mixpool: Only ban full nodes for bad UTXO sigs

If an invalid UTXO signature in a pair request message is received from any
node besides a full node, reject the message but do not ban for the error.
However, full nodes are required to check this before relaying.  Ban any
self-reporting full node that still sends these invalid messages

Because the banning logic is currently isolated to the mixpool package and
mixpool is unaware of the capabilities of the peer that provided the message,
give IsBannable an additional service flags argument.  This changes the
semantics of the bannable errors to only be bannable when all of the required
service flags are described.

mixing/mixpool/errors.go

@@ -8,6 +8,7 @@ import (
 	"errors"
 
 	"github.com/decred/dcrd/chaincfg/chainhash"
+	"github.com/decred/dcrd/wire"
 )
 
 // RuleError represents a mixpool rule violation.
@@ -31,14 +32,15 @@ func (e *RuleError) Unwrap() error {
 }
 
 type bannableError struct {
-	s string
+	s        string
+	services wire.ServiceFlag
 }
 
 func (e *bannableError) Error() string {
 	return e.s
 }
 
-func newBannableError(s string) error {
+func newBannableError(s string, services wire.ServiceFlag) error {
 	return &bannableError{
 		s: s,
 	}
@@ -48,54 +50,54 @@ func newBannableError(s string) error {
 var (
 	// ErrChangeDust is returned by AcceptMessage if a pair request's
 	// change amount is dust.
-	ErrChangeDust = newBannableError("change output is dust")
+	ErrChangeDust = newBannableError("change output is dust", 0)
 
 	// ErrLowInput is returned by AcceptMessage when not enough input value
 	// is provided by a pair request to cover the mixed output, any change
 	// output, and the minimum required fee.
-	ErrLowInput = newBannableError("not enough input value, or too low fee")
+	ErrLowInput = newBannableError("not enough input value, or too low fee", 0)
 
 	// ErrInvalidMessageCount is returned by AcceptMessage if a
 	// pair request contains an invalid message count.
-	ErrInvalidMessageCount = newBannableError("message count must be positive")
+	ErrInvalidMessageCount = newBannableError("message count must be positive", 0)
 
 	// ErrInvalidScript is returned by AcceptMessage if a pair request
 	// contains an invalid script.
-	ErrInvalidScript = newBannableError("invalid script")
+	ErrInvalidScript = newBannableError("invalid script", 0)
 
 	// ErrInvalidSessionID is returned by AcceptMessage if the message
 	// contains an invalid session id.
-	ErrInvalidSessionID = newBannableError("invalid session ID")
+	ErrInvalidSessionID = newBannableError("invalid session ID", 0)
 
 	// ErrInvalidSignature is returned by AcceptMessage if the message is
 	// not properly signed for the claimed identity.
-	ErrInvalidSignature = newBannableError("invalid message signature")
+	ErrInvalidSignature = newBannableError("invalid message signature", 0)
 
 	// ErrInvalidTotalMixAmount is returned by AcceptMessage if a pair
 	// request contains the product of the message count and mix amount
 	// that exceeds the total input value.
-	ErrInvalidTotalMixAmount = newBannableError("invalid total mix amount")
+	ErrInvalidTotalMixAmount = newBannableError("invalid total mix amount", 0)
 
 	// ErrInvalidUTXOProof is returned by AcceptMessage if a pair request
 	// fails to prove ownership of each utxo.
-	ErrInvalidUTXOProof = newBannableError("invalid UTXO ownership proof")
+	ErrInvalidUTXOProof = newBannableError("invalid UTXO ownership proof", wire.SFNodeNetwork)
 
 	// ErrMissingUTXOs is returned by AcceptMessage if a pair request
 	// message does not reference any previous UTXOs.
-	ErrMissingUTXOs = newBannableError("pair request contains no UTXOs")
+	ErrMissingUTXOs = newBannableError("pair request contains no UTXOs", 0)
 
 	// ErrPeerPositionOutOfBounds is returned by AcceptMessage if the
 	// position of a peer's own PR is outside of the possible bounds of
 	// the previously seen messages.
-	ErrPeerPositionOutOfBounds = newBannableError("peer position cannot be a valid seen PRs index")
+	ErrPeerPositionOutOfBounds = newBannableError("peer position cannot be a valid seen PRs index", 0)
 )
 
-// IsBannable returns whether the error condition is such that the peer who
-// sent the message should be immediately banned for malicious or buggy
-// behavior.
-func IsBannable(err error) bool {
+// IsBannable returns whether the error condition is such that the peer with
+// capabilities defined by services who sent the message should be immediately
+// banned for malicious or buggy behavior.
+func IsBannable(err error, services wire.ServiceFlag) bool {
 	var be *bannableError
-	return errors.As(err, &be)
+	return errors.As(err, &be) && be.services&services == services
 }
 
 var (

server.go

@@ -1734,7 +1734,7 @@ func (sp *serverPeer) onMixMessage(msg mixing.Message) {
 			nil, nil, nil, mixHashes)
 		return
 	}
-	if mixpool.IsBannable(err) {
+	if mixpool.IsBannable(err, sp.Services()) {
 		reason := fmt.Sprintf("sent malformed mix message: %s", err)
 		sp.server.BanPeer(sp, reason)
 	}