Vulnerability CVE-2023-52843
Published: 2024-05-21

Description:
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.

 References:
https://git.kernel.org/stable/c/900a4418e3f66a32db6baaf23f92b99c20ae6535
https://git.kernel.org/stable/c/9a3f9054a5227d7567cba1fb821df48ccecad10c
https://git.kernel.org/stable/c/cbdcdf42d15dac74c7287679fb2a9d955f8feb1f
https://git.kernel.org/stable/c/3a2653828ffc6101aef80bf58d5b77484239f779
https://git.kernel.org/stable/c/352887b3edd007cf9b0abc30fe9d98622acd859b
https://git.kernel.org/stable/c/f980e9a57dfb9530f1f4ee41a2420f2a256d7b29
https://git.kernel.org/stable/c/0a720d0259ad3521ec6c9e4199f9f6fc75bac77a
https://git.kernel.org/stable/c/ff5cb6a4f0c6d7fbdc84858323fb4b7af32cfd79
https://git.kernel.org/stable/c/7b3ba18703a63f6fd487183b9262b08e5632da1b
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