Affected: Linux

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…