[PATCH 3/3] selftests/filesystems: test completing a context from another user namespace

fsopen() records the caller's user namespace in fc->user_ns and hands
back an ordinary file descriptor, so the task that issues
FSCONFIG_CMD_CREATE need not be the one that created the context.
mount_capable() authorizes that for a caller holding CAP_SYS_ADMIN in an
ancestor of fc->user_ns, which any unprivileged user has over a user
namespace it just created.

binfmt_misc and overlayfs used to WARN_ON() the mismatch. Add a test for
both. Also cover the handover within one user namespace. That is a
supported thing to do and has to keep working.

Signed-off-by: Christian Brauner (Amutable) <[email protected]>
---
 tools/testing/selftests/Makefile                   |   1 +
 .../selftests/filesystems/fscontext_ns/Makefile    |  10 +
 .../filesystems/fscontext_ns/fscontext_ns_test.c   | 239 +++++++++++++++++++++
 3 files changed, 250 insertions(+)

diff --git a/tools/testing/selftests/Makefile b/tools/testing/selftests/Makefile
index 8d4db2241cc2..b622052ec3e9 100644
--- a/tools/testing/selftests/Makefile
+++ b/tools/testing/selftests/Makefile
@@ -42,6 +42,7 @@ TARGETS += filesystems/fuse
 TARGETS += filesystems/move_mount
 TARGETS += filesystems/empty_mntns
 TARGETS += filesystems/fsmount_ns
+TARGETS += filesystems/fscontext_ns
 TARGETS += firmware
 TARGETS += fpu
 TARGETS += ftrace
diff --git a/tools/testing/selftests/filesystems/fscontext_ns/Makefile b/tools/testing/selftests/filesystems/fscontext_ns/Makefile
new file mode 100644
index 000000000000..7e3506294757
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fscontext_ns/Makefile
@@ -0,0 +1,10 @@
+# SPDX-License-Identifier: GPL-2.0
+TEST_GEN_PROGS := fscontext_ns_test
+
+CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
+LDLIBS := -lcap
+
+include ../../lib.mk
+
+$(OUTPUT)/fscontext_ns_test: fscontext_ns_test.c ../utils.c
+	$(CC) $(CFLAGS) -o $@ $^ $(LDLIBS)
diff --git a/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c b/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c
new file mode 100644
index 000000000000..0f30a3e6e197
--- /dev/null
+++ b/tools/testing/selftests/filesystems/fscontext_ns/fscontext_ns_test.c
@@ -0,0 +1,239 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (C) 2026 Christian Brauner <[email protected]>
+ *
+ * Test that completing a filesystem context from another user namespace
+ * doesn't warn.
+ *
+ * fsopen() records the caller's user namespace in fc->user_ns and hands
+ * back an ordinary file descriptor. The task that issues
+ * FSCONFIG_CMD_CREATE need not be the one that created the context: the fd
+ * is inherited across fork() and exec() and it can be passed over a unix
+ * socket. vfs_cmd_create() authorizes the create with mount_capable(),
+ * which for FS_USERNS_MOUNT checks ns_capable(fc->user_ns, CAP_SYS_ADMIN),
+ * and that succeeds for a task holding CAP_SYS_ADMIN in an ancestor of
+ * fc->user_ns.
+ *
+ * binfmt_misc and overlayfs used to WARN_ON() that mismatch, which let an
+ * unprivileged user taint the kernel, flood the log and panic a kernel
+ * booted with panic_on_warn. The mount must still be refused, but it must
+ * not warn.
+ */
+#define _GNU_SOURCE
+
+#include <errno.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "../wrappers.h"
+#include "../utils.h"
+#include "../../kselftest_harness.h"
+
+#ifndef FSCONFIG_CMD_CREATE
+#define FSCONFIG_CMD_CREATE	6
+#endif
+
+/* TAINT_WARN, i.e. bit 9 of /proc/sys/kernel/tainted. */
+#define TAINT_WARN_BIT		9
+
+static bool taint_warn_set(void)
+{
+	unsigned long taint = 0;
+	FILE *f;
+
+	f = fopen("/proc/sys/kernel/tainted", "r");
+	if (!f)
+		return false;
+	if (fscanf(f, "%lu", &taint) != 1)
+		taint = 0;
+	fclose(f);
+
+	return taint & (1UL << TAINT_WARN_BIT);
+}
+
+static int send_fd(int sock, int fd)
+{
+	char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
+	char b[1] = { 'x' };
+	struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
+	struct msghdr msg = {
+		.msg_iov	= &iov,
+		.msg_iovlen	= 1,
+		.msg_control	= cmsgbuf,
+		.msg_controllen	= sizeof(cmsgbuf),
+	};
+	struct cmsghdr *cmsg;
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	cmsg->cmsg_level = SOL_SOCKET;
+	cmsg->cmsg_type = SCM_RIGHTS;
+	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+	memcpy(CMSG_DATA(cmsg), &fd, sizeof(int));
+
+	return sendmsg(sock, &msg, 0) < 0 ? -1 : 0;
+}
+
+static int recv_fd(int sock)
+{
+	char cmsgbuf[CMSG_SPACE(sizeof(int))] = {};
+	char b[1];
+	struct iovec iov = { .iov_base = b, .iov_len = sizeof(b) };
+	struct msghdr msg = {
+		.msg_iov	= &iov,
+		.msg_iovlen	= 1,
+		.msg_control	= cmsgbuf,
+		.msg_controllen	= sizeof(cmsgbuf),
+	};
+	struct cmsghdr *cmsg;
+	int fd = -1;
+
+	if (recvmsg(sock, &msg, 0) <= 0)
+		return -1;
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+	if (!cmsg || cmsg->cmsg_type != SCM_RIGHTS)
+		return -1;
+	memcpy(&fd, CMSG_DATA(cmsg), sizeof(int));
+
+	return fd;
+}
+
+/*
+ * Create a context for @fsname in a child and complete it here. With @nest
+ * the child first creates its own user namespace, so that the context is
+ * created in a descendant of the namespace completing it. The child needs a
+ * mount namespace of its own as well: fsopen() gates on may_mount(), which
+ * asks for CAP_SYS_ADMIN in the user namespace owning the caller's mount
+ * namespace.
+ *
+ * Returns the result of FSCONFIG_CMD_CREATE with errno set, or -ENODATA if
+ * the child could not create the context at all.
+ */
+static int create_from_child(const char *fsname, bool nest)
+{
+	int sock[2], fd, ret, status;
+	pid_t pid;
+
+	if (socketpair(AF_UNIX, SOCK_STREAM, 0, sock))
+		return -ENODATA;
+
+	pid = fork();
+	if (pid < 0) {
+		close(sock[0]);
+		close(sock[1]);
+		return -ENODATA;
+	}
+
+	if (pid == 0) {
+		close(sock[0]);
+
+		if (nest && unshare(CLONE_NEWUSER | CLONE_NEWNS))
+			_exit(1);
+
+		fd = sys_fsopen(fsname, 0);
+		if (fd < 0)
+			_exit(1);
+		if (send_fd(sock[1], fd))
+			_exit(1);
+		_exit(0);
+	}
+
+	close(sock[1]);
+	fd = recv_fd(sock[0]);
+	close(sock[0]);
+	wait_for_pid(pid);
+	waitpid(pid, &status, WNOHANG);
+
+	if (fd < 0)
+		return -ENODATA;
+
+	errno = 0;
+	ret = sys_fsconfig(fd, FSCONFIG_CMD_CREATE, NULL, NULL, 0);
+	status = errno;
+	close(fd);
+	errno = status;
+
+	return ret;
+}
+
+FIXTURE(fscontext_ns) {
+	bool warn_before;
+};
+
+FIXTURE_SETUP(fscontext_ns)
+{
+	self->warn_before = taint_warn_set();
+
+	if (setup_userns() != 0)
+		SKIP(return, "setup_userns failed");
+}
+
+FIXTURE_TEARDOWN(fscontext_ns)
+{
+}
+
+/*
+ * The condition the kernel used to WARN about. It has to be refused, and it
+ * has to be refused quietly: an unprivileged task reaches this.
+ */
+FIXTURE_VARIANT(fscontext_ns) {
+	const char *fsname;
+	int expected_errno;
+};
+
+FIXTURE_VARIANT_ADD(fscontext_ns, binfmt_misc) {
+	.fsname = "binfmt_misc",
+	.expected_errno = EINVAL,
+};
+
+FIXTURE_VARIANT_ADD(fscontext_ns, overlay) {
+	.fsname = "overlay",
+	.expected_errno = EIO,
+};
+
+TEST_F(fscontext_ns, create_from_descendant_userns)
+{
+	int ret;
+
+	ret = create_from_child(variant->fsname, true);
+	if (ret == -ENODATA)
+		SKIP(return, "%s unavailable", variant->fsname);
+
+	ASSERT_EQ(-1, ret);
+	ASSERT_EQ(variant->expected_errno, errno);
+
+	/*
+	 * Only meaningful if nothing had warned before us. Note that an
+	 * unrelated warning racing this test would look like a failure.
+	 */
+	if (self->warn_before)
+		TH_LOG("TAINT_WARN already set, not checking for a new warning");
+	else
+		ASSERT_FALSE(taint_warn_set());
+}
+
+/*
+ * The same handover within one user namespace is a supported thing to do and
+ * has to keep working. binfmt_misc takes no options, so the create succeeds
+ * outright and this also shows the test really drives the create path.
+ */
+TEST(create_from_same_userns)
+{
+	int ret;
+
+	if (setup_userns() != 0)
+		SKIP(return, "setup_userns failed");
+
+	ret = create_from_child("binfmt_misc", false);
+	if (ret == -ENODATA)
+		SKIP(return, "binfmt_misc unavailable");
+
+	ASSERT_EQ(0, ret);
+}
+
+TEST_HARNESS_MAIN

-- 
2.53.0