Preserve permissions when checking lower

The code was wrongly assuming that only read access was
required from the lower overlay when checking for permissions.
This allowed non-writable files to be writable in the overlay.

Fixes #316

PiperOrigin-RevId: 255263686

pkg/sentry/fs/inode_overlay.go

@@ -537,12 +537,6 @@ func overlayCheck(ctx context.Context, o *overlayEntry, p PermMask) error {
 	if o.upper != nil {
 		err = o.upper.check(ctx, p)
 	} else {
-		if p.Write {
-			// Since writes will be redirected to the upper filesystem, the lower
-			// filesystem need not be writable, but must be readable for copy-up.
-			p.Write = false
-			p.Read = true
-		}
 		err = o.lower.check(ctx, p)
 	}
 	o.copyMu.RUnlock()

runsc/boot/fs.go

@@ -85,6 +85,19 @@ func addOverlay(ctx context.Context, conf *Config, lower *fs.Inode, name string,
 	if err != nil {
 		return nil, fmt.Errorf("creating tmpfs overlay: %v", err)
 	}
+
+	// Replicate permissions and owner from lower to upper mount point.
+	attr, err := lower.UnstableAttr(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("reading attributes from lower mount point: %v", err)
+	}
+	if !upper.InodeOperations.SetPermissions(ctx, upper, attr.Perms) {
+		return nil, fmt.Errorf("error setting permission to upper mount point")
+	}
+	if err := upper.InodeOperations.SetOwner(ctx, upper, attr.Owner); err != nil {
+		return nil, fmt.Errorf("setting owner to upper mount point: %v", err)
+	}
+
 	return fs.NewOverlayRoot(ctx, upper, lower, upperFlags)
 }
 

test/syscalls/BUILD

@@ -240,7 +240,7 @@ syscall_test(
 syscall_test(test = "//test/syscalls/linux:munmap_test")
 
 syscall_test(
-    add_overlay = False,  # TODO(gvisor.dev/issue/316): enable when fixed.
+    add_overlay = True,
     test = "//test/syscalls/linux:open_create_test",
 )