Fix pollbot invalid inputs

pollbot/pollbot/http.go

@@ -87,21 +87,31 @@ func (h *HTTPSrv) handleVote(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 	vstr := r.URL.Query().Get("")
-	vote := NewVoteFromEncoded(vstr)
-	// WithoutCancel: the browser submits the vote and may close the connection;
-	// DB writes and poll result updates must complete regardless.
-	ctx := context.WithoutCancel(r.Context())
-	if err := h.db.CastVote(ctx, username, vote); err != nil {
-		h.Errorf("failed to cast vote: %s", err)
+	vote, err := NewVoteFromEncoded(vstr)
+	if err != nil {
+		h.Debug("invalid vote payload: %s", err)
 		h.showError(w)
 		return
 	}
+	// WithoutCancel: the browser submits the vote and may close the connection;
+	// DB writes and poll result updates must complete regardless.
+	ctx := context.WithoutCancel(r.Context())
 	convID, resultMsgID, numChoices, err := h.db.GetPollInfo(ctx, vote.ID)
 	if err != nil {
 		h.Errorf("failed to find poll result msg: %s", err)
 		h.showError(w)
 		return
 	}
+	if vote.Choice < 1 || vote.Choice > numChoices {
+		h.Debug("vote choice %d out of range for poll %q", vote.Choice, vote.ID)
+		h.showError(w)
+		return
+	}
+	if err := h.db.CastVote(ctx, username, vote); err != nil {
+		h.Errorf("failed to cast vote: %s", err)
+		h.showError(w)
+		return
+	}
 	tally, err := h.db.GetTally(ctx, vote.ID)
 	if err != nil {
 		h.Errorf("failed to get tally: %s", err)

pollbot/pollbot/vote.go

@@ -1,6 +1,8 @@
 package pollbot
 
 import (
+	"fmt"
+
 	"github.com/keybase/managed-bots/base"
 )
 
@@ -21,11 +23,19 @@ func NewVote(id string, choice int) Vote {
 	}
 }
 
-func NewVoteFromEncoded(sdat string) Vote {
+func NewVoteFromEncoded(sdat string) (Vote, error) {
 	var ve voteToEncode
-	dat, _ := base.URLEncoder().DecodeString(sdat)
-	_ = base.MsgpackDecode(&ve, dat)
-	return Vote(ve)
+	dat, err := base.URLEncoder().DecodeString(sdat)
+	if err != nil {
+		return Vote{}, err
+	}
+	if err := base.MsgpackDecode(&ve, dat); err != nil {
+		return Vote{}, err
+	}
+	if ve.ID == "" {
+		return Vote{}, fmt.Errorf("missing poll id")
+	}
+	return Vote(ve), nil
 }
 
 func (v Vote) Encode() string {