crypto/sha512: fix casting of d.nx in UnmarshalBinary

Fixes #29541

Change-Id: I006915b020b6e710298c32c05e3de77a7f9b54f3
GitHub-Last-Rev: c7a90a4bbe17fd1904c0942e8b343264fb85cba1
GitHub-Pull-Request: golang/go#29542
Reviewed-on: https://go-review.googlesource.com/c/156277
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/crypto/sha512/sha512.go

@@ -191,7 +191,7 @@ func (d *digest) UnmarshalBinary(b []byte) error {
 	b, d.h[7] = consumeUint64(b)
 	b = b[copy(d.x[:], b):]
 	b, d.len = consumeUint64(b)
-	d.nx = int(d.len) % chunk
+	d.nx = int(d.len % chunk)
 	return nil
 }
 

src/crypto/sha512/sha512_test.go

@@ -11,6 +11,7 @@ import (
 	"crypto/rand"
 	"encoding"
 	"encoding/hex"
+	"fmt"
 	"hash"
 	"io"
 	"testing"
@@ -831,6 +832,62 @@ func TestBlockGeneric(t *testing.T) {
 	}
 }
 
+// Tests for unmarshaling hashes that have hashed a large amount of data
+// The initial hash generation is omitted from the test, because it takes a long time.
+// The test contains some already-generated states, and their expected sums
+// Tests a problem that is outlined in Github issue #29541
+// The problem is triggered when an amount of data has been hashed for which
+// the data length has a 1 in the 32nd bit. When casted to int, this changes
+// the sign of the value, and causes the modulus operation to return a
+// different result.
+type unmarshalTest struct {
+	state string
+	sum   string
+}
+
+var largeUnmarshalTests = []unmarshalTest{
+	// Data length: 6_565_544_823
+	unmarshalTest{
+		state: "sha\aηe\x0f\x0f\xe1r]#\aoJ!.{5B\xe4\x140\x91\xdd\x00a\xe1\xb3E&\xb9\xbb\aJ\x9f^\x9f\x03ͺD\x96H\x80\xb0X\x9d\xdeʸ\f\xf7:\xd5\xe6'\xb9\x93f\xddA\xf0~\xe1\x02\x14\x00\x01\x02\x03\x04\x05\x06\a\b\t\n\v\f\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuv\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x87VCw",
+		sum:   "12d612357a1dbc74a28883dff79b83e7d2b881ae40d7a67fd7305490bc8a641cd1ce9ece598192080d6e9ac7e75d5988567a58a9812991299eb99a04ecb69523",
+	},
+	unmarshalTest{
+		state: "sha\a2\xd2\xdc\xf5\xd7\xe2\xf9\x97\xaa\xe7}Fϱ\xbc\x8e\xbf\x12h\x83Z\xa1\xc7\xf5p>bfS T\xea\xee\x1e\xa6Z\x9c\xa4ڶ\u0086\bn\xe47\x8fsGs3\xe0\xda\\\x9dqZ\xa5\xf6\xd0kM\xa1\xf2\x00\x01\x02\x03\x04\x05\x06\a\b\t\n\v\f\r\x0e\x0f\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuv\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\xa7VCw",
+		sum:   "94a04b9a901254cd94ca0313557e4be3ab1ca86e920c1f3efdc22d361e9ae12be66bc6d6dc5db79a0a4aa6eca6f293c1e9095bbae127ae405f6c325478343299",
+	},
+}
+
+func safeSum(h hash.Hash) (sum []byte, err error) {
+	defer func() {
+		if r := recover(); r != nil {
+			err = fmt.Errorf("sum panic: %v", r)
+		}
+	}()
+
+	return h.Sum(nil), nil
+}
+
+func TestLargeHashes(t *testing.T) {
+	for i, test := range largeUnmarshalTests {
+
+		h := New()
+		if err := h.(encoding.BinaryUnmarshaler).UnmarshalBinary([]byte(test.state)); err != nil {
+			t.Errorf("test %d could not unmarshal: %v", i, err)
+			continue
+		}
+
+		sum, err := safeSum(h)
+		if err != nil {
+			t.Errorf("test %d could not sum: %v", i, err)
+			continue
+		}
+
+		if fmt.Sprintf("%x", sum) != test.sum {
+			t.Errorf("test %d sum mismatch: expect %s got %x", i, test.sum, sum)
+		}
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)