CVE-2026-98829: wifi: mac80211: drop oversized fragments to avoid extra_len overflow

CVE-2026-98829: wifi: mac80211: drop oversized fragments to avoid extra_len overflow

[Greg Kroah-Hartman]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

wifi: mac80211: drop oversized fragments to avoid extra_len overflow

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.

[assign a separate drop reason]

The Linux kernel CVE team has assigned CVE-2026-98829 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 4.3 with commit 69f132236827ce7d4531846cc2b9447dd5620aff and fixed in 6.18.56 with commit 9338b2dea74cb0fc80a4c958b68e687610ba2261
	Issue introduced in 4.3 with commit 69f132236827ce7d4531846cc2b9447dd5620aff and fixed in 7.2.10 with commit 870b8cb9baa9bd859deceb33ae158011e6ba2360
	Issue introduced in 4.3 with commit 69f132236827ce7d4531846cc2b9447dd5620aff and fixed in 7.3-rc6 with commit 5e4f3509793b4db2c0835340224743db680e861f

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-98829
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/mac80211/drop.h
	net/mac80211/rx.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/9338b2dea74cb0fc80a4c958b68e687610ba2261
	https://git.kernel.org/stable/c/870b8cb9baa9bd859deceb33ae158011e6ba2360
	https://git.kernel.org/stable/c/5e4f3509793b4db2c0835340224743db680e861f