[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, 7 insertions, 0 deletions

diff --git a/src/math/big/floatmarsh.go b/src/math/big/floatmarsh.go
index d1c1dab069..990e085abe 100644
--- a/src/math/big/floatmarsh.go
+++ b/src/math/big/floatmarsh.go
@@ -8,6 +8,7 @@ package big
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 )
 
@@ -67,6 +68,9 @@ func (z *Float) GobDecode(buf []byte) error {
 		*z = Float{}
 		return nil
 	}
+	if len(buf) < 6 {
+		return errors.New("Float.GobDecode: buffer too small")
+	}
 
 	if buf[0] != floatGobVersion {
 		return fmt.Errorf("Float.GobDecode: encoding version %d not supported", buf[0])
@@ -83,6 +87,9 @@ func (z *Float) GobDecode(buf []byte) error {
 	z.prec = binary.BigEndian.Uint32(buf[2:])
 
 	if z.form == finite {
+		if len(buf) < 10 {
+			return errors.New("Float.GobDecode: buffer too small for finite form float")
+		}
 		z.exp = int32(binary.BigEndian.Uint32(buf[6:]))
 		z.mant = z.mant.setBytes(buf[10:])
 	}