Add a test to verify the issue: kprobe_write_ctx can be abused to modify
struct pt_regs of kernel functions via kprobe_write_ctx=true freplace
progs.

Without the fix, the issue is verified:

kprobe_write_ctx=true freplace prog is allowed to attach to
kprobe_write_ctx=false kprobe prog. Then, the first arg of
bpf_fentry_test1 will be set as 0, and bpf_prog_test_run_opts() gets
-EFAULT instead of 0.

With the fix, the issue is rejected at attach time.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
---
 .../selftests/bpf/prog_tests/attach_probe.c   | 64 +++++++++++++++++++
 .../selftests/bpf/progs/kprobe_write_ctx.c    | 19 ++++++
 2 files changed, 83 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
index 9e77e5da7097..4d253900c4ad 100644
--- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
+++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
@@ -220,11 +220,73 @@ static void test_attach_kprobe_write_ctx(void)
 
 	kprobe_write_ctx__destroy(skel);
 }
+
+static void test_freplace_kprobe_write_ctx(void)
+{
+	struct bpf_program *prog_kprobe, *prog_ext, *prog_fentry;
+	struct kprobe_write_ctx *skel_kprobe, *skel_ext = NULL;
+	struct bpf_link *link_kprobe = NULL, *link_ext = NULL;
+	int err, prog_fd;
+	LIBBPF_OPTS(bpf_kprobe_opts, kprobe_opts);
+	LIBBPF_OPTS(bpf_test_run_opts, topts);
+
+	skel_kprobe = kprobe_write_ctx__open();
+	if (!ASSERT_OK_PTR(skel_kprobe, "kprobe_write_ctx__open kprobe"))
+		return;
+
+	prog_kprobe = skel_kprobe->progs.kprobe_dummy;
+	bpf_program__set_autoload(prog_kprobe, true);
+
+	prog_fentry = skel_kprobe->progs.fentry;
+	bpf_program__set_autoload(prog_fentry, true);
+
+	err = kprobe_write_ctx__load(skel_kprobe);
+	if (!ASSERT_OK(err, "kprobe_write_ctx__load kprobe"))
+		goto out;
+
+	skel_ext = kprobe_write_ctx__open();
+	if (!ASSERT_OK_PTR(skel_ext, "kprobe_write_ctx__open ext"))
+		goto out;
+
+	prog_ext = skel_ext->progs.freplace_kprobe;
+	bpf_program__set_autoload(prog_ext, true);
+
+	prog_fd = bpf_program__fd(skel_kprobe->progs.kprobe_write_ctx);
+	bpf_program__set_attach_target(prog_ext, prog_fd, "kprobe_write_ctx");
+
+	err = kprobe_write_ctx__load(skel_ext);
+	if (!ASSERT_OK(err, "kprobe_write_ctx__load ext"))
+		goto out;
+
+	prog_fd = bpf_program__fd(prog_kprobe);
+	link_ext = bpf_program__attach_freplace(prog_ext, prog_fd, "kprobe_dummy");
+	ASSERT_ERR_PTR(link_ext, "bpf_program__attach_freplace link");
+	ASSERT_EQ(errno, EINVAL, "bpf_program__attach_freplace errno");
+
+	link_kprobe = bpf_program__attach_kprobe_opts(prog_kprobe, "bpf_fentry_test1",
+						      &kprobe_opts);
+	if (!ASSERT_OK_PTR(link_kprobe, "bpf_program__attach_kprobe_opts"))
+		goto out;
+
+	err = bpf_prog_test_run_opts(bpf_program__fd(prog_fentry), &topts);
+	ASSERT_OK(err, "bpf_prog_test_run_opts");
+
+out:
+	bpf_link__destroy(link_ext);
+	bpf_link__destroy(link_kprobe);
+	kprobe_write_ctx__destroy(skel_ext);
+	kprobe_write_ctx__destroy(skel_kprobe);
+}
 #else
 static void test_attach_kprobe_write_ctx(void)
 {
 	test__skip();
 }
+
+static void test_freplace_kprobe_write_ctx(void)
+{
+	test__skip();
+}
 #endif
 
 static void test_attach_probe_auto(struct test_attach_probe *skel)
@@ -434,6 +496,8 @@ void test_attach_probe(void)
 		test_attach_kprobe_long_event_name();
 	if (test__start_subtest("kprobe-write-ctx"))
 		test_attach_kprobe_write_ctx();
+	if (test__start_subtest("freplace-kprobe-write-ctx"))
+		test_freplace_kprobe_write_ctx();
 
 cleanup:
 	test_attach_probe__destroy(skel);
diff --git a/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c b/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
index f77aef0474d3..adbf52afe490 100644
--- a/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
+++ b/tools/testing/selftests/bpf/progs/kprobe_write_ctx.c
@@ -19,4 +19,23 @@ int kprobe_multi_write_ctx(struct pt_regs *ctx)
 	ctx->ax = 0;
 	return 0;
 }
+
+SEC("?kprobe")
+int kprobe_dummy(struct pt_regs *regs)
+{
+	return 0;
+}
+
+SEC("?freplace")
+int freplace_kprobe(struct pt_regs *regs)
+{
+	regs->di = 0;
+	return 0;
+}
+
+SEC("?fentry/bpf_fentry_test1")
+int BPF_PROG(fentry)
+{
+	return 0;
+}
 #endif
-- 
2.53.0