[PATCH bpf v3 0/2] net: xdp: fix bpf_xdp_shrink_data() page handling on generic XDP and veth

[PATCH bpf v3 0/2] net: xdp: fix bpf_xdp_shrink_data() page handling on generic XDP and veth

[Jiayuan Chen]

I'm always confused which tree(net or bpf) should XDP target.

bpf_xdp_shrink_data() frees a page_pool frag with the wrong memory type on
skb-backed XDP, hitting "Bad page state ... page_pool leak". Both the
generic XDP path and the veth path are affected.

Patch 1 fixes it by tagging the xdp_buff, so it no longer depends on
rxq->mem.type (which is shared on generic XDP and gets reset on veth). It
is reported by syzbot.

Patch 2 adds a selftest that reproduces the leak on both paths.


v2 -> v3: handle some feedback from AI review.
v2: https://lore.kernel.org/bpf/20260824030257.263179-1-jiayuan.chen@linux.dev/
v1 -> v2: AI found the fix was insufficient and we need a general way
          to fix them.
v1: https://lore.kernel.org/bpf/20260816031245.268898-1-jiayuan.chen@linux.dev/

Jiayuan Chen (2):
  bpf, veth: xdp: fix page_pool page leak on skb-backed XDP
  selftests/bpf: add xdp_shrink_frags

 drivers/net/veth.c                            |   6 +
 include/net/xdp.h                             |  30 +-
 net/core/dev.c                                |   6 +
 net/core/filter.c                             |   7 +
 .../bpf/prog_tests/xdp_shrink_frags.c         | 288 ++++++++++++++++++
 .../selftests/bpf/progs/xdp_shrink_frags.c    |  34 +++
 6 files changed, 370 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
 create mode 100644 tools/testing/selftests/bpf/progs/xdp_shrink_frags.c

-- 
2.43.0

[PATCH bpf v3 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

[Jiayuan Chen]

bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED,
so the page_pool page is freed with page_frag_free() and we hit
"Bad page state ... page_pool leak".

Both generic XDP and veth are affected. A non-linear skb is cow'd into
page_pool memory (skb_cow_data_for_xdp() -> skb_pp_cow_data() for generic
XDP, veth_convert_skb_to_xdp_buff() for veth), so its frags become
page_pool pages while the rxq keeps MEM_TYPE_PAGE_SHARED.

We can't just fix rxq->mem.type in place:
- generic XDP: xdp->rxq is dev->_rx[queue].xdp_rxq (see
  bpf_prog_run_generic_xdp()), a shared rxq that other CPUs may access in
  parallel, so we must not write to it.
- veth: rq->xdp_rxq.mem is shared per-queue state that veth resets on XDP
  teardown, and with GRO that reset runs without stopping in-flight NAPI,
  so a type stashed there can be clobbered under a packet still in flight.

Adding a check in __xdp_return() or bpf_xdp_shrink_data() itself is not an
option either: without recording it somewhere, both can only guess the
frag's memory type, which quickly gets confusing.

So record it in the xdp_buff. A dedicated mem_type field (mirroring
xdp_frame->mem_type) would read more naturally, but it grows xdp_buff, and
struct xdp_page_head embeds two of them, which shifts the layout the
xdp_do_redirect live-frames selftest hard-codes (MAX_PKT_SIZE). So use a
flag bit: add XDP_FLAGS_FRAGS_PAGE_POOL, the two skb-cow sites set it, and
bpf_xdp_shrink_data() frees the frag to the page_pool when it is set,
otherwise it keeps falling back to xdp->rxq->mem.type unchanged.

The flag describes the buff, so the buff <-> frame conversions have to keep
it consistent:
- buff -> frame: xdp_update_frame_from_buff() copies xdp->flags into the
  frame. veth XDP_TX/XDP_REDIRECT hand the frame to the peer, and cpumap
  and devmap run a second program on it, so the peer would inherit a stale
  tag; strip it, an xdp_frame describes its memory with ::mem_type.
- frame -> buff: xdp_convert_frame_to_buff() rebuilds a buff, and veth and
  devmap then run a program whose rxq says MEM_TYPE_PAGE_SHARED even for a
  page_pool frame (only cpumap copies mem_type into the rxq). A shrink
  there leaks the same way (this predates the series: bpf_xdp_adjust_tail()
  frees frags by rxq->mem.type), so re-derive the flag from the frame's
  ::mem_type.

Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
Fixes: 0ebab78cbcbf ("net: veth: add page_pool for page recycling")
Fixes: bf25146a5595 ("bpf: add frags support to the bpf_xdp_adjust_tail() API")
Reported-by: syzbot+237bbeed8dfe0699b7f5@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=237bbeed8dfe0699b7f5
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 drivers/net/veth.c |  6 ++++++
 include/net/xdp.h  | 30 +++++++++++++++++++++++++++++-
 net/core/dev.c     |  6 ++++++
 net/core/filter.c  |  7 +++++++
 4 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/drivers/net/veth.c b/drivers/net/veth.c
index 6ed3ee81153fb..a3fdf1959b76c 100644
--- a/drivers/net/veth.c
+++ b/drivers/net/veth.c
@@ -775,6 +775,12 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
 	if (skb_shinfo(skb)->nr_frags) {
 		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
 		xdp_buff_set_frags_flag(xdp);
+		/*
+		 * A nonlinear skb was cow'd into rq->page_pool above, so the
+		 * frags must be freed to that pool, not via the rxq's
+		 * MEM_TYPE_PAGE_SHARED.
+		 */
+		xdp_buff_set_frag_pp(xdp);
 	} else {
 		xdp_buff_clear_frags_flag(xdp);
 	}
diff --git a/include/net/xdp.h b/include/net/xdp.h
index aa742f413c358..8fd4139e9cbd3 100644
--- a/include/net/xdp.h
+++ b/include/net/xdp.h
@@ -81,6 +81,14 @@ enum xdp_buff_flags {
 	 * XDP program is not attached.
 	 */
 	XDP_FLAGS_FRAGS_UNREADABLE	= BIT(2),
+	/*
+	 * frags are page_pool memory even though rxq->mem.type is not: a
+	 * skb-backed XDP buff (generic XDP, veth) is cow'd into a page_pool.
+	 * xdp_buff only: an xdp_frame describes its memory with ::mem_type, so
+	 * this is stripped in xdp_update_frame_from_buff() and re-derived from
+	 * ::mem_type in xdp_convert_frame_to_buff().
+	 */
+	XDP_FLAGS_FRAGS_PAGE_POOL	= BIT(3),
 };
 
 struct xdp_buff {
@@ -131,6 +139,16 @@ static __always_inline void xdp_buff_set_frag_unreadable(struct xdp_buff *xdp)
 	xdp->flags |= XDP_FLAGS_FRAGS_UNREADABLE;
 }
 
+static __always_inline void xdp_buff_set_frag_pp(struct xdp_buff *xdp)
+{
+	xdp->flags |= XDP_FLAGS_FRAGS_PAGE_POOL;
+}
+
+static __always_inline bool xdp_buff_is_frag_pp(const struct xdp_buff *xdp)
+{
+	return !!(xdp->flags & XDP_FLAGS_FRAGS_PAGE_POOL);
+}
+
 static __always_inline u32 xdp_buff_get_skb_flags(const struct xdp_buff *xdp)
 {
 	return xdp->flags;
@@ -394,6 +412,15 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
 	xdp->data_meta = frame->data - frame->metasize;
 	xdp->frame_sz = frame->frame_sz;
 	xdp->flags = frame->flags;
+	/*
+	 * frame->flags never carries XDP_FLAGS_FRAGS_PAGE_POOL (it is stripped
+	 * in xdp_update_frame_from_buff()); re-derive it from the frame's own
+	 * memory type. veth and devmap rebuild a buff here and run a program
+	 * whose rxq says MEM_TYPE_PAGE_SHARED, so without this a shrink would
+	 * free a page_pool frag through page_frag_free().
+	 */
+	if (frame->mem_type == MEM_TYPE_PAGE_POOL)
+		xdp_buff_set_frag_pp(xdp);
 }
 
 static inline
@@ -420,7 +447,8 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
 	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
 	xdp_frame->metasize = metasize;
 	xdp_frame->frame_sz = xdp->frame_sz;
-	xdp_frame->flags = xdp->flags;
+	/* XDP_FLAGS_FRAGS_PAGE_POOL is xdp_buff only, don't carry it over */
+	xdp_frame->flags = xdp->flags & ~XDP_FLAGS_FRAGS_PAGE_POOL;
 
 	return 0;
 }
diff --git a/net/core/dev.c b/net/core/dev.c
index 290e0f099e6bf..94268f4d3c5f7 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -5532,6 +5532,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
 	if (skb_is_nonlinear(skb)) {
 		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
 		xdp_buff_set_frags_flag(xdp);
+		/*
+		 * A nonlinear skb was cow'd into page_pool memory by
+		 * skb_cow_data_for_xdp() before we got here, so the frags must
+		 * be freed to that pool, not via the rxq's MEM_TYPE_PAGE_SHARED.
+		 */
+		xdp_buff_set_frag_pp(xdp);
 	} else {
 		xdp_buff_clear_frags_flag(xdp);
 	}
diff --git a/net/core/filter.c b/net/core/filter.c
index 61940e7535523..d34ba56d79d8f 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -4378,6 +4378,13 @@ static bool bpf_xdp_shrink_data(struct xdp_buff *xdp, skb_frag_t *frag,
 	if (mem_type == MEM_TYPE_XSK_BUFF_POOL) {
 		netmem = 0;
 		zc_frag = bpf_xdp_shrink_data_zc(xdp, shrink, tail, release);
+	} else if (xdp_buff_is_frag_pp(xdp)) {
+		/*
+		 * Skb-backed XDP (generic XDP, veth) cow's the frags into a
+		 * page_pool while the rxq stays MEM_TYPE_PAGE_SHARED, so free
+		 * the frag to the pool, not via page_frag_free().
+		 */
+		mem_type = MEM_TYPE_PAGE_POOL;
 	}
 
 	if (release) {
-- 
2.43.0

[PATCH bpf v3 2/2] selftests/bpf: add xdp_shrink_frags

[Jiayuan Chen]

Add a test that attaches an xdp.frags program which shrinks a whole frag
away, so bpf_xdp_shrink_data() frees a page_pool frag.

test_tun triggers the page-type mismatch on the generic XDP path.
test_veth triggers the same mismatch on the veth path.
test_veth_tx bounces the cow'd buff with XDP_TX so the peer runs a frags
program on the resulting frame, covering the buff -> frame -> buff path.

A kernel without the fix will report "Bad page state ... page_pool leak".

The packet sizes only leave a frag after the cow on 4K pages, so the
subtests skip elsewhere.

Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
---
 .../bpf/prog_tests/xdp_shrink_frags.c         | 288 ++++++++++++++++++
 .../selftests/bpf/progs/xdp_shrink_frags.c    |  34 +++
 2 files changed, 322 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
 create mode 100644 tools/testing/selftests/bpf/progs/xdp_shrink_frags.c

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
new file mode 100644
index 0000000000000..ee8f9034a2684
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
@@ -0,0 +1,288 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include <network_helpers.h>
+#include <linux/if_tun.h>
+#include <linux/if_ether.h>
+#include <sys/uio.h>
+#include <net/if.h>
+#include <arpa/inet.h>
+#include "xdp_shrink_frags.skel.h"
+
+/*
+ * A generic-XDP program that shrinks into the frags frees a page_pool frag.
+ * skb-backed XDP first cow's the nonlinear skb into page_pool memory
+ * (skb_cow_data_for_xdp() for generic XDP, skb_pp_cow_data() for veth), but
+ * the shared rxq is registered as MEM_TYPE_PAGE_SHARED, so a buggy kernel
+ * frees the frag with page_frag_free() -> "Bad page state ... page_pool leak".
+ */
+
+#define TAP_NAME	"xdp_shrink0"
+#define TAP_NETNS	"xdp_shrink_tap"
+
+#define NS_NAME_MAX_LEN	32
+#define VETH_LOCAL	"xdp_shrinkA"
+#define VETH_PEER	"xdp_shrinkB"
+#define VETH_LOCAL_IP	"10.9.9.1"
+#define VETH_PEER_IP	"10.9.9.2"
+#define VETH_LOCAL_MAC	"02:00:00:00:00:01"
+
+/*
+ * skb_pp_cow_data() keeps up to one page in the linear part, so the packet
+ * sizes below only leave a frag (smaller than the 3000-byte shrink, so it is
+ * released as a whole) on 4K pages. Skip elsewhere rather than run a test
+ * that cannot tell a fixed kernel from a buggy one.
+ */
+#define PAGE_SIZE_4K	4096
+
+/*
+ * Generous, so a loaded CI does not fail the assert prematurely; normally
+ * the first check already succeeds.
+ */
+#define WAIT_ITERS	10000
+#define WAIT_US		1000
+
+static void wait_for_prog(struct xdp_shrink_frags *skel)
+{
+	int i;
+
+	for (i = 0; i < WAIT_ITERS && !skel->bss->shrink_ran; i++)
+		usleep(WAIT_US);
+}
+
+static int create_tap_napi_frags(const char *ifname)
+{
+	struct ifreq ifr = {
+		.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_NAPI | IFF_NAPI_FRAGS,
+	};
+	int fd, err;
+
+	strscpy(ifr.ifr_name, ifname);
+
+	fd = open("/dev/net/tun", O_RDWR);
+	if (fd < 0)
+		return -errno;
+
+	err = ioctl(fd, TUNSETIFF, &ifr);
+	if (err) {
+		err = -errno;
+		close(fd);
+		return err;
+	}
+
+	return fd;
+}
+
+/*
+ * Similar to flow_dissector.c: writev() an IFF_NAPI_FRAGS tap to build a
+ * nonlinear skb that tun runs through do_xdp_generic().
+ */
+static void test_tun(struct xdp_shrink_frags *skel)
+{
+	__u8 head[74], frag1[2048], frag2[2048];
+	struct ethhdr *eth = (void *)head;
+	int tap_fd = -1, ifindex, err;
+	struct netns_obj *ns = NULL;
+	struct iovec iov[3];
+	ssize_t n;
+
+	if (getpagesize() != PAGE_SIZE_4K) {
+		test__skip();
+		return;
+	}
+
+	ns = netns_new(TAP_NETNS, true);
+	if (!ASSERT_OK_PTR(ns, "netns_new"))
+		return;
+
+	tap_fd = create_tap_napi_frags(TAP_NAME);
+	if (!ASSERT_GE(tap_fd, 0, "create_tap"))
+		goto out;
+
+	SYS(out, "ip link set dev " TAP_NAME " up");
+
+	ifindex = if_nametoindex(TAP_NAME);
+	if (!ASSERT_GT(ifindex, 0, "if_nametoindex"))
+		goto out;
+
+	skel->bss->shrink_ran = 0;
+
+	err = bpf_xdp_attach(ifindex, bpf_program__fd(skel->progs.xdp_shrink),
+			     0, NULL);
+	if (!ASSERT_OK(err, "bpf_xdp_attach"))
+		goto out;
+
+	memset(head, 0, sizeof(head));
+	memset(frag1, 0x41, sizeof(frag1));
+	memset(frag2, 0x42, sizeof(frag2));
+	eth->h_proto = htons(ETH_P_IP);
+
+	iov[0].iov_base = head;  iov[0].iov_len = sizeof(head);
+	iov[1].iov_base = frag1; iov[1].iov_len = sizeof(frag1);
+	iov[2].iov_base = frag2; iov[2].iov_len = sizeof(frag2);
+
+	n = writev(tap_fd, iov, ARRAY_SIZE(iov));
+	ASSERT_EQ(n, sizeof(head) + sizeof(frag1) + sizeof(frag2), "writev");
+
+	wait_for_prog(skel);
+	/* a buggy kernel only splats "page_pool leak", it does not fail here */
+	ASSERT_GT(skel->bss->shrink_ran, 0, "xdp_prog_ran");
+
+	bpf_xdp_detach(ifindex, 0, NULL);
+out:
+	if (tap_fd >= 0)
+		close(tap_fd);
+	netns_free(ns);
+}
+
+/*
+ * Both veth ends live in their own namespace, so the traffic really crosses
+ * the pair and nothing is created in the caller's namespace.
+ */
+static int veth_setup(char *ns0, char *ns1)
+{
+	if (!ASSERT_OK(append_tid(ns0, NS_NAME_MAX_LEN), "append_tid ns0"))
+		return -1;
+	if (!ASSERT_OK(append_tid(ns1, NS_NAME_MAX_LEN), "append_tid ns1"))
+		return -1;
+
+	SYS(fail, "ip netns add %s", ns0);
+	SYS(fail_ns0, "ip netns add %s", ns1);
+	SYS(fail_ns1, "ip -n %s link add %s mtu 8000 type veth peer name %s mtu 8000",
+	    ns0, VETH_LOCAL, VETH_PEER);
+	SYS(fail_ns1, "ip -n %s link set %s netns %s", ns0, VETH_PEER, ns1);
+	SYS(fail_ns1, "ip -n %s link set %s address %s", ns0, VETH_LOCAL,
+	    VETH_LOCAL_MAC);
+	SYS(fail_ns1, "ip -n %s addr add %s/24 dev %s", ns0, VETH_LOCAL_IP,
+	    VETH_LOCAL);
+	SYS(fail_ns1, "ip -n %s link set %s up", ns0, VETH_LOCAL);
+	SYS(fail_ns1, "ip -n %s addr add %s/24 dev %s", ns1, VETH_PEER_IP,
+	    VETH_PEER);
+	SYS(fail_ns1, "ip -n %s link set %s up", ns1, VETH_PEER);
+
+	return 0;
+
+fail_ns1:
+	SYS_NOFAIL("ip netns del %s", ns1);
+fail_ns0:
+	SYS_NOFAIL("ip netns del %s", ns0);
+fail:
+	return -1;
+}
+
+static void veth_cleanup(const char *ns0, const char *ns1)
+{
+	/* Dropping the namespaces takes the veth pair and its XDP programs. */
+	SYS_NOFAIL("ip netns del %s", ns1);
+	SYS_NOFAIL("ip netns del %s", ns0);
+}
+
+static int veth_attach(const char *ns, const char *dev, int prog_fd)
+{
+	struct nstoken *nstoken;
+	int ifindex, err;
+
+	nstoken = open_netns(ns);
+	if (!ASSERT_OK_PTR(nstoken, "open_netns"))
+		return -1;
+
+	ifindex = if_nametoindex(dev);
+	if (!ASSERT_GT(ifindex, 0, "if_nametoindex")) {
+		close_netns(nstoken);
+		return -1;
+	}
+
+	err = bpf_xdp_attach(ifindex, prog_fd, 0, NULL);
+	close_netns(nstoken);
+
+	return ASSERT_OK(err, "bpf_xdp_attach") ? 0 : -1;
+}
+
+/* A large ping builds a nonlinear skb that veth cow's into its page_pool. */
+static void test_veth(struct xdp_shrink_frags *skel)
+{
+	char ns0[NS_NAME_MAX_LEN] = "xdp_shrink_ns0-";
+	char ns1[NS_NAME_MAX_LEN] = "xdp_shrink_ns1-";
+
+	if (getpagesize() != PAGE_SIZE_4K) {
+		test__skip();
+		return;
+	}
+
+	if (veth_setup(ns0, ns1))
+		return;
+
+	skel->bss->shrink_ran = 0;
+
+	if (veth_attach(ns0, VETH_LOCAL, bpf_program__fd(skel->progs.xdp_shrink)))
+		goto out;
+
+	SYS_NOFAIL("ip netns exec %s ping -q -s 5000 -c 3 -W 1 %s",
+		   ns1, VETH_LOCAL_IP);
+
+	wait_for_prog(skel);
+	/* a buggy kernel only splats "page_pool leak", it does not fail here */
+	ASSERT_GT(skel->bss->shrink_ran, 0, "xdp_prog_ran");
+out:
+	veth_cleanup(ns0, ns1);
+}
+
+/*
+ * LOCAL cow's the incoming skb and returns XDP_TX, so the buff is turned into
+ * an xdp_frame and bounced to PEER, which shrinks a frag. A page_pool tag
+ * recorded on the buff must not leak into the frame, or PEER frees a plain
+ * page (whose pp was already cleared on the XDP_TX side) as page_pool memory.
+ */
+static void test_veth_tx(struct xdp_shrink_frags *skel)
+{
+	char ns0[NS_NAME_MAX_LEN] = "xdp_shrink_tx0-";
+	char ns1[NS_NAME_MAX_LEN] = "xdp_shrink_tx1-";
+
+	if (getpagesize() != PAGE_SIZE_4K) {
+		test__skip();
+		return;
+	}
+
+	if (veth_setup(ns0, ns1))
+		return;
+
+	/*
+	 * LOCAL bounces everything (incl. ARP) with XDP_TX, so pin a static
+	 * neighbour to let the ping's payload actually reach it.
+	 */
+	SYS(out, "ip -n %s neigh add %s lladdr %s dev %s nud permanent",
+	    ns1, VETH_LOCAL_IP, VETH_LOCAL_MAC, VETH_PEER);
+
+	skel->bss->shrink_ran = 0;
+
+	if (veth_attach(ns0, VETH_LOCAL, bpf_program__fd(skel->progs.xdp_tx)))
+		goto out;
+	if (veth_attach(ns1, VETH_PEER, bpf_program__fd(skel->progs.xdp_shrink)))
+		goto out;
+
+	SYS_NOFAIL("ip netns exec %s ping -q -s 5000 -c 3 -W 1 %s",
+		   ns1, VETH_LOCAL_IP);
+
+	wait_for_prog(skel);
+	/* a buggy kernel only splats "page_pool leak", it does not fail here */
+	ASSERT_GT(skel->bss->shrink_ran, 0, "xdp_prog_ran");
+out:
+	veth_cleanup(ns0, ns1);
+}
+
+void test_xdp_shrink_frags(void)
+{
+	struct xdp_shrink_frags *skel;
+
+	skel = xdp_shrink_frags__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open_load"))
+		return;
+
+	if (test__start_subtest("tun"))
+		test_tun(skel);
+	if (test__start_subtest("veth"))
+		test_veth(skel);
+	if (test__start_subtest("veth_tx"))
+		test_veth_tx(skel);
+
+	xdp_shrink_frags__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/xdp_shrink_frags.c b/tools/testing/selftests/bpf/progs/xdp_shrink_frags.c
new file mode 100644
index 0000000000000..ad895ab699303
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/xdp_shrink_frags.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+
+int shrink_ran;
+
+SEC("xdp.frags")
+int xdp_shrink(struct xdp_md *ctx)
+{
+	/*
+	 * Runs on both skb-backed XDP paths (generic XDP via tun, and veth):
+	 * the nonlinear skb is cow'd into page_pool memory before we run, so
+	 * shrinking the tail releases a whole frag that has to go back to that
+	 * pool. The counter only tells us the program ran and the helper
+	 * succeeded -- a linear buff returns 0 as well -- it does not prove a
+	 * frag was released.
+	 */
+	if (bpf_xdp_adjust_tail(ctx, -3000) == 0)
+		__sync_fetch_and_add(&shrink_ran, 1);
+	return XDP_PASS;
+}
+
+SEC("xdp.frags")
+int xdp_tx(struct xdp_md *ctx)
+{
+	/*
+	 * Bounce the frame back. On veth this turns the buff into an
+	 * xdp_frame, which is where a buff-scoped page_pool tag must not leak
+	 * into the frame handed to the peer.
+	 */
+	return XDP_TX;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.43.0

Re: [PATCH bpf v3 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:

Pre-existing issues:
- [High] veth_xdp_rcv_one() overwrites shared per-queue rq->xdp_rxq.mem.type state, corrupting subsequent SKB processing in the same NAPI poll and causing kernel panics.
--

commit 6d121019cfea2f329267400ac7c2525a49f34f53
Author: Jiayuan Chen <jiayuan.chen@linux.dev>

    bpf, veth: xdp: fix page_pool page leak on skb-backed XDP
    
    bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
    xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
    cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED.
    This fixes the resulting page pool panics by recording the memory type in
    the xdp_buff flags instead.

> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 6ed3ee81153fb..a3fdf1959b76c 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -775,6 +775,12 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>  	if (skb_shinfo(skb)->nr_frags) {
>  		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>  		xdp_buff_set_frags_flag(xdp);
> +		/*
> +		 * A nonlinear skb was cow'd into rq->page_pool above, so the
> +		 * frags must be freed to that pool, not via the rxq's
> +		 * MEM_TYPE_PAGE_SHARED.
> +		 */
> +		xdp_buff_set_frag_pp(xdp);
>  	} else {
>  		xdp_buff_clear_frags_flag(xdp);
>  	}

[Severity: High]
This isn't a bug introduced by this patch, but can the shared per-queue memory
type state be clobbered during a mixed batch?

In veth_xdp_rcv_one(), the shared rq->xdp_rxq.mem.type state is overwritten
with the incoming frame's type without being restored:

veth_xdp_rcv_one() {
    ...
    case XDP_REDIRECT:
        orig_frame = *frame;
        xdp->rxq->mem.type = frame->mem_type;
    ...
}

If a single NAPI poll (veth_xdp_rcv) processes a batch containing a native
xdp_frame (e.g. MEM_TYPE_XSK_BUFF_POOL) followed by a standard sk_buff, the
next packet will be processed with the clobbered rxq->mem.type.

When veth_xdp_rcv_skb() processes the subsequent sk_buff, the BPF program
is executed with this clobbered state:

veth_xdp_rcv_skb() {
    ...
    act = bpf_prog_run_xdp(xdp_prog, xdp);
    ...
}

If this clobbered memory type is read by helpers like bpf_xdp_shrink_data(),
could it lead to a kernel panic? For instance, if clobbered to XSK_BUFF_POOL,
it would erroneously call xsk_buff_free() on a standard xdp_buff, resulting
in a garbage dereference.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260911135711.109338-1-jiayuan.chen@linux.dev?part=1

Re: [PATCH bpf v3 2/2] selftests/bpf: add xdp_shrink_frags

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Medium] Type punning a byte array into a struct pointer violates strict aliasing rules.
--

commit 7d6829eaaf619b188a1baf1e3f7bf91fc5a0a765
Author: Jiayuan Chen <jiayuan.chen@linux.dev>

selftests/bpf: add xdp_shrink_frags

This commit adds a BPF selftest for XDP that shrinks fragments and
triggers a page_pool leak on a generic XDP path. The test exercises
test_tun, test_veth, and test_veth_tx to cover different data path
behaviors on 4K pages.

> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
> new file mode 100644
> index 0000000000000..ee8f9034a2684
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
[ ... ]
> +static void test_tun(struct xdp_shrink_frags *skel)
> +{
> +	__u8 head[74], frag1[2048], frag2[2048];
> +	struct ethhdr *eth = (void *)head;
> +	int tap_fd = -1, ifindex, err;
> +	struct netns_obj *ns = NULL;
> +	struct iovec iov[3];
> +	ssize_t n;
[ ... ]
> +	memset(head, 0, sizeof(head));
> +	memset(frag1, 0x41, sizeof(frag1));
> +	memset(frag2, 0x42, sizeof(frag2));
> +	eth->h_proto = htons(ETH_P_IP);

[Severity: Medium]
Does this code violate strict aliasing rules by type-punning a byte array
into a struct pointer?

When test_tun() executes, it accesses an object declared as an array of
characters (__u8) through an unrelated larger struct pointer (struct ethhdr *).
Since this code is located in a test program running in user space (under
tools/), standard -fstrict-aliasing rules apply.

Could the compiler assume that eth does not alias head, leading it to reorder
or drop the write to eth->h_proto entirely during optimization?

> +
> +	iov[0].iov_base = head;  iov[0].iov_len = sizeof(head);
> +	iov[1].iov_base = frag1; iov[1].iov_len = sizeof(frag1);
> +	iov[2].iov_base = frag2; iov[2].iov_len = sizeof(frag2);
[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260911135711.109338-1-jiayuan.chen@linux.dev?part=2

Re: [PATCH bpf v3 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

[Jiayuan Chen]

On 9/11/26 10:36 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
>
> Pre-existing issues:
> - [High] veth_xdp_rcv_one() overwrites shared per-queue rq->xdp_rxq.mem.type state, corrupting subsequent SKB processing in the same NAPI poll and causing kernel panics.
> --
>
> commit 6d121019cfea2f329267400ac7c2525a49f34f53
> Author: Jiayuan Chen <jiayuan.chen@linux.dev>
>
>      bpf, veth: xdp: fix page_pool page leak on skb-backed XDP
>      
>      bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
>      xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
>      cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED.
>      This fixes the resulting page pool panics by recording the memory type in
>      the xdp_buff flags instead.
>
>> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
>> index 6ed3ee81153fb..a3fdf1959b76c 100644
>> --- a/drivers/net/veth.c
>> +++ b/drivers/net/veth.c
>> @@ -775,6 +775,12 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>>   	if (skb_shinfo(skb)->nr_frags) {
>>   		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>>   		xdp_buff_set_frags_flag(xdp);
>> +		/*
>> +		 * A nonlinear skb was cow'd into rq->page_pool above, so the
>> +		 * frags must be freed to that pool, not via the rxq's
>> +		 * MEM_TYPE_PAGE_SHARED.
>> +		 */
>> +		xdp_buff_set_frag_pp(xdp);
>>   	} else {
>>   		xdp_buff_clear_frags_flag(xdp);
>>   	}
> [Severity: High]
> This isn't a bug introduced by this patch, but can the shared per-queue memory
> type state be clobbered during a mixed batch?
>
> In veth_xdp_rcv_one(), the shared rq->xdp_rxq.mem.type state is overwritten
> with the incoming frame's type without being restored:
>
> veth_xdp_rcv_one() {
>      ...
>      case XDP_REDIRECT:
>          orig_frame = *frame;
>          xdp->rxq->mem.type = frame->mem_type;
>      ...
> }
>
> If a single NAPI poll (veth_xdp_rcv) processes a batch containing a native
> xdp_frame (e.g. MEM_TYPE_XSK_BUFF_POOL) followed by a standard sk_buff, the


The premise doesn't hold here: frame->mem_type is never 
MEM_TYPE_XSK_BUFF_POOL.


A zc buff gets copied out before it can ever become a frame. 
xdp_convert_buff_to_frame() sends

every XSK buff to xdp_convert_zc_to_xdp_frame(), and that one does:

     xdpf->mem_type = MEM_TYPE_PAGE_ORDER0;

     xsk_buff_free(xdp);


so the plain "xdp_frame->mem_type = xdp->rxq->mem.type" further down 
only ever sees non-XSK types.

I grepped the tree and the only things that ever  end up in a frame's 
mem_type are PAGE_SHARED,

PAGE_POOL and PAGE_ORDER0. So "xdp->rxq->mem.type = frame->mem_type" in 
veth_xdp_rcv_one() can't put

XSK in there, and the XSK branch in bpf_xdp_shrink_data() is simply not 
reachable from veth.


> next packet will be processed with the clobbered rxq->mem.type.
>
> When veth_xdp_rcv_skb() processes the subsequent sk_buff, the BPF program
> is executed with this clobbered state:
>
> veth_xdp_rcv_skb() {
>      ...
>      act = bpf_prog_run_xdp(xdp_prog, xdp);
>      ...
> }
>
> If this clobbered memory type is read by helpers like bpf_xdp_shrink_data(),
> could it lead to a kernel panic? For instance, if clobbered to XSK_BUFF_POOL,
> it would erroneously call xsk_buff_free() on a standard xdp_buff, resulting
> in a garbage dereference.
>

Re: [PATCH bpf v3 2/2] selftests/bpf: add xdp_shrink_frags

[Jiayuan Chen]

On 9/11/26 10:46 PM, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [Medium] Type punning a byte array into a struct pointer violates strict aliasing rules.
> --
>
> commit 7d6829eaaf619b188a1baf1e3f7bf91fc5a0a765
> Author: Jiayuan Chen <jiayuan.chen@linux.dev>
>
> selftests/bpf: add xdp_shrink_frags
>
> This commit adds a BPF selftest for XDP that shrinks fragments and
> triggers a page_pool leak on a generic XDP path. The test exercises
> test_tun, test_veth, and test_veth_tx to cover different data path
> behaviors on 4K pages.
>
>> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
>> new file mode 100644
>> index 0000000000000..ee8f9034a2684
>> --- /dev/null
>> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_shrink_frags.c
> [ ... ]
>> +static void test_tun(struct xdp_shrink_frags *skel)
>> +{
>> +	__u8 head[74], frag1[2048], frag2[2048];
>> +	struct ethhdr *eth = (void *)head;
>> +	int tap_fd = -1, ifindex, err;
>> +	struct netns_obj *ns = NULL;
>> +	struct iovec iov[3];
>> +	ssize_t n;
> [ ... ]
>> +	memset(head, 0, sizeof(head));
>> +	memset(frag1, 0x41, sizeof(frag1));
>> +	memset(frag2, 0x42, sizeof(frag2));
>> +	eth->h_proto = htons(ETH_P_IP);
> [Severity: Medium]
> Does this code violate strict aliasing rules by type-punning a byte array
> into a struct pointer?
>
> When test_tun() executes, it accesses an object declared as an array of
> characters (__u8) through an unrelated larger struct pointer (struct ethhdr *).


I don't think there is anything wrong with

     struct foo *ptr = (struct foo *)binary


it's a pretty normal thing to do, and other tests in the same directory 
do exactly the same:

     prog_tests/xdp_context_test_run.c

     __u8 pkt[...];

     struct ethhdr *eth = (void *)pkt;

     struct ipv6hdr *ip6 = (void *)(eth + 1);

     ....


Not sure why the AI complains about this one.


> Since this code is located in a test program running in user space (under
> tools/), standard -fstrict-aliasing rules apply.
>
> Could the compiler assume that eth does not alias head, leading it to reorder
> or drop the write to eth->h_proto entirely during optimization?
>
>> +
>> +	iov[0].iov_base = head;  iov[0].iov_len = sizeof(head);
>> +	iov[1].iov_base = frag1; iov[1].iov_len = sizeof(frag1);
>> +	iov[2].iov_base = frag2; iov[2].iov_len = sizeof(frag2);
> [ ... ]
>

Re: [PATCH bpf v3 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

[Lorenzo Bianconi]

[-- Attachment #1: Type: text/plain, Size: 8311 bytes --]

> bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
> xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
> cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED,
> so the page_pool page is freed with page_frag_free() and we hit
> "Bad page state ... page_pool leak".
> 
> Both generic XDP and veth are affected. A non-linear skb is cow'd into
> page_pool memory (skb_cow_data_for_xdp() -> skb_pp_cow_data() for generic
> XDP, veth_convert_skb_to_xdp_buff() for veth), so its frags become
> page_pool pages while the rxq keeps MEM_TYPE_PAGE_SHARED.
> 
> We can't just fix rxq->mem.type in place:
> - generic XDP: xdp->rxq is dev->_rx[queue].xdp_rxq (see
>   bpf_prog_run_generic_xdp()), a shared rxq that other CPUs may access in
>   parallel, so we must not write to it.
> - veth: rq->xdp_rxq.mem is shared per-queue state that veth resets on XDP
>   teardown, and with GRO that reset runs without stopping in-flight NAPI,
>   so a type stashed there can be clobbered under a packet still in flight.
> 
> Adding a check in __xdp_return() or bpf_xdp_shrink_data() itself is not an
> option either: without recording it somewhere, both can only guess the
> frag's memory type, which quickly gets confusing.
> 
> So record it in the xdp_buff. A dedicated mem_type field (mirroring
> xdp_frame->mem_type) would read more naturally, but it grows xdp_buff, and
> struct xdp_page_head embeds two of them, which shifts the layout the
> xdp_do_redirect live-frames selftest hard-codes (MAX_PKT_SIZE). So use a
> flag bit: add XDP_FLAGS_FRAGS_PAGE_POOL, the two skb-cow sites set it, and
> bpf_xdp_shrink_data() frees the frag to the page_pool when it is set,
> otherwise it keeps falling back to xdp->rxq->mem.type unchanged.

I have not looked into the details yet, but I am wondering if it is possible
to just move xdp_mem_info type (or ever the full xdp_mem_info) in xdp_buff and
remove it from xdp_rxq_info struct. What do you think?

> 
> The flag describes the buff, so the buff <-> frame conversions have to keep
> it consistent:
> - buff -> frame: xdp_update_frame_from_buff() copies xdp->flags into the
>   frame. veth XDP_TX/XDP_REDIRECT hand the frame to the peer, and cpumap
>   and devmap run a second program on it, so the peer would inherit a stale
>   tag; strip it, an xdp_frame describes its memory with ::mem_type.
> - frame -> buff: xdp_convert_frame_to_buff() rebuilds a buff, and veth and
>   devmap then run a program whose rxq says MEM_TYPE_PAGE_SHARED even for a
>   page_pool frame (only cpumap copies mem_type into the rxq). A shrink
>   there leaks the same way (this predates the series: bpf_xdp_adjust_tail()
>   frees frags by rxq->mem.type), so re-derive the flag from the frame's
>   ::mem_type.
> 
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Fixes: 0ebab78cbcbf ("net: veth: add page_pool for page recycling")
> Fixes: bf25146a5595 ("bpf: add frags support to the bpf_xdp_adjust_tail() API")
> Reported-by: syzbot+237bbeed8dfe0699b7f5@syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=237bbeed8dfe0699b7f5
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  drivers/net/veth.c |  6 ++++++
>  include/net/xdp.h  | 30 +++++++++++++++++++++++++++++-
>  net/core/dev.c     |  6 ++++++
>  net/core/filter.c  |  7 +++++++
>  4 files changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 6ed3ee81153fb..a3fdf1959b76c 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -775,6 +775,12 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>  	if (skb_shinfo(skb)->nr_frags) {
>  		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>  		xdp_buff_set_frags_flag(xdp);
> +		/*
> +		 * A nonlinear skb was cow'd into rq->page_pool above, so the
> +		 * frags must be freed to that pool, not via the rxq's
> +		 * MEM_TYPE_PAGE_SHARED.
> +		 */
> +		xdp_buff_set_frag_pp(xdp);

Even if it is not a real problem at the moment, I still think we should
set it not just if we have a non-liner skb, but for all skb returned by
skb_pp_cow_data().

>  	} else {
>  		xdp_buff_clear_frags_flag(xdp);
>  	}
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index aa742f413c358..8fd4139e9cbd3 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -81,6 +81,14 @@ enum xdp_buff_flags {
>  	 * XDP program is not attached.
>  	 */
>  	XDP_FLAGS_FRAGS_UNREADABLE	= BIT(2),
> +	/*
> +	 * frags are page_pool memory even though rxq->mem.type is not: a
> +	 * skb-backed XDP buff (generic XDP, veth) is cow'd into a page_pool.
> +	 * xdp_buff only: an xdp_frame describes its memory with ::mem_type, so
> +	 * this is stripped in xdp_update_frame_from_buff() and re-derived from
> +	 * ::mem_type in xdp_convert_frame_to_buff().
> +	 */
> +	XDP_FLAGS_FRAGS_PAGE_POOL	= BIT(3),
>  };
>  
>  struct xdp_buff {
> @@ -131,6 +139,16 @@ static __always_inline void xdp_buff_set_frag_unreadable(struct xdp_buff *xdp)
>  	xdp->flags |= XDP_FLAGS_FRAGS_UNREADABLE;
>  }
>  
> +static __always_inline void xdp_buff_set_frag_pp(struct xdp_buff *xdp)
> +{
> +	xdp->flags |= XDP_FLAGS_FRAGS_PAGE_POOL;
> +}
> +
> +static __always_inline bool xdp_buff_is_frag_pp(const struct xdp_buff *xdp)
> +{
> +	return !!(xdp->flags & XDP_FLAGS_FRAGS_PAGE_POOL);
> +}
> +
>  static __always_inline u32 xdp_buff_get_skb_flags(const struct xdp_buff *xdp)
>  {
>  	return xdp->flags;
> @@ -394,6 +412,15 @@ void xdp_convert_frame_to_buff(const struct xdp_frame *frame,
>  	xdp->data_meta = frame->data - frame->metasize;
>  	xdp->frame_sz = frame->frame_sz;
>  	xdp->flags = frame->flags;
> +	/*
> +	 * frame->flags never carries XDP_FLAGS_FRAGS_PAGE_POOL (it is stripped
> +	 * in xdp_update_frame_from_buff()); re-derive it from the frame's own
> +	 * memory type. veth and devmap rebuild a buff here and run a program
> +	 * whose rxq says MEM_TYPE_PAGE_SHARED, so without this a shrink would
> +	 * free a page_pool frag through page_frag_free().
> +	 */
> +	if (frame->mem_type == MEM_TYPE_PAGE_POOL)
> +		xdp_buff_set_frag_pp(xdp);
>  }
>  
>  static inline
> @@ -420,7 +447,8 @@ int xdp_update_frame_from_buff(const struct xdp_buff *xdp,
>  	xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>  	xdp_frame->metasize = metasize;
>  	xdp_frame->frame_sz = xdp->frame_sz;
> -	xdp_frame->flags = xdp->flags;
> +	/* XDP_FLAGS_FRAGS_PAGE_POOL is xdp_buff only, don't carry it over */
> +	xdp_frame->flags = xdp->flags & ~XDP_FLAGS_FRAGS_PAGE_POOL;

IIUC this is only necessary for the veth case, right? If so, I would suggest to
move it in veth driver (or to have a helper function called just in veth).

Regards,
Lorenzo

>  
>  	return 0;
>  }
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 290e0f099e6bf..94268f4d3c5f7 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -5532,6 +5532,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
>  	if (skb_is_nonlinear(skb)) {
>  		skb_shinfo(skb)->xdp_frags_size = skb->data_len;
>  		xdp_buff_set_frags_flag(xdp);
> +		/*
> +		 * A nonlinear skb was cow'd into page_pool memory by
> +		 * skb_cow_data_for_xdp() before we got here, so the frags must
> +		 * be freed to that pool, not via the rxq's MEM_TYPE_PAGE_SHARED.
> +		 */
> +		xdp_buff_set_frag_pp(xdp);
>  	} else {
>  		xdp_buff_clear_frags_flag(xdp);
>  	}
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 61940e7535523..d34ba56d79d8f 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4378,6 +4378,13 @@ static bool bpf_xdp_shrink_data(struct xdp_buff *xdp, skb_frag_t *frag,
>  	if (mem_type == MEM_TYPE_XSK_BUFF_POOL) {
>  		netmem = 0;
>  		zc_frag = bpf_xdp_shrink_data_zc(xdp, shrink, tail, release);
> +	} else if (xdp_buff_is_frag_pp(xdp)) {
> +		/*
> +		 * Skb-backed XDP (generic XDP, veth) cow's the frags into a
> +		 * page_pool while the rxq stays MEM_TYPE_PAGE_SHARED, so free
> +		 * the frag to the pool, not via page_frag_free().
> +		 */
> +		mem_type = MEM_TYPE_PAGE_POOL;
>  	}
>  
>  	if (release) {
> -- 
> 2.43.0
> 

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