crypto/sha3: ensure unwrapped *sha3.Digest are usable

Updates #75154

Change-Id: I57508b1360702d66a89d44b5bd2728decd075305
Reviewed-on: https://go-review.googlesource.com/c/go/+/747160
Reviewed-by: Filippo Valsorda <filippo@golang.org>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

src/crypto/hmac/hmac_test.go

@@ -7,9 +7,11 @@ package hmac
 import (
 	"crypto/internal/boring"
 	"crypto/internal/cryptotest"
+	"crypto/internal/fips140hash"
 	"crypto/md5"
 	"crypto/sha1"
 	"crypto/sha256"
+	"crypto/sha3"
 	"crypto/sha512"
 	"errors"
 	"fmt"
@@ -596,6 +598,24 @@ func TestNoClone(t *testing.T) {
 	}
 }
 
+func TestSHA3Hash(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		fn   func() hash.Hash
+	}{
+		{
+			"sha3 zero init hash",
+			func() hash.Hash { return justHash{&sha3.SHA3{}} },
+		},
+		{
+			"sha3 zero init hash by linkname",
+			func() hash.Hash { return justHash{fips140hash.Unwrap(&sha3.SHA3{})} },
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) { New(tc.fn, []byte("key")) })
+	}
+}
+
 func TestNonUniqueHash(t *testing.T) {
 	if boring.Enabled {
 		t.Skip("hash.Hash provided by boringcrypto are not comparable")

src/crypto/sha3/sha3.go

@@ -103,8 +103,9 @@ type SHA3 struct {
 }
 
 //go:linkname fips140hash_sha3Unwrap crypto/internal/fips140hash.sha3Unwrap
-func fips140hash_sha3Unwrap(sha3 *SHA3) *sha3.Digest {
-	return &sha3.s
+func fips140hash_sha3Unwrap(s *SHA3) *sha3.Digest {
+	s.init()
+	return &s.s
 }
 
 // New224 creates a new SHA3-224 hash.