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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").

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>

Greg Kroah-Hartman c088d5dd 2c863dbb

+9
+9
drivers/usb/gadget/function/f_phonet.c
··· 333 333 if (unlikely(!skb)) 334 334 break; 335 335 336 + if (unlikely(skb_shinfo(skb)->nr_frags >= MAX_SKB_FRAGS)) { 337 + /* Frame count from host exceeds frags[] capacity */ 338 + dev_kfree_skb_any(skb); 339 + if (fp->rx.skb == skb) 340 + fp->rx.skb = NULL; 341 + dev->stats.rx_length_errors++; 342 + break; 343 + } 344 + 336 345 if (skb->len == 0) { /* First fragment */ 337 346 skb->protocol = htons(ETH_P_PHONET); 338 347 skb_reset_mac_header(skb);