[release-branch.go1.13-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/+/575233

Author: katiehockman

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 {