Fix direct_aux sign mode handler

CHANGELOG.md

@@ -52,7 +52,7 @@ Ref: https://keepachangelog.com/en/1.0.0/
 * [\#10326](https://github.com/cosmos/cosmos-sdk/pull/10326) `x/authz` add query all grants by granter query.
 * [\#10024](https://github.com/cosmos/cosmos-sdk/pull/10024) `store/cachekv` performance improvement by reduced growth factor for iterator ranging by using binary searches to find dirty items when unsorted key count >= 1024
 * [\#10348](https://github.com/cosmos/cosmos-sdk/pull/10348) Add `fee.{payer,granter}` and `tip` fields to StdSignDoc for signing tipped transactions.
-* [\#10208](https://github.com/cosmos/cosmos-sdk/pull/10208) Add `SignModeTxMiddleware` for checking sign modes and `TipsTxMiddleware` for transferring tips.
+* [\#10208](https://github.com/cosmos/cosmos-sdk/pull/10208) Add `TipsTxMiddleware` for transferring tips.
 
 ### API Breaking Changes
 

types/tx/types.go

@@ -112,14 +112,14 @@ func (t *Tx) GetSigners() []sdk.AccAddress {
 	}
 
 	// ensure any specified fee payer is included in the required signers (at the end)
-	feePayer := t.AuthInfo.Fee.Payer
-	if feePayer != "" && !seen[feePayer] {
-		payerAddr, err := sdk.AccAddressFromBech32(feePayer)
+	fee := t.AuthInfo.Fee
+	if fee != nil && fee.Payer != "" && !seen[fee.Payer] {
+		payerAddr, err := sdk.AccAddressFromBech32(fee.Payer)
 		if err != nil {
 			panic(err)
 		}
 		signers = append(signers, payerAddr)
-		seen[feePayer] = true
+		seen[fee.Payer] = true
 	}
 
 	return signers

x/auth/tx/builder.go

@@ -49,10 +49,8 @@ type ExtensionOptionsTxBuilder interface {
 func newBuilder() *wrapper {
 	return &wrapper{
 		tx: &tx.Tx{
-			Body: &tx.TxBody{},
-			AuthInfo: &tx.AuthInfo{
-				Fee: &tx.Fee{},
-			},
+			Body:     &tx.TxBody{},
+			AuthInfo: &tx.AuthInfo{},
 		},
 	}
 }
@@ -133,9 +131,9 @@ func (w *wrapper) GetFee() sdk.Coins {
 }
 
 func (w *wrapper) FeePayer() sdk.AccAddress {
-	feePayer := w.tx.AuthInfo.Fee.Payer
-	if feePayer != "" {
-		payerAddr, err := sdk.AccAddressFromBech32(feePayer)
+	fee := w.tx.AuthInfo.Fee
+	if fee != nil && fee.Payer != "" {
+		payerAddr, err := sdk.AccAddressFromBech32(fee.Payer)
 		if err != nil {
 			panic(err)
 		}

x/auth/tx/direct_aux.go

@@ -44,18 +44,38 @@ func (signModeDirectAuxHandler) GetSignBytes(
 		return nil, sdkerrors.ErrInvalidRequest.Wrapf("got empty pubkey for signer #%d in %s handler", data.SignerIndex, signingtypes.SignMode_SIGN_MODE_DIRECT_AUX)
 	}
 
-	// Fee payer cannot use SIGN_MODE_DIRECT_AUX, because SIGN_MODE_DIRECT_AUX
-	// does not sign over fees, which would create malleability issues.
-	feeTx, ok := tx.(sdk.FeeTx)
-	if !ok {
-		return nil, sdkerrors.ErrInvalidType.Wrapf("expected %T, got %T", sdk.FeeTx(nil), tx)
-	}
 	addr := data.Address
 	if addr == "" {
 		return nil, sdkerrors.Wrapf(sdkerrors.ErrInvalidRequest, "got empty address in %s handler", signingtypes.SignMode_SIGN_MODE_DIRECT_AUX)
 	}
-	if feeTx.FeePayer().String() == data.Address {
-		return nil, sdkerrors.ErrUnauthorized.Wrapf("fee payer %s cannot sign with %s", feeTx.FeePayer(), signingtypes.SignMode_SIGN_MODE_DIRECT_AUX)
+
+	feePayer := protoTx.FeePayer().String()
+
+	// Fee payer cannot use SIGN_MODE_DIRECT_AUX, because SIGN_MODE_DIRECT_AUX
+	// does not sign over fees, which would create malleability issues.
+	if feePayer == data.Address {
+		tip := protoTx.tx.GetAuthInfo().GetTip()
+		var tipper string
+		if tip != nil {
+			tipper = tip.Tipper
+		}
+
+		// In general, the transactions with tips require that the fee payer and
+		// tipper are two different persons.
+		//
+		// However, recall that `protoTx.FeePayer()` is defined as
+		// `tx.AuthInfo.Fee.Payer` if not nil, or defaults to `tx.GetSigners[0]`.
+		// When fee payer is `tx.GetSigners[0]` (i.e. the tx.AuthInfo.Fee.Payer
+		// field is not set), then the tipper and the fee payer
+		// are the same person. Concretely, this happens when the tipper signs
+		// their tx before relaying it to the fee payer.
+		if tipper == feePayer {
+			if protoTx.tx.GetAuthInfo().GetFee() != nil {
+				return nil, sdkerrors.ErrUnauthorized.Wrapf("tipper %s cannot be fee payer", tipper)
+			}
+		} else {
+			return nil, sdkerrors.ErrUnauthorized.Wrapf("fee payer %s cannot sign with %s", feePayer, signingtypes.SignMode_SIGN_MODE_DIRECT_AUX)
+		}
 	}
 
 	signDocDirectAux := types.SignDocDirectAux{