CVE-2026-63831

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

mac802154: llsec: add skb_cow_data() before in-place crypto

llsec_do_encrypt_unauth(), llsec_do_encrypt_auth(),
llsec_do_decrypt_unauth(), and llsec_do_decrypt_auth() all perform
in-place cryptographic transformations on skb data. They build a
scatterlist with sg_init_one() pointing into the skb's linear data area
and then pass the same scatterlist as both src and dst to the crypto API
(e.g. crypto_skcipher_encrypt/decrypt, crypto_aead_encrypt/decrypt).

On the RX path, __ieee802154_rx_handle_packet() clones the received skb
before handing it to each subscriber via ieee802154_subif_frame(). The
cloned skb shares the same underlying data buffer via reference
counting. When llsec_do_decrypt() subsequently modifies this shared
buffer in place, it corrupts data that other clones -- potentially
belonging to other sockets or subsystems -- still reference.

On the TX path, similar data sharing can occur when an skb's head has
been cloned (skb_cloned() returns true).

The fix is to call skb_cow_data() before performing any in-place crypto
operation. skb_cow_data() ensures that the skb's data area is not
shared: if the skb head is cloned or the data spans multiple fragments,
it copies the data into a private buffer that can be safely modified in
place. This is the same pattern used by:

- ESP (net/ipv4/esp4.c, net/ipv6/esp6.c)
- MACsec (drivers/net/macsec.c)
- WireGuard (drivers/net/wireguard/receive.c)
- TIPC (net/tipc/crypto.c)

Without this guard, in-place crypto on shared skb data leads to:
- Silent data corruption of other skb clones
- Use-after-free when the crypto API scatterwalk writes through a
page that has already been freed by another clone's kfree_skb()
- Kernel crashes under concurrent 802.15.4 traffic with security
enabled (KASAN/KMSAN reports slab-use-after-free)

Found by 0sec (https://0sec.ai) using automated source analysis.