CBlockUndo& blockundo, FlatFilePos& pos, const
 
 bool UndoReadFromDisk(CBlockUndo& blockundo, const CBlockIndex* pindex)
 {
-    FlatFilePos pos = pindex->GetUndoPos();
+    const FlatFilePos pos{WITH_LOCK(::cs_main, return pindex->GetUndoPos())};
+
     if (pos.IsNull()) {
         return error("%s: no undo data available", __func__);
     }