[LTP] [PATCH STAGING v2 14/16] fchroot11: test failfs entry with no_new_privs

From: Andrea Cervesato <[email protected]>

Verify that an unprivileged process may enter failfs when it
committed to no_new_privs, since setuid binaries then pose no
confused deputy risk anymore. The test also verifies that a process
which entered failfs counts as chrooted: it can no longer create a
user namespace with unshare() to regain CAP_SYS_CHROOT.

Signed-off-by: Andrea Cervesato <[email protected]>
---
 runtest/staging                               |  1 +
 testcases/kernel/syscalls/fchroot/.gitignore  |  1 +
 testcases/kernel/syscalls/fchroot/fchroot11.c | 70 +++++++++++++++++++++++++++
 3 files changed, 72 insertions(+)

diff --git a/runtest/staging b/runtest/staging
index 9ec2a7897..137e3f99a 100644
--- a/runtest/staging
+++ b/runtest/staging
@@ -10,3 +10,4 @@ fchroot07 fchroot07
 fchroot08 fchroot08
 fchroot09 fchroot09
 fchroot10 fchroot10
+fchroot11 fchroot11
diff --git a/testcases/kernel/syscalls/fchroot/.gitignore b/testcases/kernel/syscalls/fchroot/.gitignore
index 570da2b98..0eca20548 100644
--- a/testcases/kernel/syscalls/fchroot/.gitignore
+++ b/testcases/kernel/syscalls/fchroot/.gitignore
@@ -8,3 +8,4 @@ fchroot07
 fchroot08
 fchroot09
 fchroot10
+fchroot11
diff --git a/testcases/kernel/syscalls/fchroot/fchroot11.c b/testcases/kernel/syscalls/fchroot/fchroot11.c
new file mode 100644
index 000000000..85401e5fe
--- /dev/null
+++ b/testcases/kernel/syscalls/fchroot/fchroot11.c
@@ -0,0 +1,70 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2026 SUSE LLC Andrea Cervesato <[email protected]>
+ */
+
+/*\
+ * Test unprivileged :manpage:`fchroot(2)` into failfs with no_new_privs
+ * set.
+ *
+ * An unprivileged process may enter failfs when it committed to
+ * no_new_privs, since setuid binaries then pose no confused deputy risk
+ * anymore. The test also verifies that a process which entered failfs
+ * counts as chrooted: it can no longer create a user namespace with
+ * :manpage:`unshare(2)` to regain ``CAP_SYS_CHROOT``.
+ *
+ * The user namespace check requires ``CONFIG_USER_NS``, which is enforced
+ * with ``.needs_kconfigs``.
+ *
+ * Root is required to drop to an unprivileged user in the forked child.
+ */
+
+#define _GNU_SOURCE
+#include <fcntl.h>
+#include <pwd.h>
+#include <sched.h>
+#include "tst_test.h"
+#include "lapi/fcntl.h"
+#include "lapi/prctl.h"
+#include "lapi/sched.h"
+#include "lapi/syscalls.h"
+
+static struct passwd *ltpuser;
+
+static void run(void)
+{
+	if (SAFE_FORK())
+		return;
+
+	SAFE_SETRESUID(ltpuser->pw_uid, ltpuser->pw_uid,
+		ltpuser->pw_uid);
+
+	SAFE_PRCTL(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0);
+
+	TST_EXP_PASS(tst_syscall(__NR_fchroot, FD_FAILFS_ROOT, 0),
+		"unprivileged fchroot() with no_new_privs");
+
+	TST_EXP_FAIL2(open("/etc/passwd", O_RDONLY), EOPNOTSUPP,
+		"absolute lookup after entering failfs");
+
+	TST_EXP_FAIL(unshare(CLONE_NEWUSER), EPERM,
+		"user namespace creation blocked by the failfs root");
+
+	exit(0);
+}
+
+static void setup(void)
+{
+	ltpuser = SAFE_GETPWNAM("nobody");
+}
+
+static struct tst_test test = {
+	.setup = setup,
+	.test_all = run,
+	.needs_root = 1,
+	.forks_child = 1,
+	.needs_kconfigs = (const char *[]) {
+		"CONFIG_USER_NS=y",
+		NULL,
+	}
+};

-- 
2.51.0


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