wallet: ensure that the passphrase is not deleted from memory when being used to rescan

`m_relock_mutex` is introduced so that the passphrase is not
deleted from memory when the timeout provided in
`walletpassphrase` is up, but the wallet is still rescanning.

src/wallet/rpc/backup.cpp

@@ -1655,6 +1655,10 @@ RPCHelpMan importdescriptors()
         throw JSONRPCError(RPC_WALLET_ERROR, "Wallet is currently rescanning. Abort existing rescan or wait.");
     }
 
+    // Ensure that the wallet is not locked for the remainder of this RPC, as
+    // the passphrase is used to top up the keypool.
+    LOCK(pwallet->m_relock_mutex);
+
     const UniValue& requests = main_request.params[0];
     const int64_t minimum_timestamp = 1;
     int64_t now = 0;

src/wallet/rpc/encrypt.cpp

@@ -90,7 +90,7 @@ RPCHelpMan walletpassphrase()
     std::weak_ptr<CWallet> weak_wallet = wallet;
     pwallet->chain().rpcRunLater(strprintf("lockwallet(%s)", pwallet->GetName()), [weak_wallet, relock_time] {
         if (auto shared_wallet = weak_wallet.lock()) {
-            LOCK(shared_wallet->cs_wallet);
+            LOCK2(shared_wallet->m_relock_mutex, shared_wallet->cs_wallet);
             // Skip if this is not the most recent rpcRunLater callback.
             if (shared_wallet->nRelockTime != relock_time) return;
             shared_wallet->Lock();
@@ -122,8 +122,6 @@ RPCHelpMan walletpassphrasechange()
     std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
     if (!pwallet) return UniValue::VNULL;
 
-    LOCK(pwallet->cs_wallet);
-
     if (!pwallet->IsCrypted()) {
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletpassphrasechange was called.");
     }
@@ -132,6 +130,8 @@ RPCHelpMan walletpassphrasechange()
         throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before changing the passphrase.");
     }
 
+    LOCK2(pwallet->m_relock_mutex, pwallet->cs_wallet);
+
     // TODO: get rid of these .c_str() calls by implementing SecureString::operator=(std::string)
     // Alternately, find a way to make request.params[0] mlock()'d to begin with.
     SecureString strOldWalletPass;
@@ -179,8 +179,6 @@ RPCHelpMan walletlock()
     std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
     if (!pwallet) return UniValue::VNULL;
 
-    LOCK(pwallet->cs_wallet);
-
     if (!pwallet->IsCrypted()) {
         throw JSONRPCError(RPC_WALLET_WRONG_ENC_STATE, "Error: running with an unencrypted wallet, but walletlock was called.");
     }
@@ -189,6 +187,8 @@ RPCHelpMan walletlock()
         throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before locking the wallet.");
     }
 
+    LOCK2(pwallet->m_relock_mutex, pwallet->cs_wallet);
+
     pwallet->Lock();
     pwallet->nRelockTime = 0;
 
@@ -227,8 +227,6 @@ RPCHelpMan encryptwallet()
     std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
     if (!pwallet) return UniValue::VNULL;
 
-    LOCK(pwallet->cs_wallet);
-
     if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS)) {
         throw JSONRPCError(RPC_WALLET_ENCRYPTION_FAILED, "Error: wallet does not contain private keys, nothing to encrypt.");
     }
@@ -241,6 +239,8 @@ RPCHelpMan encryptwallet()
         throw JSONRPCError(RPC_WALLET_ERROR, "Error: the wallet is currently being used to rescan the blockchain for related transactions. Please call `abortrescan` before encrypting the wallet.");
     }
 
+    LOCK2(pwallet->m_relock_mutex, pwallet->cs_wallet);
+
     // TODO: get rid of this .c_str() by implementing SecureString::operator=(std::string)
     // Alternately, find a way to make request.params[0] mlock()'d to begin with.
     SecureString strWalletPass;

src/wallet/rpc/transactions.cpp

@@ -880,6 +880,7 @@ RPCHelpMan rescanblockchain()
     std::optional<int> stop_height;
     uint256 start_block;
 
+    LOCK(pwallet->m_relock_mutex);
     {
         LOCK(pwallet->cs_wallet);
         EnsureWalletIsUnlocked(*pwallet);

src/wallet/wallet.cpp

@@ -551,7 +551,7 @@ bool CWallet::ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase,
     bool fWasLocked = IsLocked();
 
     {
-        LOCK(cs_wallet);
+        LOCK2(m_relock_mutex, cs_wallet);
         Lock();
 
         CCrypter crypter;
@@ -786,7 +786,7 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
         return false;
 
     {
-        LOCK(cs_wallet);
+        LOCK2(m_relock_mutex, cs_wallet);
         mapMasterKeys[++nMasterKeyMaxID] = kMasterKey;
         WalletBatch* encrypted_batch = new WalletBatch(GetDatabase());
         if (!encrypted_batch->TxnBegin()) {
@@ -3407,7 +3407,7 @@ bool CWallet::Lock()
         return false;
 
     {
-        LOCK(cs_wallet);
+        LOCK2(m_relock_mutex, cs_wallet);
         if (!vMasterKey.empty()) {
             memory_cleanse(vMasterKey.data(), vMasterKey.size() * sizeof(decltype(vMasterKey)::value_type));
             vMasterKey.clear();

src/wallet/wallet.h

@@ -488,6 +488,9 @@ public:
 
     // Used to prevent concurrent calls to walletpassphrase RPC.
     Mutex m_unlock_mutex;
+    // Used to prevent deleting the passphrase from memory when it is still in use.
+    RecursiveMutex m_relock_mutex;
+
     bool Unlock(const SecureString& strWalletPassphrase, bool accept_no_keys = false);
     bool ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase);
     bool EncryptWallet(const SecureString& strWalletPassphrase);