usb: gadget: f_phonet: fix skb frags[] overflow in pn_rx_complete()

A broken/bored/mean USB host can overflow the skb_shared_info->frags[] array on a Linux gadget exposing a Phonet function by sending an unbounded sequence of full-page OUT transfers.

pn_rx_complete() finalizes the skb only when req->actual < req->length, where req->length is set to PAGE_SIZE by the gadget. If the host always sends exactly PAGE_SIZE bytes per transfer, fp->rx.skb will never be reset and each completion will add another fragment via skb_add_rx_frag(). Once nr_frags exceeds MAX_SKB_FRAGS (default 17), subsequent frag stores overwrite memory adjacent to the shinfo on the heap.

Drop the skb and account a length error when the frag limit is reached, matching the fix applied in t7xx by commit f0813bcd2d9d ("net: wwan:
t7xx: fix potential skb->frags overflow in RX path").

---
 drivers/usb/gadget/function/f_phonet.c | 9 +++++++++
 1 file changed, 9 insertions(+)

index 2c007790ead2..b1ee9a7c2e94 100644
@@ -333,6 +333,15 @@ static void pn_rx_complete(struct usb_ep *ep, struct usb_request *req)
 		if (unlikely(!skb))
 			break;
 
+		if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) {
+			/* Frame count from host exceeds frags[] capacity */
+			dev_kfree_skb_any(skb);
+			if (fp->rx.skb == skb)
+				fp->rx.skb = NULL;
+			dev->stats.rx_length_errors++;
+			break;
+		}
+
 		if (skb->len == 0) { /* First fragment */
 			skb->protocol = htons(ETH_P_PHONET);
 			skb_reset_mac_header(skb);

2.53.0