[PATCH bpf-next v2 2/2] selftests/bpf: Add untrusted BTF write regression

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

Add a TCP congestion-control struct_ops load test for a write through a
BTF pointer produced by bpf_rdonly_cast().

The test expects the verifier to reject the program before the TCP CA
btf_struct_access callback can whitelist the tcp_sock field write.

Reviewed-by: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 .../selftests/bpf/prog_tests/bpf_tcp_ca.c     | 12 +++++++++
 .../bpf/progs/tcp_ca_untrusted_btf_write.c    | 26 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/tcp_ca_untrusted_btf_write.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
index fe30181e6336..eb05fc82f81b 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
@@ -14,6 +14,7 @@
 #include "tcp_ca_incompl_cong_ops.skel.h"
 #include "tcp_ca_unsupp_cong_op.skel.h"
 #include "tcp_ca_kfunc.skel.h"
+#include "tcp_ca_untrusted_btf_write.skel.h"
 #include "bpf_cc_cubic.skel.h"
 
 static const unsigned int total_bytes = 10 * 1024 * 1024;
@@ -579,6 +580,15 @@ static void test_tcp_ca_kfunc(void)
 	tcp_ca_kfunc__destroy(skel);
 }
 
+static void test_untrusted_btf_write(void)
+{
+	struct tcp_ca_untrusted_btf_write *skel;
+
+	skel = tcp_ca_untrusted_btf_write__open_and_load();
+	ASSERT_ERR_PTR(skel, "tcp_ca_untrusted_btf_write__open_and_load");
+	tcp_ca_untrusted_btf_write__destroy(skel);
+}
+
 static void test_cc_cubic(void)
 {
 	struct cb_opts cb_opts = {
@@ -637,6 +647,8 @@ void test_bpf_tcp_ca(void)
 		test_link_replace();
 	if (test__start_subtest("tcp_ca_kfunc"))
 		test_tcp_ca_kfunc();
+	if (test__start_subtest("untrusted_btf_write"))
+		test_untrusted_btf_write();
 	if (test__start_subtest("cc_cubic"))
 		test_cc_cubic();
 	if (test__start_subtest("dctcp_autoattach_map"))
diff --git a/tools/testing/selftests/bpf/progs/tcp_ca_untrusted_btf_write.c b/tools/testing/selftests/bpf/progs/tcp_ca_untrusted_btf_write.c
new file mode 100644
index 000000000000..eda4697aac80
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/tcp_ca_untrusted_btf_write.c
@@ -0,0 +1,26 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "bpf_tracing_net.h"
+#include <bpf/bpf_core_read.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+SEC("struct_ops")
+void BPF_PROG(untrusted_btf_write_init, struct sock *sk)
+{
+	struct tcp_sock *tp;
+	int v = 1;
+	void *p;
+
+	p = bpf_rdonly_cast(&v, 0);
+	tp = bpf_rdonly_cast(p, bpf_core_type_id_kernel(struct tcp_sock));
+	tp->snd_cwnd = 1;
+}
+
+SEC(".struct_ops")
+struct tcp_congestion_ops untrusted_btf_write = {
+	.init = (void *)untrusted_btf_write_init,
+	.name = "bpf_ro_btf",
+};
-- 
2.53.0