selftests/namespaces: first listns() permission test

Test that unprivileged users can only see namespaces they're currently
in. Create a namespace, drop privileges, verify we can only see our own
namespaces.

Link: https://patch.msgid.link/20251029-work-namespace-nstree-listns-v4-48-2e6f823ebdc0@kernel.org
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>

tools/testing/selftests/namespaces/.gitignore

@@ -3,3 +3,4 @@ file_handle_test
 init_ino_test
 ns_active_ref_test
 listns_test
+listns_permissions_test

tools/testing/selftests/namespaces/Makefile

@@ -2,10 +2,11 @@
 CFLAGS += -Wall -O0 -g $(KHDR_INCLUDES) $(TOOLS_INCLUDES)
 LDLIBS += -lcap
 
-TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test
+TEST_GEN_PROGS := nsid_test file_handle_test init_ino_test ns_active_ref_test listns_test listns_permissions_test
 
 include ../lib.mk
 
 $(OUTPUT)/ns_active_ref_test: ../filesystems/utils.c
 $(OUTPUT)/listns_test: ../filesystems/utils.c
+$(OUTPUT)/listns_permissions_test: ../filesystems/utils.c
 

tools/testing/selftests/namespaces/listns_permissions_test.c

@@ -0,0 +1,131 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <errno.h>
+#include <fcntl.h>
+#include <limits.h>
+#include <sched.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <linux/nsfs.h>
+#include <sys/capability.h>
+#include <sys/ioctl.h>
+#include <sys/prctl.h>
+#include <sys/stat.h>
+#include <sys/syscall.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+#include "../kselftest_harness.h"
+#include "../filesystems/utils.h"
+#include "wrappers.h"
+
+/*
+ * Test that unprivileged users can only see namespaces they're currently in.
+ * Create a namespace, drop privileges, verify we can only see our own namespaces.
+ */
+TEST(listns_unprivileged_current_only)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = CLONE_NEWNET,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 ns_ids[100];
+	ssize_t ret;
+	int pipefd[2];
+	pid_t pid;
+	int status;
+	bool found_ours;
+	int unexpected_count;
+
+	ASSERT_EQ(pipe(pipefd), 0);
+
+	pid = fork();
+	ASSERT_GE(pid, 0);
+
+	if (pid == 0) {
+		int fd;
+		__u64 our_netns_id;
+		bool found_ours;
+		int unexpected_count;
+
+		close(pipefd[0]);
+
+		/* Create user namespace to be unprivileged */
+		if (setup_userns() < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Create a network namespace */
+		if (unshare(CLONE_NEWNET) < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* Get our network namespace ID */
+		fd = open("/proc/self/ns/net", O_RDONLY);
+		if (fd < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		if (ioctl(fd, NS_GET_ID, &our_netns_id) < 0) {
+			close(fd);
+			close(pipefd[1]);
+			exit(1);
+		}
+		close(fd);
+
+		/* Now we're unprivileged - list all network namespaces */
+		ret = sys_listns(&req, ns_ids, ARRAY_SIZE(ns_ids), 0);
+		if (ret < 0) {
+			close(pipefd[1]);
+			exit(1);
+		}
+
+		/* We should only see our own network namespace */
+		found_ours = false;
+		unexpected_count = 0;
+
+		for (ssize_t i = 0; i < ret; i++) {
+			if (ns_ids[i] == our_netns_id) {
+				found_ours = true;
+			} else {
+				/* This is either init_net (which we can see) or unexpected */
+				unexpected_count++;
+			}
+		}
+
+		/* Send results to parent */
+		write(pipefd[1], &found_ours, sizeof(found_ours));
+		write(pipefd[1], &unexpected_count, sizeof(unexpected_count));
+		close(pipefd[1]);
+		exit(0);
+	}
+
+	/* Parent */
+	close(pipefd[1]);
+
+	found_ours = false;
+	unexpected_count = 0;
+	read(pipefd[0], &found_ours, sizeof(found_ours));
+	read(pipefd[0], &unexpected_count, sizeof(unexpected_count));
+	close(pipefd[0]);
+
+	waitpid(pid, &status, 0);
+	ASSERT_TRUE(WIFEXITED(status));
+	ASSERT_EQ(WEXITSTATUS(status), 0);
+
+	/* Child should have seen its own namespace */
+	ASSERT_TRUE(found_ours);
+
+	TH_LOG("Unprivileged child saw its own namespace, plus %d others (likely init_net)",
+			unexpected_count);
+}
+
+TEST_HARNESS_MAIN