wallet: throw error if legacy entries are present on loading descriptor wallets

In the wallet key-value-loading routine, most legacy type entries
require a LegacyScriptPubKeyMan instance after successful
deserialization. On a descriptor wallet, creating that (via method
`GetOrCreateLegacyScriptPubKeyMan`) fails and then leads to a
null-pointer dereference crash. Fix this by throwing an error if
if the wallet flags indicate that we have a descriptor wallet and there
is a legacy entry found.

Author: theStack

src/wallet/wallet.cpp

@@ -2919,6 +2919,10 @@ std::shared_ptr<CWallet> CWallet::Create(WalletContext& context, const std::stri
                                 "The wallet might had been created on a newer version.\n"
                                 "Please try running the latest software version.\n"), walletFile);
             return nullptr;
+        } else if (nLoadWalletRet == DBErrors::UNEXPECTED_LEGACY_ENTRY) {
+            error = strprintf(_("Unexpected legacy entry in descriptor wallet found. Loading wallet %s\n\n"
+                                "The wallet might have been tampered with or created with malicious intent.\n"), walletFile);
+            return nullptr;
         } else {
             error = strprintf(_("Error loading %s"), walletFile);
             return nullptr;

src/wallet/walletdb.cpp

@@ -315,6 +315,7 @@ class CWalletScanState {
     std::map<uint160, CHDChain> m_hd_chains;
     bool tx_corrupt{false};
     bool descriptor_unknown{false};
+    bool unexpected_legacy_entry{false};
 
     CWalletScanState() = default;
 };
@@ -332,6 +333,11 @@ ReadKeyValue(CWallet* pwallet, CDataStream& ssKey, CDataStream& ssValue,
         if (filter_fn && !filter_fn(strType)) {
             return true;
         }
+        // Legacy entries in descriptor wallets are not allowed, abort immediately
+        if (pwallet->IsWalletFlagSet(WALLET_FLAG_DESCRIPTORS) && DBKeys::LEGACY_TYPES.count(strType) > 0) {
+            wss.unexpected_legacy_entry = true;
+            return false;
+        }
         if (strType == DBKeys::NAME) {
             std::string strAddress;
             ssKey >> strAddress;
@@ -833,6 +839,12 @@ DBErrors WalletBatch::LoadWallet(CWallet* pwallet)
             std::string strType, strErr;
             if (!ReadKeyValue(pwallet, ssKey, ssValue, wss, strType, strErr))
             {
+                if (wss.unexpected_legacy_entry) {
+                    strErr = strprintf("Error: Unexpected legacy entry found in descriptor wallet %s. ", pwallet->GetName());
+                    strErr += "The wallet might have been tampered with or created with malicious intent.";
+                    pwallet->WalletLogPrintf("%s\n", strErr);
+                    return DBErrors::UNEXPECTED_LEGACY_ENTRY;
+                }
                 // losing keys is considered a catastrophic error, anything else
                 // we assume the user can live with:
                 if (IsKeyType(strType) || strType == DBKeys::DEFAULTKEY) {

src/wallet/walletdb.h

@@ -52,7 +52,8 @@ enum class DBErrors
     LOAD_FAIL,
     NEED_REWRITE,
     NEED_RESCAN,
-    UNKNOWN_DESCRIPTOR
+    UNKNOWN_DESCRIPTOR,
+    UNEXPECTED_LEGACY_ENTRY
 };
 
 namespace DBKeys {