CVE-2023-52843

In the Linux kernel, the following vulnerability has been resolved: llc:
verify mac len before reading mac header LLC reads the mac header with
eth_hdr without verifying that the skb has an Ethernet header. Syzbot was
able to enter llc_rcv on a tun device. Tun can insert packets without mac
len and with user configurable skb->protocol (passing a tun_pi header when
not configuring IFF_NO_PI). BUG: KMSAN: uninit-value in
llc_station_ac_send_test_r net/llc/llc_station.c:81 [inline] BUG: KMSAN:
uninit-value in llc_station_rcv+0x6fb/0x1290 net/llc/llc_station.c:111
llc_station_ac_send_test_r net/llc/llc_station.c:81 [inline]
llc_station_rcv+0x6fb/0x1290 net/llc/llc_station.c:111 llc_rcv+0xc5d/0x14a0
net/llc/llc_input.c:218 __netif_receive_skb_one_core net/core/dev.c:5523
[inline] __netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5637
netif_receive_skb_internal net/core/dev.c:5723 [inline]
netif_receive_skb+0x58/0x660 net/core/dev.c:5782 tun_rx_batched+0x3ee/0x980
drivers/net/tun.c:1555 tun_get_user+0x54c5/0x69c0 drivers/net/tun.c:2002
Add a mac_len test before all three eth_hdr(skb) calls under net/llc. There
are further uses in include/net/llc_pdu.h. All these are protected by a
test skb->protocol == ETH_P_802_2. Which does not protect against this tun
scenario. But the mac_len test added in this patch in llc_fixup_skb will
indirectly protect those too. That is called from llc_rcv before any other
LLC code. It is tempting to just add a blanket mac_len check in llc_rcv,
but not sure whether that could break valid LLC paths that do not assume an
Ethernet header. 802.2 LLC may be used on top of non-802.3 protocols in
principle. The below referenced commit shows that used to, on top of Token
Ring. At least one of the three eth_hdr uses goes back to before the start
of git history. But the one that syzbot exercises is introduced in this
commit. That commit is old enough (2008), that effectively all stable
kernels should receive this.