math/big: validate result of Float.GobDecode

Fixes #57946

Change-Id: Ia499ebfd8801432122f89fdf6bda4d1e7b6dd832
GitHub-Last-Rev: 29e099388680bc5b7075e0fa63499b39697579ca
GitHub-Pull-Request: golang/go#57951
Reviewed-on: https://go-review.googlesource.com/c/go/+/463017
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@google.com>

3 files changed, 37 insertions, 4 deletions

diff --git a/src/math/big/float.go b/src/math/big/float.go
index 84666d817b..2f0635a03b 100644
--- a/src/math/big/float.go
+++ b/src/math/big/float.go
@@ -365,20 +365,27 @@ func (x *Float) validate() {
 		// avoid performance bugs
 		panic("validate called but debugFloat is not set")
 	}
+	if msg := x.validate0(); msg != "" {
+		panic(msg)
+	}
+}
+
+func (x *Float) validate0() string {
 	if x.form != finite {
-		return
+		return ""
 	}
 	m := len(x.mant)
 	if m == 0 {
-		panic("nonzero finite number with empty mantissa")
+		return "nonzero finite number with empty mantissa"
 	}
 	const msb = 1 << (_W - 1)
 	if x.mant[m-1]&msb == 0 {
-		panic(fmt.Sprintf("msb not set in last word %#x of %s", x.mant[m-1], x.Text('p', 0)))
+		return fmt.Sprintf("msb not set in last word %#x of %s", x.mant[m-1], x.Text('p', 0))
 	}
 	if x.prec == 0 {
-		panic("zero precision finite number")
+		return "zero precision finite number"
 	}
+	return ""
 }
 
 // round rounds z according to z.mode to z.prec bits and sets z.acc accordingly.
diff --git a/src/math/big/floatmarsh.go b/src/math/big/floatmarsh.go
index 990e085abe..2a78c69e34 100644
--- a/src/math/big/floatmarsh.go
+++ b/src/math/big/floatmarsh.go
@@ -99,6 +99,10 @@ func (z *Float) GobDecode(buf []byte) error {
 		z.SetPrec(uint(oldPrec))
 	}
 
+	if msg := z.validate0(); msg != "" {
+		return errors.New("Float.GobDecode: " + msg)
+	}
+
 	return nil
 }
 
diff --git a/src/math/big/floatmarsh_test.go b/src/math/big/floatmarsh_test.go
index 401f45a51f..20def68a6d 100644
--- a/src/math/big/floatmarsh_test.go
+++ b/src/math/big/floatmarsh_test.go
@@ -9,6 +9,7 @@ import (
 	"encoding/gob"
 	"encoding/json"
 	"io"
+	"strings"
 	"testing"
 )
 
@@ -149,3 +150,24 @@ func TestFloatGobDecodeShortBuffer(t *testing.T) {
 		}
 	}
 }
+
+func TestFloatGobDecodeInvalid(t *testing.T) {
+	for _, tc := range []struct {
+		buf []byte
+		msg string
+	}{
+		{
+			[]byte{0x1, 0x2a, 0x20, 0x20, 0x20, 0x20, 0x0, 0x20, 0x20, 0x20, 0x0, 0x20, 0x20, 0x20, 0x20, 0x0, 0x0, 0x0, 0x0, 0xc},
+			"Float.GobDecode: msb not set in last word",
+		},
+		{
+			[]byte{1, 2, 0, 0, 0, 0, 0, 0, 0, 0, 0},
+			"Float.GobDecode: nonzero finite number with empty mantissa",
+		},
+	} {
+		err := NewFloat(0).GobDecode(tc.buf)
+		if err == nil || !strings.HasPrefix(err.Error(), tc.msg) {
+			t.Errorf("expected GobDecode error prefix: %s, got: %v", tc.msg, err)
+		}
+	}
+}