wifi: mac80211: drop oversized fragments to avoid extra_len overflow

Hi Johannes and mac80211 maintainers,

During static analysis of fragment reassembly in mac80211, we identified a potential 16-bit integer overflow in ieee80211_rx_h_defragment() that can be triggered by a sequence of oversized fragments, leading to an undersized allocation in pskb_expand_head() followed by skb_over_panic() in softirq context.

In commit 69f132236827 ("mac80211: shrink struct ieee80211_fragment_entry"), entry->extra_len was narrowed from unsigned int to u16. Since up to 16 fragments (frag numbers 0..15) can be received in an IEEE 802.11 sequence, a sequence of large incoming fragments can cause entry->extra_len to exceed 65535 bytes, wrapping around modulo 65536. When the final fragment arrives, pskb_expand_head() only allocates tailroom for the wrapped-around extra_len, and the subsequent skb_put_data() loop overflows the buffer, triggering skb_over_panic().

Because valid MSDUs in IEEE 802.11 are far below U16_MAX (standard MSDU is <= 2304 bytes, and A-MSDU cannot be fragmented), we prevent this wrap by dropping the frame and purging the queued fragments if accumulating the incoming fragment length would exceed U16_MAX.

This approach keeps struct ieee80211_fragment_entry completely untouched, preserving the structure memory reduction from the aforementioned commit.

Best regards,
Yuchao Zhang

Yuchao Zhang (1):
wifi: mac80211: drop oversized fragments to avoid extra_len overflow

 net/mac80211/rx.c | 6 ++++++
 1 file changed, 6 insertions(+)

2.53.0

In ieee80211_rx_h_defragment(), fragment payloads are accumulated into entry->extra_len as subsequent fragments arrive:

        entry->extra_len += rx->skb->len;

When the final fragment arrives, the head fragment is dequeued, and pskb_expand_head() is called with entry->extra_len to ensure sufficient tailroom for all queued fragments before copying:

        if (skb_tailroom(rx->skb) < entry->extra_len) {
                if (unlikely(pskb_expand_head(rx->skb, 0, entry->extra_len,
                                              GFP_ATOMIC))) {
                        ...
                }
        }
        while ((skb = __skb_dequeue(&entry->skb_list))) {
                skb_put_data(rx->skb, skb->data, skb->len);
                dev_kfree_skb(skb);
        }

In commit 69f132236827 ("mac80211: shrink struct ieee80211_fragment_entry"), entry->extra_len was narrowed from unsigned int to u16 in order to reduce structure memory footprint.

However, an IEEE 802.11 frame sequence can contain up to 16 fragments (frag numbers 0..15). If a sequence of large fragments arrives (e.g. from a malicious peer or rogue AP), the sum of fragment lengths can exceed 65535 bytes (for example, 15 fragments of 4400 bytes total 66000 bytes). Because extra_len is a u16, this addition overflows and wraps around modulo 65536 (e.g. 66000 wraps to 464).

Consequently, pskb_expand_head() allocates only the truncated amount of tailroom (or is skipped entirely if the head fragment already has >= 464 bytes of tailroom). When skb_put_data() subsequently iterates through the queued skb list, it appends the full payload into the undersized buffer, causing skb_put() to trigger skb_over_panic() and crash the kernel in softirq context.

A legitimate MSDU in IEEE 802.11 is at most 2304 bytes (or up to 7935/11454 bytes for A-MSDU, which is not fragmented), well below U16_MAX.

Fix this without increasing the size of struct ieee80211_fragment_entry by checking whether adding the incoming fragment length would exceed U16_MAX. If so, purge the queued fragments and drop the frame.

---
 net/mac80211/rx.c | 6 ++++++
 1 file changed, 6 insertions(+)

index 5e26be8e27d8..e7292d5febf5 100644
@@ -2499,6 +2499,12 @@ ieee80211_rx_h_defragment(struct ieee80211_rx_data *rx)
 	}
 
 	skb_pull(rx->skb, ieee80211_hdrlen(fc));
+	if (unlikely((u32)entry->extra_len + rx->skb->len > U16_MAX)) {
+		I802_DEBUG_INC(rx->local->rx_handlers_drop_defrag);
+		__skb_queue_purge(&entry->skb_list);
+		return RX_DROP_U_DEFRAG_MISMATCH;
+	}
+
 	__skb_queue_tail(&entry->skb_list, rx->skb);
 	entry->last_frag = frag;
 	entry->extra_len += rx->skb->len;

2.53.0