net: ti: icssg-prueth: Fix possible NULL pointer dereference inside emac_xmit_xdp_frame()

There is an error check inside emac_xmit_xdp_frame() function which
is called when the driver wants to transmit XDP frame, to check if
the allocated tx descriptor is NULL, if true to exit and return
ICSSG_XDP_CONSUMED implying failure in transmission.

In this case trying to free a descriptor which is NULL will result
in kernel crash due to NULL pointer dereference. Fix this error handling
and increase netdev tx_dropped stats in the caller of this function
if the function returns ICSSG_XDP_CONSUMED.

Fixes: 62aa324 ("net: ti: icssg-prueth: Add XDP support")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/all/70d8dd76-0c76-42fc-8611-9884937c82f5@stanley.mountain/
Signed-off-by: Meghana Malladi <m-malladi@ti.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250415090543.717991-3-m-malladi@ti.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: MeghanaMalladiTI

drivers/net/ethernet/ti/icssg/icssg_common.c

@@ -583,7 +583,7 @@ u32 emac_xmit_xdp_frame(struct prueth_emac *emac,
 	first_desc = k3_cppi_desc_pool_alloc(tx_chn->desc_pool);
 	if (!first_desc) {
 		netdev_dbg(ndev, "xdp tx: failed to allocate descriptor\n");
-		goto drop_free_descs;	/* drop */
+		return ICSSG_XDP_CONSUMED;	/* drop */
 	}
 
 	if (page) { /* already DMA mapped by page_pool */
@@ -671,8 +671,10 @@ static u32 emac_run_xdp(struct prueth_emac *emac, struct xdp_buff *xdp,
 
 		q_idx = smp_processor_id() % emac->tx_ch_num;
 		result = emac_xmit_xdp_frame(emac, xdpf, page, q_idx);
-		if (result == ICSSG_XDP_CONSUMED)
+		if (result == ICSSG_XDP_CONSUMED) {
+			ndev->stats.tx_dropped++;
 			goto drop;
+		}
 
 		dev_sw_netstats_rx_add(ndev, xdpf->len);
 		return result;