[release-branch.go1.17] math/big: check buffer lengths in GobDecode

In Float.GobDecode and Rat.GobDecode, check buffer sizes before
indexing slices.

Updates #53871
Fixes #54094

Change-Id: I1b652c32c2bc7a0e8aa7620f7be9b2740c568b0a
Reviewed-on: https://go-review.googlesource.com/c/go/+/417774
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Tatiana Bradley <tatiana@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
(cherry picked from commit 055113ef364337607e3e72ed7d48df67fde6fc66)
Reviewed-on: https://go-review.googlesource.com/c/go/+/419814
Reviewed-by: Julie Qiu <julieqiu@google.com>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/math/big/ratmarsh_test.go b/src/math/big/ratmarsh_test.go
index 351d109f8d..55a9878bb8 100644
--- a/src/math/big/ratmarsh_test.go
+++ b/src/math/big/ratmarsh_test.go
@@ -123,3 +123,15 @@ func TestRatXMLEncoding(t *testing.T) {
 		}
 	}
 }
+
+func TestRatGobDecodeShortBuffer(t *testing.T) {
+	for _, tc := range [][]byte{
+		[]byte{0x2},
+		[]byte{0x2, 0x0, 0x0, 0x0, 0xff},
+	} {
+		err := NewRat(1, 2).GobDecode(tc)
+		if err == nil {
+			t.Error("expected GobDecode to return error for malformed input")
+		}
+	}
+}