[PATCH bpf v5 0/2] bpf: Fix generic devmap egress skb sharing

[PATCH bpf v5 0/2] bpf: Fix generic devmap egress skb sharing

[Sun Jian]

Generic XDP devmap multi redirect can leave cloned skbs sharing packet
data. When a devmap egress program mutates packet data, another
destination sharing the same data may observe that mutation.

Fix this by making cloned skbs private before running the generic devmap
egress program. The private copy is made in dev_map_generic_redirect()
so dev_map_bpf_prog_run_skb() can keep returning the XDP action directly.

Add selftest coverage for the last-destination case, where the final
destination runs on the original skb while earlier destinations use 
cloned skbs. The test records the source MAC observed by an earlier
destination and checks that it is neither the sentinel value left in the 
result map nor the MAC written by the final destination.

---

v5:
- Move the skb_copy() check back to dev_map_generic_redirect() to keep
  dev_map_bpf_prog_run_skb() returning only the XDP action.
- Preserve mac_len after skb_copy().
- Use __be64 temporary values when updating mac_map from userspace.
- Initialize rx_mac with a sentinel in the last-destination test instead
  of relying on -ENOENT for ARRAY map lookups.
- Adjust the last-destination test topology so the checked earlier
  destination is not the ingress/source veth.
- Split the last-destination check into two assertions: one for store_mac_1
  updating rx_mac and one for detecting last-destination rewrite leakage.

v4: https://lore.kernel.org/bpf/20260611080850.536996-1-sun.jian.kdev@gmail.com/T/#mf830f03d362f33e0941d1b0e425169698fce76e5
- Preserve mac_len after skb_copy().
- Separate errno return from XDP action output in
  dev_map_bpf_prog_run_skb().
- Zero-initialize net_config in the new selftest.

v3: https://lore.kernel.org/bpf/20260611043317.512843-1-sun.jian.kdev@gmail.com/
- Split the kernel fix and selftest into separate patches.
- Move the private-copy logic into dev_map_bpf_prog_run_skb().
- Use deterministic DEVMAP_HASH keys in the last-destination selftest.
- Fix the Fixes tag.

v2: https://lore.kernel.org/bpf/08c35c70-a59e-4e0e-91db-22b5ec30b611@linux.dev/
- Move the private-copy step into dev_map_generic_redirect() so the
  last-destination path is covered as well.
- Use skb_copy() instead of skb_unshare() to keep caller ownership
  unchanged on allocation failure.
- Add a generic XDP last-destination selftest case.

v1: https://lore.kernel.org/bpf/CABFUUZFimdrZdq=NWi+N-0sJZWvMwY=f4iF6-3TVMS8=m07Zmw@mail.gmail.com/

Sun Jian (2):
  bpf: Run generic devmap egress prog on private skb
  selftests/bpf: Cover generic devmap egress last-dst rewrite

 kernel/bpf/devmap.c                           |  12 ++
 .../selftests/bpf/prog_tests/test_xdp_veth.c  | 166 +++++++++++++++++-
 2 files changed, 175 insertions(+), 3 deletions(-)


base-commit: e7ae89a0c97ce2b68b0983cd01eda67cf373517d
-- 
2.43.0

[PATCH bpf v5 1/2] bpf: Run generic devmap egress prog on private skb

[Sun Jian]

Generic XDP devmap multi redirect uses skb_clone() for intermediate
destinations and sends the last destination with the original skb. This
can leave multiple destinations sharing the same packet data.

This becomes visible after generic devmap egress-program support was
added: a devmap egress program may mutate packet data, and another
destination sharing the same data can observe that mutation.

Native XDP broadcast redirect does not have this issue because
xdpf_clone() copies the frame data for each destination. Generic XDP
should provide the same per-destination isolation before running a
devmap egress program.

Fix this by making cloned skbs private before running the generic devmap
egress program. Use skb_copy() instead of skb_unshare() so allocation
failure does not consume the skb and the existing caller error paths keep
their ownership semantics.

Fixes: 2ea5eabaf04a ("bpf: devmap: Implement devmap prog execution for generic XDP")
Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 kernel/bpf/devmap.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
index cc0a43ebab6b..28bd44591ce4 100644
--- a/kernel/bpf/devmap.c
+++ b/kernel/bpf/devmap.c
@@ -706,6 +706,18 @@ int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,
 	if (unlikely(err))
 		return err;
 
+	if (dst->xdp_prog && skb_cloned(skb)) {
+		struct sk_buff *nskb;
+
+		nskb = skb_copy(skb, GFP_ATOMIC);
+		if (!nskb)
+			return -ENOMEM;
+
+		nskb->mac_len = skb->mac_len;
+		consume_skb(skb);
+		skb = nskb;
+	}
+
 	/* Redirect has already succeeded semantically at this point, so we just
 	 * return 0 even if packet is dropped. Helper below takes care of
 	 * freeing skb.
-- 
2.43.0

[PATCH bpf v5 2/2] selftests/bpf: Cover generic devmap egress last-dst rewrite

[Sun Jian]

Strengthen xdp_veth_egress to check that each destination observes the
MAC selected for its own egress ifindex, instead of only checking that
the observed MAC differs from a single magic value.

Add a generic XDP last-destination test where an earlier destination does
not have a devmap egress program while the final destination does. This
covers the case where the final destination runs on the original skb and
could otherwise rewrite packet data still shared with an earlier cloned
skb.

Use deterministic DEVMAP_HASH keys for the egress map so the intended
last destination is stable. Initialize the result map with a sentinel
value and check that store_mac_1 overwrites it before checking that the
earlier destination did not observe the MAC written by the final
destination.

Suggested-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 .../selftests/bpf/prog_tests/test_xdp_veth.c  | 166 +++++++++++++++++-
 1 file changed, 163 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/test_xdp_veth.c b/tools/testing/selftests/bpf/prog_tests/test_xdp_veth.c
index 3e98a1665936..1675b32753a8 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_xdp_veth.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_xdp_veth.c
@@ -456,7 +456,11 @@ static void xdp_veth_egress(u32 flags)
 			.remote_flags = flags,
 		}
 	};
-	const char magic_mac[6] = { 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0xFF};
+	const unsigned char egress_macs[VETH_PAIRS_COUNT][ETH_ALEN] = {
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x01 },
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x02 },
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x03 },
+	};
 	struct xdp_redirect_multi_kern *xdp_redirect_multi_kern;
 	struct bpf_object *bpf_objs[VETH_EGRESS_SKEL_NB];
 	struct xdp_redirect_map *xdp_redirect_map;
@@ -512,7 +516,13 @@ static void xdp_veth_egress(u32 flags)
 						 &net_config, prog_cfg, i))
 			goto destroy_xdp_redirect_map;
 
-		err = bpf_map_update_elem(mac_map, &ifindex, magic_mac, 0);
+		{
+			__be64 mac = 0;
+
+			memcpy(&mac, egress_macs[i], ETH_ALEN);
+			err = bpf_map_update_elem(mac_map, &ifindex, &mac, 0);
+		}
+
 		if (!ASSERT_OK(err, "bpf_map_update_elem"))
 			goto destroy_xdp_redirect_map;
 
@@ -531,15 +541,162 @@ static void xdp_veth_egress(u32 flags)
 
 	for (i = 0; i < 2; i++) {
 		u32 key = i;
+		__be64 expected = 0;
 		u64 res;
 
 		err = bpf_map_lookup_elem(res_map, &key, &res);
 		if (!ASSERT_OK(err, "get MAC res"))
 			goto destroy_xdp_redirect_map;
 
-		ASSERT_STRNEQ((const char *)&res, magic_mac, ETH_ALEN, "compare mac");
+		/* store_mac_1/2 run on the second/third remote veths. */
+		memcpy(&expected, egress_macs[i + 1], ETH_ALEN);
+		ASSERT_EQ(res, expected, "compare mac");
+	}
+
+destroy_xdp_redirect_map:
+	close_netns(nstoken);
+	xdp_redirect_map__destroy(xdp_redirect_map);
+destroy_xdp_redirect_multi_kern:
+	xdp_redirect_multi_kern__destroy(xdp_redirect_multi_kern);
+destroy_xdp_dummy:
+	xdp_dummy__destroy(xdp_dummy);
+
+	cleanup_network(&net_config);
+}
+
+static void xdp_veth_egress_last_dst(u32 flags)
+{
+	struct prog_configuration prog_cfg[VETH_PAIRS_COUNT] = {
+		{
+			.local_name = "xdp_redirect_map_all_prog",
+			.remote_name = "xdp_dummy_prog",
+			.local_flags = flags,
+			.remote_flags = flags,
+		},
+		{
+			.local_name = "xdp_redirect_map_all_prog",
+			.remote_name = "store_mac_1",
+			.local_flags = flags,
+			.remote_flags = flags,
+		},
+		{
+			.local_name = "xdp_redirect_map_all_prog",
+			.remote_name = "xdp_dummy_prog",
+			.local_flags = flags,
+			.remote_flags = flags,
+		}
+	};
+	const unsigned char egress_macs[VETH_PAIRS_COUNT][ETH_ALEN] = {
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x01 },
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x02 },
+		{ 0xAA, 0xBB, 0xCC, 0xDD, 0xEE, 0x03 },
+	};
+	struct xdp_redirect_multi_kern *xdp_redirect_multi_kern;
+	struct bpf_object *bpf_objs[VETH_EGRESS_SKEL_NB];
+	struct xdp_redirect_map *xdp_redirect_map;
+	struct net_configuration net_config = {};
+	int mac_map, egress_map, res_map;
+	struct nstoken *nstoken = NULL;
+	struct xdp_dummy *xdp_dummy;
+	__be64 sentinel_mac = 0;
+	__be64 last_mac = 0;
+	__be64 res;
+	u32 key;
+	int err;
+	int i;
+
+	xdp_dummy = xdp_dummy__open_and_load();
+	if (!ASSERT_OK_PTR(xdp_dummy, "xdp_dummy__open_and_load"))
+		return;
+
+	xdp_redirect_multi_kern = xdp_redirect_multi_kern__open_and_load();
+	if (!ASSERT_OK_PTR(xdp_redirect_multi_kern, "xdp_redirect_multi_kern__open_and_load"))
+		goto destroy_xdp_dummy;
+
+	xdp_redirect_map = xdp_redirect_map__open_and_load();
+	if (!ASSERT_OK_PTR(xdp_redirect_map, "xdp_redirect_map__open_and_load"))
+		goto destroy_xdp_redirect_multi_kern;
+
+	if (!ASSERT_OK(create_network(&net_config), "create network"))
+		goto destroy_xdp_redirect_map;
+
+	mac_map = bpf_map__fd(xdp_redirect_multi_kern->maps.mac_map);
+	if (!ASSERT_OK_FD(mac_map, "open mac_map"))
+		goto destroy_xdp_redirect_map;
+
+	egress_map = bpf_map__fd(xdp_redirect_multi_kern->maps.map_egress);
+	if (!ASSERT_OK_FD(egress_map, "open map_egress"))
+		goto destroy_xdp_redirect_map;
+
+	bpf_objs[0] = xdp_dummy->obj;
+	bpf_objs[1] = xdp_redirect_multi_kern->obj;
+	bpf_objs[2] = xdp_redirect_map->obj;
+
+	nstoken = open_netns(net_config.ns0_name);
+	if (!ASSERT_OK_PTR(nstoken, "open NS0"))
+		goto destroy_xdp_redirect_map;
+
+	for (i = 0; i < VETH_PAIRS_COUNT; i++) {
+		struct bpf_devmap_val devmap_val = {};
+		int ifindex = if_nametoindex(net_config.veth_cfg[i].local_veth);
+		u32 key = i;
+
+		SYS(destroy_xdp_redirect_map,
+		    "ip -n %s neigh add %s lladdr 00:00:00:00:00:01 dev %s",
+		    net_config.veth_cfg[i].namespace, IP_NEIGH,
+		    net_config.veth_cfg[i].remote_veth);
+
+		if (attach_programs_to_veth_pair(bpf_objs, VETH_EGRESS_SKEL_NB,
+						 &net_config, prog_cfg, i))
+			goto destroy_xdp_redirect_map;
+
+		{
+			__be64 mac = 0;
+
+			memcpy(&mac, egress_macs[i], ETH_ALEN);
+			err = bpf_map_update_elem(mac_map, &ifindex, &mac, 0);
+		}
+
+		if (!ASSERT_OK(err, "bpf_map_update_elem"))
+			goto destroy_xdp_redirect_map;
+
+		devmap_val.ifindex = ifindex;
+		devmap_val.bpf_prog.fd = -1;
+
+		if (i == VETH_PAIRS_COUNT - 1)
+			devmap_val.bpf_prog.fd =
+				bpf_program__fd(xdp_redirect_multi_kern->progs.xdp_devmap_prog);
+
+		err = bpf_map_update_elem(egress_map, &key, &devmap_val, 0);
+		if (!ASSERT_OK(err, "bpf_map_update_elem"))
+			goto destroy_xdp_redirect_map;
 	}
 
+	res_map = bpf_map__fd(xdp_redirect_map->maps.rx_mac);
+	if (!ASSERT_OK_FD(res_map, "open rx_map"))
+		goto destroy_xdp_redirect_map;
+
+	memcpy(&sentinel_mac, egress_macs[VETH_PAIRS_COUNT - 1], ETH_ALEN);
+	memcpy(&last_mac, egress_macs[VETH_PAIRS_COUNT - 1], ETH_ALEN);
+
+	key = 0;
+	err = bpf_map_update_elem(res_map, &key, &sentinel_mac, 0);
+	if (!ASSERT_OK(err, "init rx mac"))
+		goto destroy_xdp_redirect_map;
+
+	SYS_NOFAIL("ip netns exec %s ping %s -i 0.1 -c 4 -W1 > /dev/null ",
+		   net_config.veth_cfg[0].namespace, IP_NEIGH);
+
+	err = bpf_map_lookup_elem(res_map, &key, &res);
+	if (!ASSERT_OK(err, "get MAC res"))
+		goto destroy_xdp_redirect_map;
+
+	if (!ASSERT_NEQ(res, sentinel_mac, "rx_mac overwritten by store_mac_1"))
+		goto destroy_xdp_redirect_map;
+
+	if (!ASSERT_NEQ(res, last_mac, "earlier dst not rewritten by last dst"))
+		goto destroy_xdp_redirect_map;
+
 destroy_xdp_redirect_map:
 	close_netns(nstoken);
 	xdp_redirect_map__destroy(xdp_redirect_map);
@@ -596,4 +753,7 @@ void test_xdp_veth_egress(void)
 
 	if (test__start_subtest("SKB_MODE/egress"))
 		xdp_veth_egress(XDP_FLAGS_SKB_MODE);
+
+	if (test__start_subtest("SKB_MODE/egress_last_dst"))
+		xdp_veth_egress_last_dst(XDP_FLAGS_SKB_MODE);
 }
-- 
2.43.0