[PATCH net] igb: only strip Rx timestamp header on the first buffer of a frame

[Tjerk Kusters]

Hi,

The patch is attached (0001-igb-only-strip-Rx-timestamp-header-on-the-first-buff.patch)
as my mail setup cannot send it inline via git send-email; apologies for the
attachment.

Summary of the bug it fixes:

When Rx hardware timestamping is enabled (e.g. ptp4l, which configures
HWTSTAMP_FILTER_ALL), the igb NIC prepends a 16-byte timestamp header to the
first Rx buffer of every received frame. igb_clean_rx_irq() strips this header
inside its per-buffer loop, so for a frame that spans more than one Rx buffer
(a jumbo frame) the strip runs once per buffer.

The timestamp header only exists on the first buffer, but igb_ptp_rx_pktstamp()
is called for every buffer and only checks that PTP is enabled and that the
first 8 bytes (reserved dwords) are zero. On a continuation buffer whose
payload happens to begin with 8 zero bytes, both checks pass, the payload is
mistaken for a timestamp header, and 16 bytes of real data are stripped from
that buffer. A frame spanning N buffers whose continuation buffers start with
zero bytes loses 16 * (N - 1) bytes from its tail, silently.

This is easily triggered by a GigE Vision camera streaming dark frames (mostly
0x00 pixel data) over jumbo UDP with PTP active on the receiver: all-zero
frames arrive truncated, frames with non-zero content are fine.

The fix only attempts the strip on the first buffer of a frame (skb == NULL in
igb_clean_rx_irq), which is the only buffer that can contain a timestamp
header. A content-based check cannot reliably distinguish a continuation
buffer that begins with zero bytes from a real header.

Testing: reproduced on Linux 7.0.11 and 6.14.9 with an Intel I350-T2, MTU 8228,
ptp4l active. A random,zero,zero,random burst of 1024 jumbo UDP datagrams
captured on the receiver showed 512 of 1024 truncated (both all-zero bursts)
on the stock driver, and 0 truncated with this fix applied. The bug is still
present in mainline (checked v7.1-rc7).

Fixes: 5379260852b0 ("igb: Fix XDP with PTP enabled")

Thanks,
T Kusters
From fee3e3452dfcd7e109332369672a3e0090cadeb3 Mon Sep 17 00:00:00 2001
From: T Kusters <tkusters@xxxxxxxx>
Date: Tue, 9 Jun 2026 14:06:24 +0200
Subject: [PATCH net] igb: only strip Rx timestamp header on the first buffer
 of a frame

When Rx hardware timestamping is enabled (e.g. ptp4l, which configures
HWTSTAMP_FILTER_ALL), the NIC prepends a 16-byte timestamp header to the
first Rx buffer of every received frame. igb_clean_rx_irq() strips this
header inside its per-buffer loop:

	if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
		ts_hdr_len = igb_ptp_rx_pktstamp(rx_ring->q_vector,
						 pktbuf, &timestamp);
		pkt_offset += ts_hdr_len;
		size -= ts_hdr_len;
	}

For a frame that spans more than one Rx buffer (e.g. a jumbo frame), this
block runs once per buffer. The timestamp header only exists at the start
of the first buffer, but igb_ptp_rx_pktstamp() is called for every buffer.

On a continuation buffer the data is packet payload, not a timestamp
header. igb_ptp_rx_pktstamp() already has two guards against acting on a
non-header buffer: it returns 0 if PTP is disabled, and returns 0 if the
reserved dwords (the first 8 bytes) are non-zero. Neither is sufficient
here: PTP is enabled, and a continuation buffer whose payload happens to
begin with 8 zero bytes passes the reserved-dword check. In that case the
payload is mistaken for a valid timestamp header and igb_ptp_rx_pktstamp()
returns IGB_TS_HDR_LEN, so the caller strips 16 bytes of real data from
that buffer. A frame spanning N buffers whose continuation buffers start
with zero bytes therefore loses 16 * (N - 1) bytes from its tail.

This is easily triggered by a GigE Vision camera streaming dark frames
(mostly 0x00 pixel data) over jumbo UDP with PTP active on the receiver:
the all-zero frames arrive truncated while frames with non-zero content
are fine. There is no error indication.

No content-based check can reliably tell a continuation buffer that begins
with zero bytes from a real timestamp header, because both are all zero.
Fix it structurally instead: only attempt the strip on the first buffer of
a frame, which is the only buffer that can contain a timestamp header. In
igb_clean_rx_irq() skb is NULL until the first buffer has been processed,
so guarding the strip with !skb restricts it to the first buffer
regardless of payload content.

Fixes: 5379260852b0 ("igb: Fix XDP with PTP enabled")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: T Kusters <tkusters@xxxxxxxx>
---
 drivers/net/ethernet/intel/igb/igb_main.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/intel/igb/igb_main.c b/drivers/net/ethernet/intel/igb/igb_main.c
index ce91dda00ec0..abb55cd589a9 100644
--- a/drivers/net/ethernet/intel/igb/igb_main.c
+++ b/drivers/net/ethernet/intel/igb/igb_main.c
@@ -9061,7 +9061,8 @@ static int igb_clean_rx_irq(struct igb_q_vector *q_vector, const int budget)
 		pktbuf = page_address(rx_buffer->page) + rx_buffer->page_offset;
 
 		/* pull rx packet timestamp if available and valid */
-		if (igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
+		if (!skb &&
+		    igb_test_staterr(rx_desc, E1000_RXDADV_STAT_TSIP)) {
 			int ts_hdr_len;
 
 			ts_hdr_len = igb_ptp_rx_pktstamp(rx_ring->q_vector,
-- 
2.27.0