selftests/bpf: Cover metadata access from a modified skb clone

Demonstrate that, when processing an skb clone, the metadata gets truncated
if the program contains a direct write to either the payload or the
metadata, due to an implicit unclone in the prologue, and otherwise the
dynptr to the metadata is limited to being read-only.

Signed-off-by: Jakub Sitnicki <[email protected]>
Signed-off-by: Martin KaFai Lau <[email protected]>
Link: https://patch.msgid.link/20250814-skb-metadata-thru-dynptr-v7-9-8a39e636e0fb@cloudflare.com

Author: jsitnicki

tools/testing/selftests/bpf/config

@@ -61,6 +61,7 @@ CONFIG_MPLS_IPTUNNEL=y
 CONFIG_MPLS_ROUTING=y
 CONFIG_MPTCP=y
 CONFIG_NET_ACT_GACT=y
+CONFIG_NET_ACT_MIRRED=y
 CONFIG_NET_ACT_SKBMOD=y
 CONFIG_NET_CLS=y
 CONFIG_NET_CLS_ACT=y

tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c

@@ -9,6 +9,7 @@
 #define TX_NETNS "xdp_context_tx"
 #define RX_NETNS "xdp_context_rx"
 #define TAP_NAME "tap0"
+#define DUMMY_NAME "dum0"
 #define TAP_NETNS "xdp_context_tuntap"
 
 #define TEST_PAYLOAD_LEN 32
@@ -156,6 +157,22 @@ static int send_test_packet(int ifindex)
 	return -1;
 }
 
+static int write_test_packet(int tap_fd)
+{
+	__u8 packet[sizeof(struct ethhdr) + TEST_PAYLOAD_LEN];
+	int n;
+
+	/* The ethernet header doesn't need to be valid for this test */
+	memset(packet, 0, sizeof(struct ethhdr));
+	memcpy(packet + sizeof(struct ethhdr), test_payload, TEST_PAYLOAD_LEN);
+
+	n = write(tap_fd, packet, sizeof(packet));
+	if (!ASSERT_EQ(n, sizeof(packet), "write packet"))
+		return -1;
+
+	return 0;
+}
+
 static void assert_test_result(const struct bpf_map *result_map)
 {
 	int err;
@@ -276,7 +293,6 @@ static void test_tuntap(struct bpf_program *xdp_prog,
 	LIBBPF_OPTS(bpf_tc_hook, tc_hook, .attach_point = BPF_TC_INGRESS);
 	LIBBPF_OPTS(bpf_tc_opts, tc_opts, .handle = 1, .priority = 1);
 	struct netns_obj *ns = NULL;
-	__u8 packet[sizeof(struct ethhdr) + TEST_PAYLOAD_LEN];
 	int tap_fd = -1;
 	int tap_ifindex;
 	int ret;
@@ -322,19 +338,82 @@ static void test_tuntap(struct bpf_program *xdp_prog,
 	if (!ASSERT_GE(ret, 0, "bpf_xdp_attach"))
 		goto close;
 
-	/* The ethernet header is not relevant for this test and doesn't need to
-	 * be meaningful.
-	 */
-	struct ethhdr eth = { 0 };
+	ret = write_test_packet(tap_fd);
+	if (!ASSERT_OK(ret, "write_test_packet"))
+		goto close;
 
-	memcpy(packet, &eth, sizeof(eth));
-	memcpy(packet + sizeof(eth), test_payload, TEST_PAYLOAD_LEN);
+	assert_test_result(result_map);
+
+close:
+	if (tap_fd >= 0)
+		close(tap_fd);
+	netns_free(ns);
+}
+
+/* Write a packet to a tap dev and copy it to ingress of a dummy dev */
+static void test_tuntap_mirred(struct bpf_program *xdp_prog,
+			       struct bpf_program *tc_prog,
+			       bool *test_pass)
+{
+	LIBBPF_OPTS(bpf_tc_hook, tc_hook, .attach_point = BPF_TC_INGRESS);
+	LIBBPF_OPTS(bpf_tc_opts, tc_opts, .handle = 1, .priority = 1);
+	struct netns_obj *ns = NULL;
+	int dummy_ifindex;
+	int tap_fd = -1;
+	int tap_ifindex;
+	int ret;
 
-	ret = write(tap_fd, packet, sizeof(packet));
-	if (!ASSERT_EQ(ret, sizeof(packet), "write packet"))
+	*test_pass = false;
+
+	ns = netns_new(TAP_NETNS, true);
+	if (!ASSERT_OK_PTR(ns, "netns_new"))
+		return;
+
+	/* Setup dummy interface */
+	SYS(close, "ip link add name " DUMMY_NAME " type dummy");
+	SYS(close, "ip link set dev " DUMMY_NAME " up");
+
+	dummy_ifindex = if_nametoindex(DUMMY_NAME);
+	if (!ASSERT_GE(dummy_ifindex, 0, "if_nametoindex"))
 		goto close;
 
-	assert_test_result(result_map);
+	tc_hook.ifindex = dummy_ifindex;
+	ret = bpf_tc_hook_create(&tc_hook);
+	if (!ASSERT_OK(ret, "bpf_tc_hook_create"))
+		goto close;
+
+	tc_opts.prog_fd = bpf_program__fd(tc_prog);
+	ret = bpf_tc_attach(&tc_hook, &tc_opts);
+	if (!ASSERT_OK(ret, "bpf_tc_attach"))
+		goto close;
+
+	/* Setup TAP interface */
+	tap_fd = open_tuntap(TAP_NAME, true);
+	if (!ASSERT_GE(tap_fd, 0, "open_tuntap"))
+		goto close;
+
+	SYS(close, "ip link set dev " TAP_NAME " up");
+
+	tap_ifindex = if_nametoindex(TAP_NAME);
+	if (!ASSERT_GE(tap_ifindex, 0, "if_nametoindex"))
+		goto close;
+
+	ret = bpf_xdp_attach(tap_ifindex, bpf_program__fd(xdp_prog), 0, NULL);
+	if (!ASSERT_GE(ret, 0, "bpf_xdp_attach"))
+		goto close;
+
+	/* Copy all packets received from TAP to dummy ingress */
+	SYS(close, "tc qdisc add dev " TAP_NAME " clsact");
+	SYS(close, "tc filter add dev " TAP_NAME " ingress "
+		   "protocol all matchall "
+		   "action mirred ingress mirror dev " DUMMY_NAME);
+
+	/* Receive a packet on TAP */
+	ret = write_test_packet(tap_fd);
+	if (!ASSERT_OK(ret, "write_test_packet"))
+		goto close;
+
+	ASSERT_TRUE(*test_pass, "test_pass");
 
 close:
 	if (tap_fd >= 0)
@@ -385,6 +464,30 @@ void test_xdp_context_tuntap(void)
 			    skel->progs.ing_cls_dynptr_offset_oob,
 			    skel->progs.ing_cls,
 			    skel->maps.test_result);
+	if (test__start_subtest("clone_data_meta_empty_on_data_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_data_meta_empty_on_data_write,
+				   &skel->bss->test_pass);
+	if (test__start_subtest("clone_data_meta_empty_on_meta_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_data_meta_empty_on_meta_write,
+				   &skel->bss->test_pass);
+	if (test__start_subtest("clone_dynptr_empty_on_data_slice_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_dynptr_empty_on_data_slice_write,
+				   &skel->bss->test_pass);
+	if (test__start_subtest("clone_dynptr_empty_on_meta_slice_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_dynptr_empty_on_meta_slice_write,
+				   &skel->bss->test_pass);
+	if (test__start_subtest("clone_dynptr_rdonly_before_data_dynptr_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_dynptr_rdonly_before_data_dynptr_write,
+				   &skel->bss->test_pass);
+	if (test__start_subtest("clone_dynptr_rdonly_before_meta_dynptr_write"))
+		test_tuntap_mirred(skel->progs.ing_xdp,
+				   skel->progs.clone_dynptr_rdonly_before_meta_dynptr_write,
+				   &skel->bss->test_pass);
 
 	test_xdp_meta__destroy(skel);
 }

tools/testing/selftests/bpf/progs/test_xdp_meta.c

@@ -26,6 +26,8 @@ struct {
 	__uint(value_size, META_SIZE);
 } test_result SEC(".maps");
 
+bool test_pass;
+
 SEC("tc")
 int ing_cls(struct __sk_buff *ctx)
 {
@@ -301,4 +303,193 @@ int ing_xdp(struct xdp_md *ctx)
 	return XDP_PASS;
 }
 
+/*
+ * Check that skb->data_meta..skb->data is empty if prog writes to packet
+ * _payload_ using packet pointers. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_data_meta_empty_on_data_write(struct __sk_buff *ctx)
+{
+	struct ethhdr *eth = ctx_ptr(ctx, data);
+
+	if (eth + 1 > ctx_ptr(ctx, data_end))
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	/* Expect no metadata */
+	if (ctx->data_meta != ctx->data)
+		goto out;
+
+	/* Packet write to trigger unclone in prologue */
+	eth->h_proto = 42;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
+/*
+ * Check that skb->data_meta..skb->data is empty if prog writes to packet
+ * _metadata_ using packet pointers. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_data_meta_empty_on_meta_write(struct __sk_buff *ctx)
+{
+	struct ethhdr *eth = ctx_ptr(ctx, data);
+	__u8 *md = ctx_ptr(ctx, data_meta);
+
+	if (eth + 1 > ctx_ptr(ctx, data_end))
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	if (md + 1 > ctx_ptr(ctx, data)) {
+		/* Expect no metadata */
+		test_pass = true;
+	} else {
+		/* Metadata write to trigger unclone in prologue */
+		*md = 42;
+	}
+out:
+	return TC_ACT_SHOT;
+}
+
+/*
+ * Check that skb_meta dynptr is writable but empty if prog writes to packet
+ * _payload_ using a dynptr slice. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_dynptr_empty_on_data_slice_write(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	struct ethhdr *eth;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	eth = bpf_dynptr_slice_rdwr(&data, 0, NULL, sizeof(*eth));
+	if (!eth)
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	/* Expect no metadata */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (bpf_dynptr_is_rdonly(&meta) || bpf_dynptr_size(&meta) > 0)
+		goto out;
+
+	/* Packet write to trigger unclone in prologue */
+	eth->h_proto = 42;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
+/*
+ * Check that skb_meta dynptr is writable but empty if prog writes to packet
+ * _metadata_ using a dynptr slice. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_dynptr_empty_on_meta_slice_write(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	const struct ethhdr *eth;
+	__u8 *md;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	eth = bpf_dynptr_slice(&data, 0, NULL, sizeof(*eth));
+	if (!eth)
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	/* Expect no metadata */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (bpf_dynptr_is_rdonly(&meta) || bpf_dynptr_size(&meta) > 0)
+		goto out;
+
+	/* Metadata write to trigger unclone in prologue */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	md = bpf_dynptr_slice_rdwr(&meta, 0, NULL, sizeof(*md));
+	if (md)
+		*md = 42;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
+/*
+ * Check that skb_meta dynptr is read-only before prog writes to packet payload
+ * using dynptr_write helper. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_dynptr_rdonly_before_data_dynptr_write(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	const struct ethhdr *eth;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	eth = bpf_dynptr_slice(&data, 0, NULL, sizeof(*eth));
+	if (!eth)
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	/* Expect read-only metadata before unclone */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (!bpf_dynptr_is_rdonly(&meta) || bpf_dynptr_size(&meta) != META_SIZE)
+		goto out;
+
+	/* Helper write to payload will unclone the packet */
+	bpf_dynptr_write(&data, offsetof(struct ethhdr, h_proto), "x", 1, 0);
+
+	/* Expect no metadata after unclone */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (bpf_dynptr_is_rdonly(&meta) || bpf_dynptr_size(&meta) != 0)
+		goto out;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
+/*
+ * Check that skb_meta dynptr is read-only if prog writes to packet
+ * metadata using dynptr_write helper. Applies only to cloned skbs.
+ */
+SEC("tc")
+int clone_dynptr_rdonly_before_meta_dynptr_write(struct __sk_buff *ctx)
+{
+	struct bpf_dynptr data, meta;
+	const struct ethhdr *eth;
+
+	bpf_dynptr_from_skb(ctx, 0, &data);
+	eth = bpf_dynptr_slice(&data, 0, NULL, sizeof(*eth));
+	if (!eth)
+		goto out;
+	/* Ignore non-test packets */
+	if (eth->h_proto != 0)
+		goto out;
+
+	/* Expect read-only metadata */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (!bpf_dynptr_is_rdonly(&meta) || bpf_dynptr_size(&meta) != META_SIZE)
+		goto out;
+
+	/* Metadata write. Expect failure. */
+	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
+	if (bpf_dynptr_write(&meta, 0, "x", 1, 0) != -EINVAL)
+		goto out;
+
+	test_pass = true;
+out:
+	return TC_ACT_SHOT;
+}
+
 char _license[] SEC("license") = "GPL";