From 2017d88dbc096381d4f348d2fb08bfb3c2b7ed73 Mon Sep 17 00:00:00 2001
From: Katie Hockman <katie@golang.org>
Date: Mon, 14 Oct 2019 16:42:21 -0400
Subject: [PATCH] [release-branch.go1.12-security] crypto/dsa: prevent bad
 public keys from causing panic

dsa.Verify might currently use a nil s inverse in a
multiplication if the public key contains a non-prime Q,
causing a panic. Change this to check that the mod
inverse exists before using it.

Fixes CVE-2019-17596

Change-Id: I94d5f3cc38f1b5d52d38dcb1d253c71b7fd1cae7
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/572809
Reviewed-by: Filippo Valsorda <valsorda@google.com>
(cherry picked from commit 9119dfb0511326d4485b248b83d4fde19c95d0f7)
Reviewed-on: https://team-review.git.corp.google.com/c/golang/go-private/+/575232
---
 src/crypto/dsa/dsa.go | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/src/crypto/dsa/dsa.go b/src/crypto/dsa/dsa.go
index 575314b1b4689..2fc4f1f05bfb0 100644
--- a/src/crypto/dsa/dsa.go
+++ b/src/crypto/dsa/dsa.go
@@ -279,6 +279,9 @@ func Verify(pub *PublicKey, hash []byte, r, s *big.Int) bool {
 	}
 
 	w := new(big.Int).ModInverse(s, pub.Q)
+	if w == nil {
+		return false
+	}
 
 	n := pub.Q.BitLen()
 	if n&7 != 0 {