selftests/ovl: add third selftest for "override_creds"

Add a simple test to verify that the new "override_creds" option works.

Link: https://lore.kernel.org/r/20250219-work-overlayfs-v3-4-46af55e4ceda@kernel.org
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: brauner

tools/testing/selftests/filesystems/overlayfs/set_layers_via_fds.c

@@ -8,6 +8,7 @@
 #include <string.h>
 #include <sys/socket.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/mount.h>
 #include <unistd.h>
 
@@ -442,4 +443,83 @@ TEST_F(set_layers_via_fds, set_override_creds_invalid)
 	ASSERT_EQ(close(fd_userns2), 0);
 }
 
+TEST_F(set_layers_via_fds, set_override_creds_nomknod)
+{
+	int fd_context, fd_tmpfs, fd_overlay;
+	int layer_fds[] = { [0 ... 3] = -EBADF };
+	pid_t pid;
+	int pidfd;
+
+	ASSERT_EQ(unshare(CLONE_NEWNS), 0);
+	ASSERT_EQ(sys_mount(NULL, "/", NULL, MS_SLAVE | MS_REC, NULL), 0);
+
+	fd_context = sys_fsopen("tmpfs", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_CMD_CREATE, NULL, NULL, 0), 0);
+	fd_tmpfs = sys_fsmount(fd_context, 0, 0);
+	ASSERT_GE(fd_tmpfs, 0);
+	ASSERT_EQ(close(fd_context), 0);
+
+	ASSERT_EQ(mkdirat(fd_tmpfs, "w", 0755), 0);
+	ASSERT_EQ(mkdirat(fd_tmpfs, "u", 0755), 0);
+	ASSERT_EQ(mkdirat(fd_tmpfs, "l1", 0755), 0);
+	ASSERT_EQ(mkdirat(fd_tmpfs, "l2", 0755), 0);
+
+	layer_fds[0] = openat(fd_tmpfs, "w", O_DIRECTORY);
+	ASSERT_GE(layer_fds[0], 0);
+
+	layer_fds[1] = openat(fd_tmpfs, "u", O_DIRECTORY);
+	ASSERT_GE(layer_fds[1], 0);
+
+	layer_fds[2] = openat(fd_tmpfs, "l1", O_DIRECTORY);
+	ASSERT_GE(layer_fds[2], 0);
+
+	layer_fds[3] = openat(fd_tmpfs, "l2", O_DIRECTORY);
+	ASSERT_GE(layer_fds[3], 0);
+
+	ASSERT_EQ(sys_move_mount(fd_tmpfs, "", -EBADF, "/tmp", MOVE_MOUNT_F_EMPTY_PATH), 0);
+	ASSERT_EQ(close(fd_tmpfs), 0);
+
+	fd_context = sys_fsopen("overlay", 0);
+	ASSERT_GE(fd_context, 0);
+
+	ASSERT_NE(sys_fsconfig(fd_context, FSCONFIG_SET_FD, "lowerdir", NULL, layer_fds[2]), 0);
+
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FD, "workdir",   NULL, layer_fds[0]), 0);
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FD, "upperdir",  NULL, layer_fds[1]), 0);
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FD, "lowerdir+", NULL, layer_fds[2]), 0);
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FD, "lowerdir+", NULL, layer_fds[3]), 0);
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "userxattr", NULL, 0), 0);
+
+	pid = create_child(&pidfd, 0);
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		if (!cap_down(CAP_MKNOD))
+			_exit(EXIT_FAILURE);
+
+		if (!cap_down(CAP_SYS_ADMIN))
+			_exit(EXIT_FAILURE);
+
+		if (sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "override_creds", NULL, 0))
+			_exit(EXIT_FAILURE);
+
+		_exit(EXIT_SUCCESS);
+	}
+	ASSERT_EQ(sys_waitid(P_PID, pid, NULL, WEXITED), 0);
+	ASSERT_GE(close(pidfd), 0);
+
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_CMD_CREATE, NULL, NULL, 0), 0);
+
+	fd_overlay = sys_fsmount(fd_context, 0, 0);
+	ASSERT_GE(fd_overlay, 0);
+
+	ASSERT_EQ(sys_move_mount(fd_overlay, "", -EBADF, "/set_layers_via_fds", MOVE_MOUNT_F_EMPTY_PATH), 0);
+	ASSERT_EQ(mknodat(fd_overlay, "dev-zero", S_IFCHR | 0644, makedev(1, 5)), -1);
+	ASSERT_EQ(errno, EPERM);
+
+	ASSERT_EQ(close(fd_context), 0);
+	ASSERT_EQ(close(fd_overlay), 0);
+}
+
 TEST_HARNESS_MAIN

tools/testing/selftests/filesystems/utils.c

@@ -472,3 +472,30 @@ int caps_down(void)
 	cap_free(caps);
 	return fret;
 }
+
+/* cap_down - lower an effective cap */
+int cap_down(cap_value_t down)
+{
+	bool fret = false;
+	cap_t caps = NULL;
+	cap_value_t cap = down;
+	int ret = -1;
+
+	caps = cap_get_proc();
+	if (!caps)
+		goto out;
+
+	ret = cap_set_flag(caps, CAP_EFFECTIVE, 1, &cap, 0);
+	if (ret)
+		goto out;
+
+	ret = cap_set_proc(caps);
+	if (ret)
+		goto out;
+
+	fret = true;
+
+out:
+	cap_free(caps);
+	return fret;
+}

tools/testing/selftests/filesystems/utils.h

@@ -24,6 +24,7 @@ extern int get_userns_fd(unsigned long nsid, unsigned long hostid,
 			 unsigned long range);
 
 extern int caps_down(void);
+extern int cap_down(cap_value_t down);
 
 extern bool switch_ids(uid_t uid, gid_t gid);