Re: [PATCH v6 bpf-next 2/2] selftests/bpf: Add a kernel flag test for LSM bpf hook

Blaise Boscaccy
2025-03-10T18:11:41Z

Song Liu <song@kernel.org> writes:

> On Mon, Mar 10, 2025 at 10:43 AM Blaise Boscaccy
> <bboscaccy@linux.microsoft.com> wrote:
>>
>> Song Liu <song@kernel.org> writes:
>>
>> > On Fri, Mar 7, 2025 at 5:33 PM Blaise Boscaccy
>> > <bboscaccy@linux.microsoft.com> wrote:
>> >>
>> >> This test exercises the kernel flag added to security_bpf by
>> >> effectively blocking light-skeletons from loading while allowing
>> >> normal skeletons to function as-is. Since this should work with any
>> >> arbitrary BPF program, an existing program from LSKELS_EXTRA was
>> >> used as a test payload.
>> >>
>> >> Signed-off-by: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
>> >> ---
>> >> .../selftests/bpf/prog_tests/kernel_flag.c | 43 +++++++++++++++++++
>> >> .../selftests/bpf/progs/test_kernel_flag.c | 28 ++++++++++++
>> >> 2 files changed, 71 insertions(+)
>> >> create mode 100644 tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>> >> create mode 100644 tools/testing/selftests/bpf/progs/test_kernel_flag.c
>> >>
>> >> diff --git a/tools/testing/selftests/bpf/prog_tests/kernel_flag.c b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>> >> new file mode 100644
>> >> index 0000000000000..479ad5de3737e
>> >> --- /dev/null
>> >> +++ b/tools/testing/selftests/bpf/prog_tests/kernel_flag.c
>> >> @@ -0,0 +1,43 @@
>> >> +// SPDX-License-Identifier: GPL-2.0
>> >> +/* Copyright (c) 2025 Microsoft */
>> >> +#include <test_progs.h>
>> >> +#include "kfunc_call_test.skel.h"
>> >> +#include "kfunc_call_test.lskel.h"
>> >> +#include "test_kernel_flag.skel.h"
>> >> +
>> >> +void test_kernel_flag(void)
>> >> +{
>> >> + struct test_kernel_flag *lsm_skel;
>> >> + struct kfunc_call_test *skel = NULL;
>> >> + struct kfunc_call_test_lskel *lskel = NULL;
>> >> + int ret;
>> >> +
>> >> + lsm_skel = test_kernel_flag__open_and_load();
>> >> + if (!ASSERT_OK_PTR(lsm_skel, "lsm_skel"))
>> >> + return;
>> >> +
>> >> + ret = test_kernel_flag__attach(lsm_skel);
>> >> + if (!ASSERT_OK(ret, "test_kernel_flag__attach"))
>> >> + goto close_prog;
>> >> +
>> >> + lsm_skel->bss->monitored_pid = getpid();
>> >
>> > We usually set monitored_pid before attaching the program.
>> >
>>
>> Okay, copy that.
>>
>> >> +
>> >> + /* Test with skel. This should pass the gatekeeper */
>> >> + skel = kfunc_call_test__open_and_load();
>> >> + if (!ASSERT_OK_PTR(skel, "skel"))
>> >> + goto close_prog;
>> >> +
>> >> + /* Test with lskel. This should fail due to blocking kernel-based bpf() invocations */
>> >> + lskel = kfunc_call_test_lskel__open_and_load();
>> >> + if (!ASSERT_ERR_PTR(lskel, "lskel"))
>> >> + goto close_prog;
>> >> +
>> >> +close_prog:
>> >> + if (skel)
>> >> + kfunc_call_test__destroy(skel);
>> >> + if (lskel)
>> >> + kfunc_call_test_lskel__destroy(lskel);
>> >> +
>> >> + lsm_skel->bss->monitored_pid = 0;
>> >> + test_kernel_flag__destroy(lsm_skel);
>> >> +}
>> >> diff --git a/tools/testing/selftests/bpf/progs/test_kernel_flag.c b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
>> >> new file mode 100644
>> >> index 0000000000000..9ca01aadb6656
>> >> --- /dev/null
>> >> +++ b/tools/testing/selftests/bpf/progs/test_kernel_flag.c
>> >> @@ -0,0 +1,28 @@
>> >> +// SPDX-License-Identifier: GPL-2.0
>> >> +
>> >> +/*
>> >> + * Copyright (C) 2025 Microsoft Corporation
>> >> + *
>> >> + * Author: Blaise Boscaccy <bboscaccy@linux.microsoft.com>
>> >> + */
>> >> +
>> >> +#include "vmlinux.h"
>> >> +#include <errno.h>
>> >> +#include <bpf/bpf_helpers.h>
>> >> +#include <bpf/bpf_tracing.h>
>> >> +
>> >> +char _license[] SEC("license") = "GPL";
>> >> +
>> >> +__u32 monitored_pid;
>> >> +
>> >> +SEC("lsm.s/bpf")
>> >> +int BPF_PROG(bpf, int cmd, union bpf_attr *attr, unsigned int size, bool kernel)
>> >> +{
>> >> + __u32 pid;
>> >> +
>> >> + pid = bpf_get_current_pid_tgid() >> 32;
>> >> + if (!kernel || pid != monitored_pid)
>> >> + return 0;
>> >
>> > We are blocking lskel load for the pid. This could make
>> > parallel testing (test_progs -j) flaky. We should probably
>> > change the logic to filtering on monitored_tiid.
>> >
>>
>> Curious on this for my own edification. The
>>
>> pid = bpf_get_current_pid_tgid() >> 32;
>>
>> is used extensively in the current test suite in a bunch of other
>> tests. Why does that not cause an issue with the other tests during
>> parallel testing?
>
> We are blindly blocking all security_bpf() with kernel=true here, so
> any lskel load in parallel with this test may fail. On the other hand,
> existing tests only block some operations under certain conditions.
> For example, test_cgroup1_hierarchy.c only blocks operations for
> target_ancestor_cgid.
>
> Does this make sense?
>

Not quite. This is only blocking security_bpf where kernel=true and
pid=monitored_pid.

> Thanks,
> Song