net: tun: bound receive headroom

tun_get_user() uses tun->align both as skb headroom and when choosing how
much packet data to keep linear. OVS can propagate an oversized headroom
request from another port to TUN or TAP.

When align is larger than the usable space in a one-page skb head,
SKB_MAX_HEAD(align) underflows and the result becomes negative when stored
in good_linear. That value later wraps when assigned to the size_t linear
variable, and tun_alloc_skb() can place skb->data outside the allocated
head.

Bound the headroom stored by TUN to the one-page skb-head budget and the
largest non-sentinel 16-bit skb header offset. Leave one linear byte for
raw TUN and a complete Ethernet header for TAP, including NET_IP_ALIGN.

Also pull the raw-TUN protocol byte and the TAP Ethernet header before
accessing them, so these checks remain safe for nonlinear skbs supplied by
other allocation paths.

Fixes: eaea34b23c46 ("net/tun: implement ndo_set_rx_headroom")
Cc: stable@vger.kernel.org
Signed-off-by: Asim Viladi Oglu Manizada <manizada@pm.me>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260812012139.2134643-1-manizada@pm.me
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

0 files changed, 0 insertions, 0 deletions