serve docker: reject volume names resolving to the base directory itself GHSA-p6vx-hf7p-98j6

An empty or "." volume name joined onto the base directory resolves to the
base directory itself. newVolume does not call validate, so such a name
would mount a remote over the base directory and shadow every other
volume's mountpoint.

Require the resolved mountpoint to be a strict descendant of the base
directory so these degenerate names are refused.

cmd/serve/docker/docker_test.go

@@ -245,6 +245,16 @@ func TestDockerPluginVolumeNameConfined(t *testing.T) {
 	// The volume must not have been registered.
 	_, err = drv.Get(&docker.GetRequest{Name: escape})
 	assert.Error(t, err)
+
+	// A name that resolves to the base directory itself (empty or ".")
+	// must also be rejected, since mounting there would shadow every
+	// other volume.
+	for _, name := range []string{"", ".", filepath.Join("a", "..")} {
+		volReq.Name = name
+		err = drv.Create(volReq)
+		assertErrorContains(t, err, "resolves outside the base directory",
+			"name %q should be rejected", name)
+	}
 }
 
 // TestDockerPluginRestoreStateConfined checks that a persisted state file

cmd/serve/docker/volume.go

@@ -56,16 +56,18 @@ type VolInfo struct {
 }
 
 // volumeMountPath returns the mountpoint for a volume called name below
-// root, together with whether that path stays confined within root.
+// root, together with whether that path is a strict descendant of root.
 //
 // filepath.Join cleans its result, collapsing any ".." components in the
 // volume name, so a crafted name could otherwise resolve to an arbitrary
-// host path outside root. Callers must reject names for which confined is
-// false rather than creating a directory and mounting there.
+// host path outside root, or to root itself (for an empty or "." name)
+// where a mount would shadow every other volume. Callers must reject names
+// for which confined is false rather than creating a directory and mounting
+// there.
 func volumeMountPath(root, name string) (path string, confined bool) {
 	path = filepath.Join(root, name)
 	root = filepath.Clean(root)
-	return path, path == root || strings.HasPrefix(path, root+string(filepath.Separator))
+	return path, strings.HasPrefix(path, root+string(filepath.Separator))
 }
 
 func newVolume(ctx context.Context, name string, volOpt VolOpts, drv *Driver) (*Volume, error) {