[PATCH bpf v3 1/2] bpf: enforce that struct_ops programs be GPL-only

With the introduction of the struct_ops program type, it became possible to
implement kernel functionality in BPF, making it viable to use BPF in place
of a regular kernel module for these particular operations.

Thus far, the only user of this mechanism is for implementing TCP
congestion control algorithms. These are clearly marked as GPL-only when
implemented as modules (as seen by the use of EXPORT_SYMBOL_GPL for
tcp_register_congestion_control()), so it seems like an oversight that this
was not carried over to BPF implementations. Since this is the only user
of the struct_ops mechanism, just enforcing GPL-only for the struct_ops
program type seems like the simplest way to fix this.

v3: No change
v2: Move check to the top of check_struct_ops_btf_id().

Fixes: 0baf26b0fcd7 ("bpf: tcp: Support tcp_congestion_ops in bpf")
Acked-by: Martin KaFai Lau <redacted>
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 kernel/bpf/verifier.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 44e4ec1640f1..3a738724a380 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c

@@ -12158,6 +12158,11 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
 	u32 btf_id, member_idx;
 	const char *mname;
 
+	if (!prog->gpl_compatible) {
+		verbose(env, "struct ops programs must have a GPL compatible license\n");
+		return -EINVAL;
+	}
+
 	btf_id = prog->aux->attach_btf_id;
 	st_ops = bpf_struct_ops_find(btf_id);
 	if (!st_ops) {

-- 
2.31.0

This adds a selftest to check that the verifier rejects a TCP CC struct_ops
with a non-GPL license.

v3:
- Rename prog to bpf_tcp_nogpl
- Use ASSERT macros instead of CHECK
- Skip unneeded initialisation, unconditionally close skeleton
v2:
- Use a minimal struct_ops BPF program instead of rewriting bpf_dctcp's
  license in memory.
- Check for the verifier reject message instead of just the return code.

Acked-by: Martin KaFai Lau <redacted>
Signed-off-by: Toke Høiland-Jørgensen <redacted>
---
 .../selftests/bpf/prog_tests/bpf_tcp_ca.c     | 44 +++++++++++++++++++
 .../selftests/bpf/progs/bpf_tcp_nogpl.c       | 19 ++++++++
 2 files changed, 63 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_tcp_nogpl.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 37c5494a0381..e25917f04602 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_tcp_ca.c

@@ -6,6 +6,7 @@
 #include <test_progs.h>
 #include "bpf_dctcp.skel.h"
 #include "bpf_cubic.skel.h"
+#include "bpf_tcp_nogpl.skel.h"
 
 #define min(a, b) ((a) < (b) ? (a) : (b))
 

@@ -227,10 +228,53 @@ static void test_dctcp(void)
 	bpf_dctcp__destroy(dctcp_skel);
 }
 
+static char *err_str;
+static bool found;
+
+static int libbpf_debug_print(enum libbpf_print_level level,
+			      const char *format, va_list args)
+{
+	char *log_buf;
+
+	if (level != LIBBPF_WARN ||
+	    strcmp(format, "libbpf: \n%s\n")) {
+		vprintf(format, args);
+		return 0;
+	}
+
+	log_buf = va_arg(args, char *);
+	if (!log_buf)
+		goto out;
+	if (err_str && strstr(log_buf, err_str) != NULL)
+		found = true;
+out:
+	printf(format, log_buf);
+	return 0;
+}
+
+static void test_invalid_license(void)
+{
+	libbpf_print_fn_t old_print_fn;
+	struct bpf_tcp_nogpl *skel;
+
+	err_str = "struct ops programs must have a GPL compatible license";
+	found = false;
+	old_print_fn = libbpf_set_print(libbpf_debug_print);
+
+	skel = bpf_tcp_nogpl__open_and_load();
+	ASSERT_NULL(skel, "bpf_tcp_nogpl");
+	ASSERT_EQ(found, true, "expected_err_msg");
+
+	bpf_tcp_nogpl__destroy(skel);
+	libbpf_set_print(old_print_fn);
+}
+
 void test_bpf_tcp_ca(void)
 {
 	if (test__start_subtest("dctcp"))
 		test_dctcp();
 	if (test__start_subtest("cubic"))
 		test_cubic();
+	if (test__start_subtest("invalid_license"))
+		test_invalid_license();
 }

diff --git a/tools/testing/selftests/bpf/progs/bpf_tcp_nogpl.c b/tools/testing/selftests/bpf/progs/bpf_tcp_nogpl.c
new file mode 100644
index 000000000000..2ecd833dcd41
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_tcp_nogpl.c

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <linux/types.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "X";
+
+void BPF_STRUCT_OPS(nogpltcp_init, struct sock *sk)
+{
+}
+
+SEC(".struct_ops")
+struct tcp_congestion_ops bpf_nogpltcp = {
+	.init           = (void *)nogpltcp_init,
+	.name           = "bpf_nogpltcp",
+};

-- 
2.31.0

Hello:

This series was applied to bpf/bpf.git (refs/heads/master):

On Fri, 26 Mar 2021 11:03:13 +0100 you wrote:

With the introduction of the struct_ops program type, it became possible to
implement kernel functionality in BPF, making it viable to use BPF in place
of a regular kernel module for these particular operations.

Thus far, the only user of this mechanism is for implementing TCP
congestion control algorithms. These are clearly marked as GPL-only when
implemented as modules (as seen by the use of EXPORT_SYMBOL_GPL for
tcp_register_congestion_control()), so it seems like an oversight that this
was not carried over to BPF implementations. Since this is the only user
of the struct_ops mechanism, just enforcing GPL-only for the struct_ops
program type seems like the simplest way to fix this.

[...]

Here is the summary with links:
  - [bpf,v3,1/2] bpf: enforce that struct_ops programs be GPL-only
    https://git.kernel.org/bpf/bpf/c/12aa8a9467b3
  - [bpf,v3,2/2] bpf/selftests: test that kernel rejects a TCP CC with an invalid license
    https://git.kernel.org/bpf/bpf/c/d8e8052e42d0

You are awesome, thank you!
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