archive/tar: terminate when reading malformed sparse files

Fixes #10968.

Change-Id: I027bc571a71629ac49c2a0ff101b2950af6e7531
Reviewed-on: https://go-review.googlesource.com/10482
Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: David Symonds <dsymonds@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/archive/tar/reader.go

@@ -791,6 +791,9 @@ func (sfr *sparseFileReader) Read(b []byte) (n int, err error) {
 		// Otherwise, we're at the end of the file
 		return 0, io.EOF
 	}
+	if sfr.tot < sfr.sp[0].offset {
+		return 0, io.ErrUnexpectedEOF
+	}
 	if sfr.pos < sfr.sp[0].offset {
 		// We're in a hole
 		n = sfr.readHole(b, sfr.sp[0].offset)

src/archive/tar/reader_test.go

@@ -757,3 +757,22 @@ func TestNegativeHdrSize(t *testing.T) {
 	}
 	io.Copy(ioutil.Discard, r)
 }
+
+// This used to hang in (*sparseFileReader).readHole due to missing
+// verification of sparse offsets against file size.
+func TestIssue10968(t *testing.T) {
+	f, err := os.Open("testdata/issue10968.tar")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer f.Close()
+	r := NewReader(f)
+	_, err = r.Next()
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = io.Copy(ioutil.Discard, r)
+	if err != io.ErrUnexpectedEOF {
+		t.Fatalf("expected %q, got %q", io.ErrUnexpectedEOF, err)
+	}
+}