[PATCH net] net: ethernet: ti: am65-cpsw-nuss: Fix port_id extraction from SRC TAG

[Siddharth Vadapalli <s-vadapalli@ti.com>]

On the packet reception path, the ID of the MAC Port on which the packet
was received, is embedded in the RX DMA Descriptor's metadata. The ID is
extracted using the helper function cppi5_desc_get_tags_ids() which fills
in the 16-bit Source Tag into the 'port_id' variable. However, it is only
the lower 8-bits of the 16-bit Source Tag that represent the MAC Port ID,
while the upper 8-bits are Hardware-Reserved and carry an arbitrary value.
With the existing logic, sporadic kernel crash is observed due to the
subsequent driver code accessing out-of-bound memory because of an invalid
port_id.

Hence, fix the port_id extraction logic to use only the lower 8-bits of the
Source Tag as the MAC Port ID.

Fixes: 93a76530316a ("net: ethernet: ti: introduce am65x/j721e gigabit eth subsystem driver")
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
---

Hello,

This patch is based on commit
f9a2394a2348 Merge tag 'mm-hotfixes-stable-2026-08-06-18-44' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
of Mainline Linux.

I have observed it in practice that the RESERVED field contains non-zero
value (such as 0xFF on one instance) causing port_id to be set to an
invalid value (such as 0xFF01 in that instance) resulting in the subsequent
driver code accessing out-of-bound / incorrect memory and corrupting it and
causing a kernel crash. To summarize, this patch is fixing a real issue and
not a hypothetical one.

Regards,
Siddharth.

 drivers/net/ethernet/ti/am65-cpsw-nuss.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/net/ethernet/ti/am65-cpsw-nuss.c b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
index 434a31080855..6330540979d7 100644
--- a/drivers/net/ethernet/ti/am65-cpsw-nuss.c
+++ b/drivers/net/ethernet/ti/am65-cpsw-nuss.c
@@ -1310,6 +1310,8 @@ static int am65_cpsw_nuss_rx_packets(struct am65_cpsw_rx_flow *flow,
 	k3_udma_glue_rx_cppi5_to_dma_addr(rx_chn->rx_chn, &buf_dma);
 	pkt_len = cppi5_hdesc_get_pktlen(desc_rx);
 	cppi5_desc_get_tags_ids(&desc_rx->hdr, &port_id, NULL);
+	/* Port ID is contained in the lower 8-bits of the 16-bit Source Tag */
+	port_id &= 0xFF;
 	dev_dbg(dev, "%s rx port_id:%d\n", __func__, port_id);
 	port = am65_common_get_port(common, port_id);
 	ndev = port->ndev;
-- 
2.51.1