fix: Prevent early loan impairment and due-date manipulation

Stop impairLoan and unimpairLoan from rewriting
sfNextPaymentDueDate when the amendment is active. Previously a
colluding broker could repeatedly impair and unimpair an overdue
loan to keep pushing the due date forward, permanently blocking
default eligibility and suppressing late-interest / late-fee
accrual.

Author: Tapanito

include/xrpl/tx/transactors/lending/LendingHelpers.h

@@ -219,6 +219,10 @@ computeFullPaymentInterest(
     std::uint32_t startDate,
     TenthBips32 closeInterestRate);
 
+/// Returns true if the loan's next payment due date has passed.
+[[nodiscard]] bool
+isPaymentLate(ReadView const& view, SLE::const_ref loanSle);
+
 namespace detail {
 // These classes and functions should only be accessed by LendingHelper
 // functions and unit tests

src/libxrpl/tx/transactors/lending/LendingHelpers.cpp

@@ -64,6 +64,12 @@ isRounded(Asset const& asset, Number const& value, std::int32_t scale)
         roundToAsset(asset, value, scale, Number::upward);
 }
 
+[[nodiscard]] bool
+isPaymentLate(ReadView const& view, SLE::const_ref loanSle)
+{
+    return hasExpired(view, loanSle->at(sfNextPaymentDueDate));
+}
+
 namespace detail {
 
 void
@@ -675,11 +681,6 @@ computeLatePayment(
     TenthBips16 managementFeeRate,
     beast::Journal j)
 {
-    // Check if the due date has passed. If not, reject the payment as
-    // being too soon
-    if (!hasExpired(view, nextDueDate))
-        return Unexpected(tecTOO_SOON);
-
     // Calculate the penalty interest based on how long the payment is overdue.
     auto const latePaymentInterest = loanLatePaymentInterest(
         principalOutstanding, lateInterestRate, view.parentCloseTime(), nextDueDate);
@@ -1614,7 +1615,7 @@ loanMakePayment(
 
     // -------------------------------------------------------------
     // A late payment not flagged as late overrides all other options.
-    if (paymentType != LoanPaymentType::late && hasExpired(view, nextDueDateProxy))
+    if (paymentType != LoanPaymentType::late && isPaymentLate(view, loan))
     {
         // If the payment is late, and the late flag was not set, it's not
         // valid
@@ -1708,6 +1709,10 @@ loanMakePayment(
     // late payment handling
     if (paymentType == LoanPaymentType::late)
     {
+        // Check if the due date has passed. If not, reject the payment as being too soon
+        if (!isPaymentLate(view, loan))
+            return Unexpected(tecTOO_SOON);
+
         TenthBips32 const lateInterestRate{loan->at(sfLateInterestRate)};
         Number const latePaymentFee = loan->at(sfLatePaymentFee);
 

src/libxrpl/tx/transactors/lending/LoanManage.cpp

@@ -282,6 +282,12 @@ LoanManage::impairLoan(
     Asset const& vaultAsset,
     beast::Journal j)
 {
+    if (view.rules().enabled(featureLendingProtocolV1_1) && !isPaymentLate(view, loanSle))
+    {
+        JLOG(j.warn()) << "Cannot impair a loan that is not late";
+        return tecTOO_SOON;
+    }
+
     Number const lossUnrealized = owedToVault(loanSle);
 
     // The vault may be at a different scale than the loan. Reduce rounding
@@ -296,20 +302,22 @@ LoanManage::impairLoan(
     {
         // Having a loss greater than the vault's unavailable assets
         // will leave the vault in an invalid / inconsistent state.
-        JLOG(j.warn()) << "Vault unrealized loss is too large, and will "
-                          "corrupt the vault.";
+        JLOG(j.warn()) << "Vault unrealized loss is too large, and will corrupt the vault.";
         return tecLIMIT_EXCEEDED;
     }
     view.update(vaultSle);
 
     // Update the Loan object
     loanSle->setFlag(lsfLoanImpaired);
-    auto loanNextDueProxy = loanSle->at(sfNextPaymentDueDate);
-    if (!hasExpired(view, loanNextDueProxy))
+
+    if (!view.rules().enabled(featureLendingProtocolV1_1))
     {
-        // loan payment is not yet late -
-        // move the next payment due date to now
-        loanNextDueProxy = view.parentCloseTime().time_since_epoch().count();
+        auto loanNextDueProxy = loanSle->at(sfNextPaymentDueDate);
+        if (!isPaymentLate(view, loanSle))
+        {
+            // loan payment is not yet late move the next payment due date to now
+            loanNextDueProxy = view.parentCloseTime().time_since_epoch().count();
+        }
     }
     view.update(loanSle);
 
@@ -346,19 +354,24 @@ LoanManage::unimpairLoan(
 
     // Update the Loan object
     loanSle->clearFlag(lsfLoanImpaired);
-    auto const paymentInterval = loanSle->at(sfPaymentInterval);
-    auto const normalPaymentDueDate =
-        std::max(loanSle->at(sfPreviousPaymentDueDate), loanSle->at(sfStartDate)) + paymentInterval;
-    if (!hasExpired(view, normalPaymentDueDate))
+    if (!view.rules().enabled(featureLendingProtocolV1_1))
     {
-        // loan was unimpaired within the payment interval
-        loanSle->at(sfNextPaymentDueDate) = normalPaymentDueDate;
-    }
-    else
-    {
-        // loan was unimpaired after the original payment due date
-        loanSle->at(sfNextPaymentDueDate) =
-            view.parentCloseTime().time_since_epoch().count() + paymentInterval;
+        auto const paymentInterval = loanSle->at(sfPaymentInterval);
+        auto const normalPaymentDueDate =
+            std::max(loanSle->at(sfPreviousPaymentDueDate), loanSle->at(sfStartDate)) +
+            paymentInterval;
+
+        if (!hasExpired(view, normalPaymentDueDate))
+        {
+            // loan was unimpaired within the payment interval
+            loanSle->at(sfNextPaymentDueDate) = normalPaymentDueDate;
+        }
+        else
+        {
+            // loan was unimpaired after the original payment due date
+            loanSle->at(sfNextPaymentDueDate) =
+                view.parentCloseTime().time_since_epoch().count() + paymentInterval;
+        }
     }
     view.update(loanSle);