CVE-2026-46306

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

flow_dissector: do not dissect PPPoE PFC frames

RFC 2516 Section 7 states that Protocol Field Compression (PFC) is NOT

RECOMMENDED for PPPoE. In practice, pppd does not support negotiating

PFC for PPPoE sessions, and the flow dissector driver has assumed an

uncompressed frame until the blamed commit.

During the review process of that commit [1], support for PFC is

suggested. However, having a compressed (1-byte) protocol field means

the subsequent PPP payload is shifted by one byte, causing 4-byte

misalignment for the network header and an unaligned access exception

on some architectures.

The exception can be reproduced by sending a PPPoE PFC frame to an

ethernet interface of a MIPS board, with RPS enabled, even if no PPPoE

session is active on that interface:

$ 0 : 00000000 80c40000 00000000 85144817

$ 4 : 00000008 00000100 80a75758 81dc9bb8

$ 8 : 00000010 8087ae2c 0000003d 00000000

$12 : 000000e0 00000039 00000000 00000000

$16 : 85043240 80a75758 81dc9bb8 00006488

$20 : 0000002f 00000007 85144810 80a70000

$24 : 81d1bda0 00000000

$28 : 81dc8000 81dc9aa8 00000000 805ead08

Hi : 00009d51

Lo : 2163358a

epc : 805e91f0 __skb_flow_dissect+0x1b0/0x1b50

ra : 805ead08 __skb_get_hash_net+0x74/0x12c

Status: 11000403 KERNEL EXL IE

Cause : 40800010 (ExcCode 04)

BadVA : 85144817

PrId : 0001992f (MIPS 1004Kc)

Call Trace:

[<805e91f0>] __skb_flow_dissect+0x1b0/0x1b50

[<805ead08>] __skb_get_hash_net+0x74/0x12c

[<805ef330>] get_rps_cpu+0x1b8/0x3fc

[<805fca70>] netif_receive_skb_list_internal+0x324/0x364

[<805fd120>] napi_complete_done+0x68/0x2a4

[<8058de5c>] mtk_napi_rx+0x228/0xfec

[<805fd398>] __napi_poll+0x3c/0x1c4

[<805fd754>] napi_threaded_poll_loop+0x234/0x29c

[<805fd848>] napi_threaded_poll+0x8c/0xb0

[<80053544>] kthread+0x104/0x12c

[<80002bd8>] ret_from_kernel_thread+0x14/0x1c

Code: 02d51821 1060045b 00000000 <8c640000> 3084000f 2c820005 144001a2 00042080 8e220000

To reduce the attack surface and maintain performance, do not process

PPPoE PFC frames.

[1] https://lore.kernel.org/r/[email protected]