selftests/bpf: Add tests verifying bpf lsm userns_create hook

The LSM hook userns_create was introduced to provide LSM's an
opportunity to block or allow unprivileged user namespace creation. This
test serves two purposes: it provides a test eBPF implementation, and
tests the hook successfully blocks or allows user namespace creation.

This tests 3 cases:

        1. Unattached bpf program does not block unpriv user namespace
           creation.
        2. Attached bpf program allows user namespace creation given
           CAP_SYS_ADMIN privileges.
        3. Attached bpf program denies user namespace creation for a
           user without CAP_SYS_ADMIN.

Acked-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Frederick Lawler <fred@cloudflare.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

diff --git a/tools/testing/selftests/bpf/prog_tests/deny_namespace.c b/tools/testing/selftests/bpf/prog_tests/deny_namespace.c
new file mode 100644 (file)
index 0000000..1bc6241
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/deny_namespace.c
@@ -0,0 +1,102 @@
+// SPDX-License-Identifier: GPL-2.0
+#define _GNU_SOURCE
+#include <test_progs.h>
+#include "test_deny_namespace.skel.h"
+#include <sched.h>
+#include "cap_helpers.h"
+#include <stdio.h>
+
+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);
+}
+
+/* negative return value -> some internal error
+ * positive return value -> userns creation failed
+ * 0                     -> userns creation succeeded
+ */
+static int create_user_ns(void)
+{
+       pid_t pid;
+
+       pid = fork();
+       if (pid < 0)
+               return -1;
+
+       if (pid == 0) {
+               if (unshare(CLONE_NEWUSER))
+                       _exit(EXIT_FAILURE);
+               _exit(EXIT_SUCCESS);
+       }
+
+       return wait_for_pid(pid);
+}
+
+static void test_userns_create_bpf(void)
+{
+       __u32 cap_mask = 1ULL << CAP_SYS_ADMIN;
+       __u64 old_caps = 0;
+
+       cap_enable_effective(cap_mask, &old_caps);
+
+       ASSERT_OK(create_user_ns(), "priv new user ns");
+
+       cap_disable_effective(cap_mask, &old_caps);
+
+       ASSERT_EQ(create_user_ns(), EPERM, "unpriv new user ns");
+
+       if (cap_mask & old_caps)
+               cap_enable_effective(cap_mask, NULL);
+}
+
+static void test_unpriv_userns_create_no_bpf(void)
+{
+       __u32 cap_mask = 1ULL << CAP_SYS_ADMIN;
+       __u64 old_caps = 0;
+
+       cap_disable_effective(cap_mask, &old_caps);
+
+       ASSERT_OK(create_user_ns(), "no-bpf unpriv new user ns");
+
+       if (cap_mask & old_caps)
+               cap_enable_effective(cap_mask, NULL);
+}
+
+void test_deny_namespace(void)
+{
+       struct test_deny_namespace *skel = NULL;
+       int err;
+
+       if (test__start_subtest("unpriv_userns_create_no_bpf"))
+               test_unpriv_userns_create_no_bpf();
+
+       skel = test_deny_namespace__open_and_load();
+       if (!ASSERT_OK_PTR(skel, "skel load"))
+               goto close_prog;
+
+       err = test_deny_namespace__attach(skel);
+       if (!ASSERT_OK(err, "attach"))
+               goto close_prog;
+
+       if (test__start_subtest("userns_create_bpf"))
+               test_userns_create_bpf();
+
+       test_deny_namespace__detach(skel);
+
+close_prog:
+       test_deny_namespace__destroy(skel);
+}

diff --git a/tools/testing/selftests/bpf/progs/test_deny_namespace.c b/tools/testing/selftests/bpf/progs/test_deny_namespace.c
new file mode 100644 (file)
index 0000000..09ad5a4
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_deny_namespace.c
@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include <errno.h>
+#include <linux/capability.h>
+
+struct kernel_cap_struct {
+       __u32 cap[_LINUX_CAPABILITY_U32S_3];
+} __attribute__((preserve_access_index));
+
+struct cred {
+       struct kernel_cap_struct cap_effective;
+} __attribute__((preserve_access_index));
+
+char _license[] SEC("license") = "GPL";
+
+SEC("lsm.s/userns_create")
+int BPF_PROG(test_userns_create, const struct cred *cred, int ret)
+{
+       struct kernel_cap_struct caps = cred->cap_effective;
+       int cap_index = CAP_TO_INDEX(CAP_SYS_ADMIN);
+       __u32 cap_mask = CAP_TO_MASK(CAP_SYS_ADMIN);
+
+       if (ret)
+               return 0;
+
+       ret = -EPERM;
+       if (caps.cap[cap_index] & cap_mask)
+               return 0;
+
+       return -EPERM;
+}