Fix breakouts from git root during build

Signed-off-by: Tõnis Tiigi <tonistiigi@gmail.com>

utils/git.go

@@ -10,6 +10,7 @@ import (
 	"path/filepath"
 	"strings"
 
+	"github.com/docker/docker/pkg/symlink"
 	"github.com/docker/docker/pkg/urlutil"
 )
 
@@ -69,7 +70,11 @@ func checkoutGit(fragment, root string) (string, error) {
 	}
 
 	if len(refAndDir) > 1 && len(refAndDir[1]) != 0 {
-		newCtx := filepath.Join(root, refAndDir[1])
+		newCtx, err := symlink.FollowSymlinkInScope(filepath.Join(root, refAndDir[1]), root)
+		if err != nil {
+			return "", fmt.Errorf("Error setting git context, %q not within git root: %s", refAndDir[1], err)
+		}
+
 		fi, err := os.Stat(newCtx)
 		if err != nil {
 			return "", err

utils/git_test.go

@@ -103,6 +103,14 @@ func TestCheckoutGit(t *testing.T) {
 		t.Fatal(err)
 	}
 
+	if err = os.Symlink("../subdir", filepath.Join(gitDir, "parentlink")); err != nil {
+		t.Fatal(err)
+	}
+
+	if err = os.Symlink("/subdir", filepath.Join(gitDir, "absolutelink")); err != nil {
+		t.Fatal(err)
+	}
+
 	if _, err = gitWithinDir(gitDir, "add", "-A"); err != nil {
 		t.Fatal(err)
 	}
@@ -147,6 +155,9 @@ func TestCheckoutGit(t *testing.T) {
 		{":Dockerfile", "", true}, // not a directory error
 		{"master:nosubdir", "", true},
 		{"master:subdir", "FROM scratch\nEXPOSE 5000", false},
+		{"master:parentlink", "FROM scratch\nEXPOSE 5000", false},
+		{"master:absolutelink", "FROM scratch\nEXPOSE 5000", false},
+		{"master:../subdir", "", true},
 		{"test", "FROM scratch\nEXPOSE 3000", false},
 		{"test:", "FROM scratch\nEXPOSE 3000", false},
 		{"test:subdir", "FROM busybox\nEXPOSE 5000", false},