Merge tag 'vfs-6.15-rc1.overlayfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs

Pull vfs overlayfs updates from Christian Brauner:
 "Currently overlayfs uses the mounter's credentials for its
  override_creds() calls. That provides a consistent permission model.

  This patches allows a caller to instruct overlayfs to use its
  credentials instead. The caller must be located in the same user
  namespace hierarchy as the user namespace the overlayfs instance will
  be mounted in. This provides a consistent and simple security model.

  With this it is possible to e.g., mount an overlayfs instance where
  the mounter must have CAP_SYS_ADMIN but the credentials used for
  override_creds() have dropped CAP_SYS_ADMIN. It also allows the usage
  of custom fs{g,u}id different from the callers and other tweaks"

* tag 'vfs-6.15-rc1.overlayfs' of git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs:
  selftests/ovl: add third selftest for "override_creds"
  selftests/ovl: add second selftest for "override_creds"
  selftests/filesystems: add utils.{c,h}
  selftests/ovl: add first selftest for "override_creds"
  ovl: allow to specify override credentials

7 files changed, 926 insertions, 12 deletions

diff --git a/Documentation/filesystems/overlayfs.rst b/Documentation/filesystems/overlayfs.rst
index 6245b67ae9e0..2db379b4b31e 100644
--- a/Documentation/filesystems/overlayfs.rst
+++ b/Documentation/filesystems/overlayfs.rst
@@ -292,13 +292,27 @@ rename or unlink will of course be noticed and handled).
 Permission model
 ----------------
 
+An overlay filesystem stashes credentials that will be used when
+accessing lower or upper filesystems.
+
+In the old mount api the credentials of the task calling mount(2) are
+stashed. In the new mount api the credentials of the task creating the
+superblock through FSCONFIG_CMD_CREATE command of fsconfig(2) are
+stashed.
+
+Starting with kernel v6.15 it is possible to use the "override_creds"
+mount option which will cause the credentials of the calling task to be
+recorded. Note that "override_creds" is only meaningful when used with
+the new mount api as the old mount api combines setting options and
+superblock creation in a single mount(2) syscall.
+
 Permission checking in the overlay filesystem follows these principles:
 
  1) permission check SHOULD return the same result before and after copy up
 
  2) task creating the overlay mount MUST NOT gain additional privileges
 
- 3) non-mounting task MAY gain additional privileges through the overlay,
+ 3) task[*] MAY gain additional privileges through the overlay,
     compared to direct access on underlying lower or upper filesystems
 
 This is achieved by performing two permission checks on each access:
@@ -306,7 +320,7 @@ This is achieved by performing two permission checks on each access:
  a) check if current task is allowed access based on local DAC (owner,
     group, mode and posix acl), as well as MAC checks
 
- b) check if mounting task would be allowed real operation on lower or
+ b) check if stashed credentials would be allowed real operation on lower or
     upper layer based on underlying filesystem permissions, again including
     MAC checks
 
@@ -315,10 +329,10 @@ are copied up.  On the other hand it can result in server enforced
 permissions (used by NFS, for example) being ignored (3).
 
 Check (b) ensures that no task gains permissions to underlying layers that
-the mounting task does not have (2).  This also means that it is possible
+the stashed credentials do not have (2).  This also means that it is possible
 to create setups where the consistency rule (1) does not hold; normally,
-however, the mounting task will have sufficient privileges to perform all
-operations.
+however, the stashed credentials will have sufficient privileges to
+perform all operations.
 
 Another way to demonstrate this model is drawing parallels between::
 
diff --git a/fs/overlayfs/params.c b/fs/overlayfs/params.c
index 1115c22deca0..6759f7d040c8 100644
--- a/fs/overlayfs/params.c
+++ b/fs/overlayfs/params.c
@@ -59,6 +59,7 @@ enum ovl_opt {
 	Opt_metacopy,
 	Opt_verity,
 	Opt_volatile,
+	Opt_override_creds,
 };
 
 static const struct constant_table ovl_parameter_bool[] = {
@@ -155,6 +156,7 @@ const struct fs_parameter_spec ovl_parameter_spec[] = {
 	fsparam_enum("metacopy",            Opt_metacopy, ovl_parameter_bool),
 	fsparam_enum("verity",              Opt_verity, ovl_parameter_verity),
 	fsparam_flag("volatile",            Opt_volatile),
+	fsparam_flag_no("override_creds",   Opt_override_creds),
 	{}
 };
 
@@ -662,6 +664,29 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
 	case Opt_userxattr:
 		config->userxattr = true;
 		break;
+	case Opt_override_creds: {
+		const struct cred *cred = NULL;
+
+		if (result.negated) {
+			swap(cred, ofs->creator_cred);
+			put_cred(cred);
+			break;
+		}
+
+		if (!current_in_userns(fc->user_ns)) {
+			err = -EINVAL;
+			break;
+		}
+
+		cred = prepare_creds();
+		if (cred)
+			swap(cred, ofs->creator_cred);
+		else
+			err = -ENOMEM;
+
+		put_cred(cred);
+		break;
+	}
 	default:
 		pr_err("unrecognized mount option \"%s\" or missing value\n",
 		       param->key);
diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
index 86ae6f6da36b..61e21c3129e8 100644
--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -1305,6 +1305,7 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 {
 	struct ovl_fs *ofs = sb->s_fs_info;
 	struct ovl_fs_context *ctx = fc->fs_private;
+	const struct cred *old_cred = NULL;
 	struct dentry *root_dentry;
 	struct ovl_entry *oe;
 	struct ovl_layer *layers;
@@ -1318,10 +1319,15 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 	sb->s_d_op = &ovl_dentry_operations;
 
 	err = -ENOMEM;
-	ofs->creator_cred = cred = prepare_creds();
+	if (!ofs->creator_cred)
+		ofs->creator_cred = cred = prepare_creds();
+	else
+		cred = (struct cred *)ofs->creator_cred;
 	if (!cred)
 		goto out_err;
 
+	old_cred = ovl_override_creds(sb);
+
 	err = ovl_fs_params_verify(ctx, &ofs->config);
 	if (err)
 		goto out_err;
@@ -1481,11 +1487,19 @@ int ovl_fill_super(struct super_block *sb, struct fs_context *fc)
 
 	sb->s_root = root_dentry;
 
+	ovl_revert_creds(old_cred);
 	return 0;
 
 out_free_oe:
 	ovl_free_entry(oe);
 out_err:
+	/*
+	 * Revert creds before calling ovl_free_fs() which will call
+	 * put_cred() and put_cred() requires that the cred's that are
+	 * put are not the caller's creds, i.e., current->cred.
+	 */
+	if (old_cred)
+		ovl_revert_creds(old_cred);
 	ovl_free_fs(ofs);
 	sb->s_fs_info = NULL;
 	return err;
diff --git a/tools/testing/selftests/filesystems/overlayfs/Makefile b/tools/testing/selftests/filesystems/overlayfs/Makefile
index e8d1adb021af..6c661232b3b5 100644
--- a/tools/testing/selftests/filesystems/overlayfs/Makefile
+++ b/tools/testing/selftests/filesystems/overlayfs/Makefile
@@ -1,7 +1,14 @@
 # SPDX-License-Identifier: GPL-2.0
 
-TEST_GEN_PROGS := dev_in_maps set_layers_via_fds
+CFLAGS += -Wall
+CFLAGS += $(KHDR_INCLUDES)
+LDLIBS += -lcap
 
-CFLAGS := -Wall -Werror
+LOCAL_HDRS += wrappers.h log.h
+
+TEST_GEN_PROGS := dev_in_maps
+TEST_GEN_PROGS += set_layers_via_fds
 
 include ../../lib.mk
+
+$(OUTPUT)/set_layers_via_fds: ../utils.c
diff --git a/tools/testing/selftests/filesystems/overlayfs/set_layers_via_fds.c b/tools/testing/selftests/filesystems/overlayfs/set_layers_via_fds.c
index e65d95d97846..5074e64e74a8 100644
--- a/tools/testing/selftests/filesystems/overlayfs/set_layers_via_fds.c
+++ b/tools/testing/selftests/filesystems/overlayfs/set_layers_via_fds.c
@@ -6,30 +6,40 @@
 #include <sched.h>
 #include <stdio.h>
 #include <string.h>
+#include <sys/socket.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/mount.h>
 #include <unistd.h>
 
 #include "../../kselftest_harness.h"
+#include "../../pidfd/pidfd.h"
 #include "log.h"
+#include "../utils.h"
 #include "wrappers.h"
 
 FIXTURE(set_layers_via_fds) {
+	int pidfd;
 };
 
 FIXTURE_SETUP(set_layers_via_fds)
 {
-	ASSERT_EQ(mkdir("/set_layers_via_fds", 0755), 0);
-	ASSERT_EQ(mkdir("/set_layers_via_fds_tmpfs", 0755), 0);
+	self->pidfd = -EBADF;
+	EXPECT_EQ(mkdir("/set_layers_via_fds", 0755), 0);
+	EXPECT_EQ(mkdir("/set_layers_via_fds_tmpfs", 0755), 0);
 }
 
 FIXTURE_TEARDOWN(set_layers_via_fds)
 {
+	if (self->pidfd >= 0) {
+		EXPECT_EQ(sys_pidfd_send_signal(self->pidfd, SIGKILL, NULL, 0), 0);
+		EXPECT_EQ(close(self->pidfd), 0);
+	}
 	umount2("/set_layers_via_fds", 0);
-	ASSERT_EQ(rmdir("/set_layers_via_fds"), 0);
+	EXPECT_EQ(rmdir("/set_layers_via_fds"), 0);
 
 	umount2("/set_layers_via_fds_tmpfs", 0);
-	ASSERT_EQ(rmdir("/set_layers_via_fds_tmpfs"), 0);
+	EXPECT_EQ(rmdir("/set_layers_via_fds_tmpfs"), 0);
 }
 
 TEST_F(set_layers_via_fds, set_layers_via_fds)
@@ -218,6 +228,304 @@ TEST_F(set_layers_via_fds, set_500_layers_via_fds)
 	ASSERT_EQ(close(fd_overlay), 0);
 }
 
+TEST_F(set_layers_via_fds, set_override_creds)
+{
+	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_STRING, "metacopy", "on", 0), 0);
+
+	pid = create_child(&pidfd, 0);
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		if (sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "override_creds", NULL, 0)) {
+			TH_LOG("sys_fsconfig should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		_exit(EXIT_SUCCESS);
+	}
+	ASSERT_GE(sys_waitid(P_PID, pid, NULL, WEXITED), 0);
+	ASSERT_GE(close(pidfd), 0);
+
+	pid = create_child(&pidfd, 0);
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		if (sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "nooverride_creds", NULL, 0)) {
+			TH_LOG("sys_fsconfig should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		_exit(EXIT_SUCCESS);
+	}
+	ASSERT_GE(sys_waitid(P_PID, pid, NULL, WEXITED), 0);
+	ASSERT_GE(close(pidfd), 0);
+
+	pid = create_child(&pidfd, 0);
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		if (sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "override_creds", NULL, 0)) {
+			TH_LOG("sys_fsconfig should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		_exit(EXIT_SUCCESS);
+	}
+	ASSERT_GE(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(close(fd_context), 0);
+	ASSERT_EQ(close(fd_overlay), 0);
+}
+
+TEST_F(set_layers_via_fds, set_override_creds_invalid)
+{
+	int fd_context, fd_tmpfs, fd_overlay, ret;
+	int layer_fds[] = { [0 ... 3] = -EBADF };
+	pid_t pid;
+	int fd_userns1, fd_userns2;
+	int ipc_sockets[2];
+	char c;
+	const unsigned int predictable_fd_context_nr = 123;
+
+	fd_userns1 = get_userns_fd(0, 0, 10000);
+	ASSERT_GE(fd_userns1, 0);
+
+	fd_userns2 = get_userns_fd(0, 1234, 10000);
+	ASSERT_GE(fd_userns2, 0);
+
+	ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, ipc_sockets);
+	ASSERT_GE(ret, 0);
+
+	pid = create_child(&self->pidfd, 0);
+	ASSERT_GE(pid, 0);
+	if (pid == 0) {
+		if (close(ipc_sockets[0])) {
+			TH_LOG("close should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		if (!switch_userns(fd_userns2, 0, 0, false)) {
+			TH_LOG("switch_userns should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		if (read_nointr(ipc_sockets[1], &c, 1) != 1) {
+			TH_LOG("read_nointr should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		if (close(ipc_sockets[1])) {
+			TH_LOG("close should have succeeded");
+			_exit(EXIT_FAILURE);
+		}
+
+		if (!sys_fsconfig(predictable_fd_context_nr, FSCONFIG_SET_FLAG, "override_creds", NULL, 0)) {
+			TH_LOG("sys_fsconfig should have failed");
+			_exit(EXIT_FAILURE);
+		}
+
+		_exit(EXIT_SUCCESS);
+	}
+
+	ASSERT_EQ(close(ipc_sockets[1]), 0);
+	ASSERT_EQ(switch_userns(fd_userns1, 0, 0, false), true);
+	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_EQ(dup3(fd_context, predictable_fd_context_nr, 0), predictable_fd_context_nr);
+	ASSERT_EQ(close(fd_context), 0);
+	fd_context = predictable_fd_context_nr;
+	ASSERT_EQ(write_nointr(ipc_sockets[0], "1", 1), 1);
+	ASSERT_EQ(close(ipc_sockets[0]), 0);
+
+	ASSERT_EQ(wait_for_pid(pid), 0);
+	ASSERT_EQ(close(self->pidfd), 0);
+	self->pidfd = -EBADF;
+
+	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);
+
+	for (int i = 0; i < ARRAY_SIZE(layer_fds); i++)
+		ASSERT_EQ(close(layer_fds[i]), 0);
+
+	ASSERT_EQ(sys_fsconfig(fd_context, FSCONFIG_SET_FLAG, "userxattr", NULL, 0), 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(close(fd_context), 0);
+	ASSERT_EQ(close(fd_overlay), 0);
+	ASSERT_EQ(close(fd_userns1), 0);
+	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_F(set_layers_via_fds, set_500_layers_via_opath_fds)
 {
 	int fd_context, fd_tmpfs, fd_overlay, fd_work, fd_upper, fd_lower;
diff --git a/tools/testing/selftests/filesystems/utils.c b/tools/testing/selftests/filesystems/utils.c
new file mode 100644
index 000000000000..e553c89c5b19
--- /dev/null
+++ b/tools/testing/selftests/filesystems/utils.c
@@ -0,0 +1,501 @@
+// SPDX-License-Identifier: GPL-2.0
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <fcntl.h>
+#include <sys/types.h>
+#include <dirent.h>
+#include <grp.h>
+#include <linux/limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/eventfd.h>
+#include <sys/fsuid.h>
+#include <sys/prctl.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/xattr.h>
+
+#include "utils.h"
+
+#define MAX_USERNS_LEVEL 32
+
+#define syserror(format, ...)                           \
+	({                                              \
+		fprintf(stderr, "%m - " format "\n", ##__VA_ARGS__); \
+		(-errno);                               \
+	})
+
+#define syserror_set(__ret__, format, ...)                    \
+	({                                                    \
+		typeof(__ret__) __internal_ret__ = (__ret__); \
+		errno = labs(__ret__);                        \
+		fprintf(stderr, "%m - " format "\n", ##__VA_ARGS__);       \
+		__internal_ret__;                             \
+	})
+
+#define STRLITERALLEN(x) (sizeof(""x"") - 1)
+
+#define INTTYPE_TO_STRLEN(type)             \
+	(2 + (sizeof(type) <= 1             \
+		  ? 3                       \
+		  : sizeof(type) <= 2       \
+			? 5                 \
+			: sizeof(type) <= 4 \
+			      ? 10          \
+			      : sizeof(type) <= 8 ? 20 : sizeof(int[-2 * (sizeof(type) > 8)])))
+
+#define list_for_each(__iterator, __list) \
+	for (__iterator = (__list)->next; __iterator != __list; __iterator = __iterator->next)
+
+typedef enum idmap_type_t {
+	ID_TYPE_UID,
+	ID_TYPE_GID
+} idmap_type_t;
+
+struct id_map {
+	idmap_type_t map_type;
+	__u32 nsid;
+	__u32 hostid;
+	__u32 range;
+};
+
+struct list {
+	void *elem;
+	struct list *next;
+	struct list *prev;
+};
+
+struct userns_hierarchy {
+	int fd_userns;
+	int fd_event;
+	unsigned int level;
+	struct list id_map;
+};
+
+static inline void list_init(struct list *list)
+{
+	list->elem = NULL;
+	list->next = list->prev = list;
+}
+
+static inline int list_empty(const struct list *list)
+{
+	return list == list->next;
+}
+
+static inline void __list_add(struct list *new, struct list *prev, struct list *next)
+{
+	next->prev = new;
+	new->next = next;
+	new->prev = prev;
+	prev->next = new;
+}
+
+static inline void list_add_tail(struct list *head, struct list *list)
+{
+	__list_add(list, head->prev, head);
+}
+
+static inline void list_del(struct list *list)
+{
+	struct list *next, *prev;
+
+	next = list->next;
+	prev = list->prev;
+	next->prev = prev;
+	prev->next = next;
+}
+
+static ssize_t read_nointr(int fd, void *buf, size_t count)
+{
+	ssize_t ret;
+
+	do {
+		ret = read(fd, buf, count);
+	} while (ret < 0 && errno == EINTR);
+
+	return ret;
+}
+
+static ssize_t write_nointr(int fd, const void *buf, size_t count)
+{
+	ssize_t ret;
+
+	do {
+		ret = write(fd, buf, count);
+	} while (ret < 0 && errno == EINTR);
+
+	return ret;
+}
+
+#define __STACK_SIZE (8 * 1024 * 1024)
+static pid_t do_clone(int (*fn)(void *), void *arg, int flags)
+{
+	void *stack;
+
+	stack = malloc(__STACK_SIZE);
+	if (!stack)
+		return -ENOMEM;
+
+#ifdef __ia64__
+	return __clone2(fn, stack, __STACK_SIZE, flags | SIGCHLD, arg, NULL);
+#else
+	return clone(fn, stack + __STACK_SIZE, flags | SIGCHLD, arg, NULL);
+#endif
+}
+
+static int get_userns_fd_cb(void *data)
+{
+	for (;;)
+		pause();
+	_exit(0);
+}
+
+static int wait_for_pid(pid_t pid)
+{
+	int status, ret;
+
+again:
+	ret = waitpid(pid, &status, 0);
+	if (ret == -1) {
+		if (errno == EINTR)
+			goto again;
+
+		return -1;
+	}
+
+	if (!WIFEXITED(status))
+		return -1;
+
+	return WEXITSTATUS(status);
+}
+
+static int write_id_mapping(idmap_type_t map_type, pid_t pid, const char *buf, size_t buf_size)
+{
+	int fd = -EBADF, setgroups_fd = -EBADF;
+	int fret = -1;
+	int ret;
+	char path[STRLITERALLEN("/proc/") + INTTYPE_TO_STRLEN(pid_t) +
+		  STRLITERALLEN("/setgroups") + 1];
+
+	if (geteuid() != 0 && map_type == ID_TYPE_GID) {
+		ret = snprintf(path, sizeof(path), "/proc/%d/setgroups", pid);
+		if (ret < 0 || ret >= sizeof(path))
+			goto out;
+
+		setgroups_fd = open(path, O_WRONLY | O_CLOEXEC);
+		if (setgroups_fd < 0 && errno != ENOENT) {
+			syserror("Failed to open \"%s\"", path);
+			goto out;
+		}
+
+		if (setgroups_fd >= 0) {
+			ret = write_nointr(setgroups_fd, "deny\n", STRLITERALLEN("deny\n"));
+			if (ret != STRLITERALLEN("deny\n")) {
+				syserror("Failed to write \"deny\" to \"/proc/%d/setgroups\"", pid);
+				goto out;
+			}
+		}
+	}
+
+	ret = snprintf(path, sizeof(path), "/proc/%d/%cid_map", pid, map_type == ID_TYPE_UID ? 'u' : 'g');
+	if (ret < 0 || ret >= sizeof(path))
+		goto out;
+
+	fd = open(path, O_WRONLY | O_CLOEXEC);
+	if (fd < 0) {
+		syserror("Failed to open \"%s\"", path);
+		goto out;
+	}
+
+	ret = write_nointr(fd, buf, buf_size);
+	if (ret != buf_size) {
+		syserror("Failed to write %cid mapping to \"%s\"",
+			 map_type == ID_TYPE_UID ? 'u' : 'g', path);
+		goto out;
+	}
+
+	fret = 0;
+out:
+	close(fd);
+	close(setgroups_fd);
+
+	return fret;
+}
+
+static int map_ids_from_idmap(struct list *idmap, pid_t pid)
+{
+	int fill, left;
+	char mapbuf[4096] = {};
+	bool had_entry = false;
+	idmap_type_t map_type, u_or_g;
+
+	if (list_empty(idmap))
+		return 0;
+
+	for (map_type = ID_TYPE_UID, u_or_g = 'u';
+	     map_type <= ID_TYPE_GID; map_type++, u_or_g = 'g') {
+		char *pos = mapbuf;
+		int ret;
+		struct list *iterator;
+
+
+		list_for_each(iterator, idmap) {
+			struct id_map *map = iterator->elem;
+			if (map->map_type != map_type)
+				continue;
+
+			had_entry = true;
+
+			left = 4096 - (pos - mapbuf);
+			fill = snprintf(pos, left, "%u %u %u\n", map->nsid, map->hostid, map->range);
+			/*
+			 * The kernel only takes <= 4k for writes to
+			 * /proc/<pid>/{g,u}id_map
+			 */
+			if (fill <= 0 || fill >= left)
+				return syserror_set(-E2BIG, "Too many %cid mappings defined", u_or_g);
+
+			pos += fill;
+		}
+		if (!had_entry)
+			continue;
+
+		ret = write_id_mapping(map_type, pid, mapbuf, pos - mapbuf);
+		if (ret < 0)
+			return syserror("Failed to write mapping: %s", mapbuf);
+
+		memset(mapbuf, 0, sizeof(mapbuf));
+	}
+
+	return 0;
+}
+
+static int get_userns_fd_from_idmap(struct list *idmap)
+{
+	int ret;
+	pid_t pid;
+	char path_ns[STRLITERALLEN("/proc/") + INTTYPE_TO_STRLEN(pid_t) +
+		     STRLITERALLEN("/ns/user") + 1];
+
+	pid = do_clone(get_userns_fd_cb, NULL, CLONE_NEWUSER | CLONE_NEWNS);
+	if (pid < 0)
+		return -errno;
+
+	ret = map_ids_from_idmap(idmap, pid);
+	if (ret < 0)
+		return ret;
+
+	ret = snprintf(path_ns, sizeof(path_ns), "/proc/%d/ns/user", pid);
+	if (ret < 0 || (size_t)ret >= sizeof(path_ns))
+		ret = -EIO;
+	else
+		ret = open(path_ns, O_RDONLY | O_CLOEXEC | O_NOCTTY);
+
+	(void)kill(pid, SIGKILL);
+	(void)wait_for_pid(pid);
+	return ret;
+}
+
+int get_userns_fd(unsigned long nsid, unsigned long hostid, unsigned long range)
+{
+	struct list head, uid_mapl, gid_mapl;
+	struct id_map uid_map = {
+		.map_type	= ID_TYPE_UID,
+		.nsid		= nsid,
+		.hostid		= hostid,
+		.range		= range,
+	};
+	struct id_map gid_map = {
+		.map_type	= ID_TYPE_GID,
+		.nsid		= nsid,
+		.hostid		= hostid,
+		.range		= range,
+	};
+
+	list_init(&head);
+	uid_mapl.elem = &uid_map;
+	gid_mapl.elem = &gid_map;
+	list_add_tail(&head, &uid_mapl);
+	list_add_tail(&head, &gid_mapl);
+
+	return get_userns_fd_from_idmap(&head);
+}
+
+bool switch_ids(uid_t uid, gid_t gid)
+{
+	if (setgroups(0, NULL))
+		return syserror("failure: setgroups");
+
+	if (setresgid(gid, gid, gid))
+		return syserror("failure: setresgid");
+
+	if (setresuid(uid, uid, uid))
+		return syserror("failure: setresuid");
+
+	/* Ensure we can access proc files from processes we can ptrace. */
+	if (prctl(PR_SET_DUMPABLE, 1, 0, 0, 0))
+		return syserror("failure: make dumpable");
+
+	return true;
+}
+
+static int create_userns_hierarchy(struct userns_hierarchy *h);
+
+static int userns_fd_cb(void *data)
+{
+	struct userns_hierarchy *h = data;
+	char c;
+	int ret;
+
+	ret = read_nointr(h->fd_event, &c, 1);
+	if (ret < 0)
+		return syserror("failure: read from socketpair");
+
+	/* Only switch ids if someone actually wrote a mapping for us. */
+	if (c == '1') {
+		if (!switch_ids(0, 0))
+			return syserror("failure: switch ids to 0");
+	}
+
+	ret = write_nointr(h->fd_event, "1", 1);
+	if (ret < 0)
+		return syserror("failure: write to socketpair");
+
+	ret = create_userns_hierarchy(++h);
+	if (ret < 0)
+		return syserror("failure: userns level %d", h->level);
+
+	return 0;
+}
+
+static int create_userns_hierarchy(struct userns_hierarchy *h)
+{
+	int fret = -1;
+	char c;
+	int fd_socket[2];
+	int fd_userns = -EBADF, ret = -1;
+	ssize_t bytes;
+	pid_t pid;
+	char path[256];
+
+	if (h->level == MAX_USERNS_LEVEL)
+		return 0;
+
+	ret = socketpair(AF_LOCAL, SOCK_STREAM | SOCK_CLOEXEC, 0, fd_socket);
+	if (ret < 0)
+		return syserror("failure: create socketpair");
+
+	/* Note the CLONE_FILES | CLONE_VM when mucking with fds and memory. */
+	h->fd_event = fd_socket[1];
+	pid = do_clone(userns_fd_cb, h, CLONE_NEWUSER | CLONE_FILES | CLONE_VM);
+	if (pid < 0) {
+		syserror("failure: userns level %d", h->level);
+		goto out_close;
+	}
+
+	ret = map_ids_from_idmap(&h->id_map, pid);
+	if (ret < 0) {
+		kill(pid, SIGKILL);
+		syserror("failure: writing id mapping for userns level %d for %d", h->level, pid);
+		goto out_wait;
+	}
+
+	if (!list_empty(&h->id_map))
+		bytes = write_nointr(fd_socket[0], "1", 1); /* Inform the child we wrote a mapping. */
+	else
+		bytes = write_nointr(fd_socket[0], "0", 1); /* Inform the child we didn't write a mapping. */
+	if (bytes < 0) {
+		kill(pid, SIGKILL);
+		syserror("failure: write to socketpair");
+		goto out_wait;
+	}
+
+	/* Wait for child to set*id() and become dumpable. */
+	bytes = read_nointr(fd_socket[0], &c, 1);
+	if (bytes < 0) {
+		kill(pid, SIGKILL);
+		syserror("failure: read from socketpair");
+		goto out_wait;
+	}
+
+	snprintf(path, sizeof(path), "/proc/%d/ns/user", pid);
+	fd_userns = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd_userns < 0) {
+		kill(pid, SIGKILL);
+		syserror("failure: open userns level %d for %d", h->level, pid);
+		goto out_wait;
+	}
+
+	fret = 0;
+
+out_wait:
+	if (!wait_for_pid(pid) && !fret) {
+		h->fd_userns = fd_userns;
+		fd_userns = -EBADF;
+	}
+
+out_close:
+	if (fd_userns >= 0)
+		close(fd_userns);
+	close(fd_socket[0]);
+	close(fd_socket[1]);
+	return fret;
+}
+
+/* caps_down - lower all effective caps */
+int caps_down(void)
+{
+	bool fret = false;
+	cap_t caps = NULL;
+	int ret = -1;
+
+	caps = cap_get_proc();
+	if (!caps)
+		goto out;
+
+	ret = cap_clear_flag(caps, CAP_EFFECTIVE);
+	if (ret)
+		goto out;
+
+	ret = cap_set_proc(caps);
+	if (ret)
+		goto out;
+
+	fret = true;
+
+out:
+	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;
+}
diff --git a/tools/testing/selftests/filesystems/utils.h b/tools/testing/selftests/filesystems/utils.h
new file mode 100644
index 000000000000..7f1df2a3e94c
--- /dev/null
+++ b/tools/testing/selftests/filesystems/utils.h
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __IDMAP_UTILS_H
+#define __IDMAP_UTILS_H
+
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <errno.h>
+#include <linux/types.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+#include <sys/capability.h>
+#include <sys/fsuid.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+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);
+
+static inline bool switch_userns(int fd, uid_t uid, gid_t gid, bool drop_caps)
+{
+	if (setns(fd, CLONE_NEWUSER))
+		return false;
+
+	if (!switch_ids(uid, gid))
+		return false;
+
+	if (drop_caps && !caps_down())
+		return false;
+
+	return true;
+}
+
+#endif /* __IDMAP_UTILS_H */