[PATCH net v3 0/5] xsk: fix meta and publish of cq issues

[PATCH net v3 0/5] xsk: fix meta and publish of cq issues

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

The series is the product of previous review from sashiko[1].

1) META
patch 1: address TOCTOU around metadata.

2) PUBLISH of CQ
patch 2: make sure xsk_addr->addrs[] can be published to cq when
         overflow occurs.
patch 3: keep cleaning up the continuation descs (more than 17) and
         publish its address when overflow occurs.
patch 4: like patch 3, but only handles the invalid descs cases.

[1]: https://lore.kernel.org/all/20260502200722.53960-1-kerneljasonxing@gmail.com/

---
V3
Link: https://lore.kernel.org/all/20260515123018.80147-1-kerneljasonxing@gmail.com/
1. avoid breaking previous usage of sendto, and siliently handle
overflow case (Stan, sashiko)
2. add one particular exception process in patch 4 (sashiko)
3. adjust the selftest to make sure it passes in either virutal or
physical machines, which includes add usleep to support physical machine.

V2
Link: https://lore.kernel.org/all/20260510012310.88570-1-kerneljasonxing@gmail.com/
1. adjust selftests (Jakub)
2. add READ_ONCE in patch 1 (Stan)

Jason Xing (5):
  xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()
  xsk: fix buffer leak in xsk_drop_skb() for AF_XDP multi-buffer Tx
  xsk: drain continuation descs after overflow in xsk_build_skb()
  xsk: drain continuation descs on invalid descriptor in
    __xsk_generic_xmit()
  selftests/xsk: drain CQ to wait for TX completion

 include/net/xdp_sock.h                        |  1 +
 net/xdp/xsk.c                                 | 44 +++++++++++++----
 .../selftests/bpf/prog_tests/test_xsk.c       | 48 +++++++++++--------
 3 files changed, 63 insertions(+), 30 deletions(-)

-- 
2.43.7

[PATCH net v3 1/5] xsk: cache csum_start/csum_offset to fix TOCTOU in xsk_skb_metadata()

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

The TX metadata area resides in the UMEM buffer which is memory-mapped
and concurrently writable by userspace. In xsk_skb_metadata(),
csum_start and csum_offset are read from shared memory for bounds
validation, then read again for skb assignment. A malicious userspace
application can race to overwrite these values between the two reads,
bypassing the bounds check and causing out-of-bounds memory access
during checksum computation in the transmit path.

Fix this by reading csum_start and csum_offset into local variables
once, then using the local copies for both validation and assignment.

Note that other metadata fields (flags, launch_time) and the cached
csum fields may be mutually inconsistent due to concurrent userspace
writes, but this is benign: the only security-critical invariant is
that each field's validated value is the same one used, which local
caching guarantees.

Closes: https://lore.kernel.org/all/20260503200927.73EA1C2BCB4@smtp.kernel.org/
Fixes: 48eb03dd2630 ("xsk: Add TX timestamp and TX checksum offload support")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 5e5786cd9af5..f8c8a8c9dfba 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -802,6 +802,7 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
 			    u32 hr)
 {
 	struct xsk_tx_metadata *meta = NULL;
+	u16 csum_start, csum_offset;
 
 	if (unlikely(pool->tx_metadata_len == 0))
 		return -EINVAL;
@@ -811,13 +812,15 @@ static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
 		return -EINVAL;
 
 	if (meta->flags & XDP_TXMD_FLAGS_CHECKSUM) {
-		if (unlikely(meta->request.csum_start +
-			     meta->request.csum_offset +
+		csum_start = READ_ONCE(meta->request.csum_start);
+		csum_offset = READ_ONCE(meta->request.csum_offset);
+
+		if (unlikely(csum_start + csum_offset +
 			     sizeof(__sum16) > desc->len))
 			return -EINVAL;
 
-		skb->csum_start = hr + meta->request.csum_start;
-		skb->csum_offset = meta->request.csum_offset;
+		skb->csum_start = hr + csum_start;
+		skb->csum_offset = csum_offset;
 		skb->ip_summed = CHECKSUM_PARTIAL;
 
 		if (unlikely(pool->tx_sw_csum)) {
-- 
2.43.7

[PATCH net v3 2/5] xsk: fix buffer leak in xsk_drop_skb() for AF_XDP multi-buffer Tx

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

This patch is inspired by the check[1] from sashiko. It says when
overflow happens, the address of cq to be published is invalid.
Actually the severer thing is the whole process of publishing the
address of cq in this particular case is not right: it should truely
publish the address and advance the cached_prod in cq as long as it
reads descriptors from txq.

The following is the full analysis.
xsk_drop_skb() is called in three places, which all discard a partially
built multi-buffer skb:
1) xsk_build_skb() -EOVERFLOW error path: packet exceeds MAX_SKB_FRAGS
2) __xsk_generic_xmit() post-loop cleanup: an invalid descriptor in
   the TX ring prevents the partial packet from completing
3) xsk_release(): socket close while xs->skb holds an incomplete packet

In all three cases, the TX descriptors for the already-processed frags
have been consumed from the TX ring (xskq_cons_release), and CQ slots
have been reserved. However, xsk_drop_skb() calls xsk_consume_skb()
which cancels the CQ reservations via xsk_cq_cancel_locked(). Since
the buffer addresses never appear in the completion queue, userspace
permanently loses track of these buffers.

Fix this by letting consume_skb() trigger the existing xsk_destruct_skb
destructor, which already submits buffer addresses to the CQ via
xsk_cq_submit_addr_locked().

Note that cancelling the descriptors back to the TX ring (via
xskq_cons_cancel_n) is not a appropriate option because an oversized
packet that always exceeds MAX_SKB_FRAGS would be retried indefinitely,
which is an obviously deadlock bug in the TX path.

Also move the desc->addr assignment in xsk_build_skb() above the
overflow check so that the current descriptor's address is recorded
before a potential -EOVERFLOW jump to free_err, consistent with the
zerocopy path in xsk_build_skb_zerocopy().

[1]: https://lore.kernel.org/all/20260425041726.85FB3C2BCB2@smtp.kernel.org/

Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index f8c8a8c9dfba..0a6203c42576 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -793,8 +793,11 @@ static void xsk_consume_skb(struct sk_buff *skb)
 
 static void xsk_drop_skb(struct sk_buff *skb)
 {
-	xdp_sk(skb->sk)->tx->invalid_descs += xsk_get_num_desc(skb);
-	xsk_consume_skb(skb);
+	struct xdp_sock *xs = xdp_sk(skb->sk);
+
+	xs->tx->invalid_descs += xsk_get_num_desc(skb);
+	consume_skb(skb);
+	xs->skb = NULL;
 }
 
 static int xsk_skb_metadata(struct sk_buff *skb, void *buffer,
@@ -876,7 +879,7 @@ static struct sk_buff *xsk_build_skb_zerocopy(struct xdp_sock *xs,
 			return ERR_PTR(-ENOMEM);
 
 		/* in case of -EOVERFLOW that could happen below,
-		 * xsk_consume_skb() will release this node as whole skb
+		 * xsk_drop_skb() will release this node as whole skb
 		 * would be dropped, which implies freeing all list elements
 		 */
 		xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
@@ -968,6 +971,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 				goto free_err;
 			}
 
+			xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
+
 			if (unlikely(nr_frags == (MAX_SKB_FRAGS - 1) && xp_mb_desc(desc))) {
 				err = -EOVERFLOW;
 				goto free_err;
@@ -985,8 +990,6 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 
 			skb_add_rx_frag(skb, nr_frags, page, 0, len, PAGE_SIZE);
 			refcount_add(PAGE_SIZE, &xs->sk.sk_wmem_alloc);
-
-			xsk_addr->addrs[xsk_addr->num_descs] = desc->addr;
 		}
 	}
 
-- 
2.43.7

[PATCH net v3 3/5] xsk: drain continuation descs after overflow in xsk_build_skb()

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

When a multi-buffer packet exceeds MAX_SKB_FRAGS and triggers -EOVERFLOW,
only the current descriptor is released from the TX ring. The remaining
continuation descriptors of the same packet stay in the ring. Since
xs->skb is set to NULL after the drop, the TX loop picks up these
leftover frags and misinterprets each one as the beginning of a new
packet, corrupting the packet stream.

Fix this by adding a drain_cont flag to xdp_sock. When overflow occurs
and the dropped descriptor has XDP_PKT_CONTD set, the flag is raised.
The main TX loop in __xsk_generic_xmit() then handles continuation
descriptors one at a time: each gets a normal CQ reservation (with
backpressure), its address is submitted to the completion queue, and
the descriptor is released from the TX ring. When the last fragment
(without XDP_PKT_CONTD) is processed, the flag is cleared and the
function returns -EOVERFLOW so the next call starts with a fresh
budget for normal packets. This behavior roughly follows how xmit path
treats overflow packets previously: stop sending packets when detecting
the desc has problems. Here, it is stopped only when this group of descs
from the same skb are completed.

This reuses the existing CQ backpressure and budget mechanisms, so if
the CQ is full the function returns -EAGAIN and userspace drains the
CQ before retrying. Zero buffer leakage, zero packet stream corruption.

Closes: https://lore.kernel.org/all/20260425041726.85FB3C2BCB2@smtp.kernel.org/
Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 include/net/xdp_sock.h |  1 +
 net/xdp/xsk.c          | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/net/xdp_sock.h b/include/net/xdp_sock.h
index ebac60a3d8a1..8b51876efbed 100644
--- a/include/net/xdp_sock.h
+++ b/include/net/xdp_sock.h
@@ -80,6 +80,7 @@ struct xdp_sock {
 	 * call of __xsk_generic_xmit().
 	 */
 	struct sk_buff *skb;
+	bool drain_cont;
 
 	struct list_head map_list;
 	/* Protects map_list */
diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 0a6203c42576..298194b7335e 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -1016,6 +1016,8 @@ static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
 			xs->tx->invalid_descs++;
 		}
 		xskq_cons_release(xs->tx);
+		if (xp_mb_desc(desc))
+			xs->drain_cont = true;
 	} else {
 		/* Let application retry */
 		xsk_cq_cancel_locked(xs->pool, 1);
@@ -1062,6 +1064,23 @@ static int __xsk_generic_xmit(struct sock *sk)
 			goto out;
 		}
 
+		if (unlikely(xs->drain_cont)) {
+			unsigned long flags;
+			u32 idx;
+
+			spin_lock_irqsave(&xs->pool->cq_prod_lock, flags);
+			idx = xskq_get_prod(xs->pool->cq);
+			xskq_prod_write_addr(xs->pool->cq, idx, desc.addr);
+			xskq_prod_submit_n(xs->pool->cq, 1);
+			spin_unlock_irqrestore(&xs->pool->cq_prod_lock, flags);
+
+			xs->tx->invalid_descs++;
+			xskq_cons_release(xs->tx);
+			if (!xp_mb_desc(&desc))
+				xs->drain_cont = false;
+			continue;
+		}
+
 		skb = xsk_build_skb(xs, &desc);
 		if (IS_ERR(skb)) {
 			err = PTR_ERR(skb);
-- 
2.43.7

[PATCH net v3 4/5] xsk: drain continuation descs on invalid descriptor in __xsk_generic_xmit()

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

When the TX loop in __xsk_generic_xmit() encounters an invalid
descriptor mid-packet (e.g. an out-of-bounds address), the partial
skb is dropped and the offending descriptor is released. However,
remaining continuation descriptors belonging to the same multi-buffer
packet still sit in the TX ring. Since xs->skb becomes NULL after the
drop, the next iteration treats the leftover continuation fragment as
a brand-new packet, corrupting the packet stream.

Fix this by setting the drain_cont flag when the released descriptor
has XDP_PKT_CONTD set. On the next call to __xsk_generic_xmit(), the
drain logic introduced in the previous patch handles the remaining
fragments with normal CQ backpressure.

There is one subtle case: if a subsequent continuation descriptor also
has an invalid address, xskq_cons_peek_desc() rejects it and the
while loop is never entered, so the in-loop drain path cannot clear
drain_cont. The post-loop code already handles this: it sees
xskq_has_descs() is true (the failed descriptor was read but not
released by peek), releases it, and checks its XDP_PKT_CONTD flag.
Add an else branch so that when the released descriptor is the
last fragment (no XDP_PKT_CONTD), drain_cont is cleared. This
prevents the next valid packet from being incorrectly drained.

Fixes: cf24f5a5feea ("xsk: add support for AF_XDP multi-buffer on Tx path")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/xdp/xsk.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 298194b7335e..cd451b285645 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -1122,6 +1122,7 @@ static int __xsk_generic_xmit(struct sock *sk)
 		if (xs->skb)
 			xsk_drop_skb(xs->skb);
 		xskq_cons_release(xs->tx);
+		xs->drain_cont = xp_mb_desc(&desc);
 	}
 
 out:
-- 
2.43.7

[PATCH net v3 5/5] selftests/xsk: drain CQ to wait for TX completion

[Jason Xing]

From: Jason Xing <kernelxing@tencent.com>

After the kernel xsk drain_cont patches, dropped multi-buffer
descriptors get their buffer addresses published to the completion
queue (CQ) via the skb destructor instead of being cancelled. As a
result, the CQ entries observed by user space no longer match the
software-side accounting based on valid_frags only:
__send_pkts() bumps xsk->outstanding_tx by valid_frags, while
complete_pkts() decrements it by every CQ entry it consumes,
including those produced by drops/drains. This makes
outstanding_tx underflow and causes wait_for_tx_completion() to
exit while valid descriptors are still sitting in the TX ring,
which in turn makes receive_pkts() time out for the
ALIGNED_INV_DESC_MULTI_BUFF, UNALIGNED_INV_DESC_MULTI_BUFF and
TOO_MANY_FRAGS subtests.

Fix this with two changes to the TX completion path:
- complete_pkts(): tolerate extra CQ completions by clamping
  outstanding_tx to zero instead of failing.
- wait_for_tx_completion(): after the outstanding_tx loop finishes,
  add a drain loop that kicks TX and consumes remaining CQ entries.
  After the drain loop exits, do a short usleep and one final
  complete_pkts() call so that real hardware (e.g. ice) has enough
  time to post late CQ entries before we conclude the ring is
  fully drained.

Adjust the multi-buffer invalid-desc tests so that the last
descriptor of every invalid packet has XDP_PKT_CONTD cleared.
Without this, the kernel drain_cont logic would consume
descriptors past the packet boundary and eat into the next valid
packet, breaking pkt_nb validation. Concretely:
- XSK_DESC__INVALID_OPTION is changed from 0xffff to 0xfffe so it
  no longer asserts the XDP_PKT_CONTD bit (bit 0).
- testapp_invalid_desc_mb() clears XDP_PKT_CONTD on the trailing
  descriptor of the invalid-address and invalid-length packets.
- testapp_too_many_frags() appends one extra terminating
  descriptor so the over-sized invalid packet ends with
  XDP_PKT_CONTD cleared, preventing the drain from spilling into
  the trailing sync packet.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 .../selftests/bpf/prog_tests/test_xsk.c       | 48 +++++++++++--------
 1 file changed, 27 insertions(+), 21 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/test_xsk.c b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
index 7950c504ed28..1f196c8ebc73 100644
--- a/tools/testing/selftests/bpf/prog_tests/test_xsk.c
+++ b/tools/testing/selftests/bpf/prog_tests/test_xsk.c
@@ -31,7 +31,7 @@
 #define POLL_TMOUT			1000
 #define THREAD_TMOUT			3
 #define UMEM_HEADROOM_TEST_SIZE		128
-#define XSK_DESC__INVALID_OPTION	(0xffff)
+#define XSK_DESC__INVALID_OPTION	(0xfffe)
 #define XSK_UMEM__INVALID_FRAME_SIZE	(MAX_ETH_JUMBO_SIZE + 1)
 #define XSK_UMEM__LARGE_FRAME_SIZE	(3 * 1024)
 #define XSK_UMEM__MAX_FRAME_SIZE	(4 * 1024)
@@ -950,17 +950,11 @@ static int complete_pkts(struct xsk_socket_info *xsk, int batch_size)
 
 	rcvd = xsk_ring_cons__peek(&xsk->umem->cq, batch_size, &idx);
 	if (rcvd) {
-		if (rcvd > xsk->outstanding_tx) {
-			u64 addr = *xsk_ring_cons__comp_addr(&xsk->umem->cq, idx + rcvd - 1);
-
-			ksft_print_msg("[%s] Too many packets completed\n", __func__);
-			ksft_print_msg("Last completion address: %llx\n",
-				       (unsigned long long)addr);
-			return TEST_FAILURE;
-		}
-
 		xsk_ring_cons__release(&xsk->umem->cq, rcvd);
-		xsk->outstanding_tx -= rcvd;
+		if (rcvd > xsk->outstanding_tx)
+			xsk->outstanding_tx = 0;
+		else
+			xsk->outstanding_tx -= rcvd;
 	}
 
 	return TEST_PASS;
@@ -1274,6 +1268,8 @@ static int __send_pkts(struct ifobject *ifobject, struct xsk_socket_info *xsk, b
 static int wait_for_tx_completion(struct xsk_socket_info *xsk)
 {
 	struct timeval tv_end, tv_now, tv_timeout = {THREAD_TMOUT, 0};
+	unsigned int rcvd;
+	u32 idx;
 	int ret;
 
 	ret = gettimeofday(&tv_now, NULL);
@@ -1293,6 +1289,17 @@ static int wait_for_tx_completion(struct xsk_socket_info *xsk)
 		complete_pkts(xsk, xsk->batch_size);
 	}
 
+	do {
+		if (xsk_ring_prod__needs_wakeup(&xsk->tx))
+			kick_tx(xsk);
+		rcvd = xsk_ring_cons__peek(&xsk->umem->cq, xsk->batch_size, &idx);
+		if (rcvd)
+			xsk_ring_cons__release(&xsk->umem->cq, rcvd);
+	} while (rcvd);
+
+	usleep(100);
+	complete_pkts(xsk, xsk->batch_size);
+
 	return TEST_PASS;
 }
 
@@ -2075,10 +2082,10 @@ int testapp_invalid_desc_mb(struct test_spec *test)
 		{0, 0, 0, false, 0},
 		/* Invalid address in the second frame */
 		{0, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, XDP_PKT_CONTD},
-		{umem_size, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, XDP_PKT_CONTD},
+		{umem_size, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, 0},
 		/* Invalid len in the middle */
 		{0, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, XDP_PKT_CONTD},
-		{0, XSK_UMEM__INVALID_FRAME_SIZE, 0, false, XDP_PKT_CONTD},
+		{0, XSK_UMEM__INVALID_FRAME_SIZE, 0, false, 0},
 		/* Invalid options in the middle */
 		{0, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, XDP_PKT_CONTD},
 		{0, XSK_UMEM__LARGE_FRAME_SIZE, 0, false, XSK_DESC__INVALID_OPTION},
@@ -2229,7 +2236,7 @@ int testapp_too_many_frags(struct test_spec *test)
 		max_frags += 1;
 	}
 
-	pkts = calloc(2 * max_frags + 2, sizeof(struct pkt));
+	pkts = calloc(2 * max_frags + 3, sizeof(struct pkt));
 	if (!pkts)
 		return TEST_FAILURE;
 
@@ -2247,20 +2254,19 @@ int testapp_too_many_frags(struct test_spec *test)
 	}
 	pkts[max_frags].options = 0;
 
-	/* An invalid packet with the max amount of frags but signals packet
-	 * continues on the last frag
-	 */
-	for (i = max_frags + 1; i < 2 * max_frags + 1; i++) {
+	/* An invalid packet with too many frags */
+	for (i = max_frags + 1; i < 2 * max_frags + 2; i++) {
 		pkts[i].len = MIN_PKT_SIZE;
 		pkts[i].options = XDP_PKT_CONTD;
 		pkts[i].valid = false;
 	}
+	pkts[2 * max_frags + 1].options = 0;
 
 	/* Valid packet for synch */
-	pkts[2 * max_frags + 1].len = MIN_PKT_SIZE;
-	pkts[2 * max_frags + 1].valid = true;
+	pkts[2 * max_frags + 2].len = MIN_PKT_SIZE;
+	pkts[2 * max_frags + 2].valid = true;
 
-	if (pkt_stream_generate_custom(test, pkts, 2 * max_frags + 2)) {
+	if (pkt_stream_generate_custom(test, pkts, 2 * max_frags + 3)) {
 		free(pkts);
 		return TEST_FAILURE;
 	}
-- 
2.43.7