[PATCH net 07/12] net: systemport: Fix potential packet length underflow in bcm_sysport_desc_rx()

In bcm_sysport_desc_rx(), the packet length 'len' extracted from the RSB
is only validated against RX_BUF_LENGTH. If a malformed or corrupted
frame is received with 'len' smaller than the prepended Receive Status
Block (sizeof(*rsb)) plus 2 padding bytes (and optional FCS), calling
skb_pull() will trigger a BUG() in __skb_pull() when pulling beyond
skb->len. Furthermore, subtracting (sizeof(*rsb) + 2) from 'len' (u16)
will underflow, resulting in corrupted packet stats and potential
out-of-bounds operations.

Fix this by ensuring 'len' is at least sizeof(*rsb) + 2 (plus
ETH_FCS_LEN if CRC forward is enabled) before proceeding.

Fixes: 80105befdb4b ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Assisted-by: LLM
Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>
---
 drivers/net/ethernet/broadcom/bcmsysport.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/net/ethernet/broadcom/bcmsysport.c b/drivers/net/ethernet/broadcom/bcmsysport.c
index b69833d34bd5..95cead1df160 100644
--- a/drivers/net/ethernet/broadcom/bcmsysport.c
+++ b/drivers/net/ethernet/broadcom/bcmsysport.c

@@ -781,8 +781,9 @@ static unsigned int bcm_sysport_desc_rx(struct bcm_sysport_priv *priv,
 			  p_index, priv->rx_c_index, priv->rx_read_ptr,
 			  len, status);
 
-		if (unlikely(len > RX_BUF_LENGTH)) {
-			netif_err(priv, rx_status, ndev, "oversized packet\n");
+		if (unlikely(len > RX_BUF_LENGTH ||
+			     len < sizeof(*rsb) + 2 + (priv->crc_fwd ? ETH_FCS_LEN : 0))) {
+			netif_err(priv, rx_status, ndev, "invalid packet size: %d\n", len);
 			ndev->stats.rx_length_errors++;
 			ndev->stats.rx_errors++;
 			dev_kfree_skb_any(skb);

-- 
2.34.1