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 f0813bc ("net: wwan:
t7xx: fix potential skb->frags overflow in RX path").

Cc: stable <stable@kernel.org>
Assisted-by: gregkh_clanker_t1000
Link: https://patch.msgid.link/2026040705-fruit-unloved-0701@gregkh
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: gregkh

drivers/usb/gadget/function/f_phonet.c

@@ -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);