CVE-2026-46306: flow_dissector: do not dissect PPPoE PFC frames

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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/20220630231016.GA392@debian.home

The Linux kernel CVE team has assigned CVE-2026-46306 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 6.1.175 with commit 18ae9eacfc95cc715c0606b2c86e8aa8a86cf3e3
	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 6.6.140 with commit db104b0d8a7856397c0469d83a4289adf7c54863
	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 6.12.88 with commit 6044392d9cace3a3672b02c8bc7d38b502e51734
	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 6.18.30 with commit 0d00b9015069712944934bab09eaa6c542143049
	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 7.0.7 with commit 7c93f353eab4ea911e394630f07d72e040a729d8
	Issue introduced in 6.0 with commit 46126db9c86110e5fc1e369b9bb89735ddefdae4 and fixed in 7.1-rc1 with commit d6c19b31a3c1d519fabdcf0aa239e6b6109b9473

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-46306
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/core/flow_dissector.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/e7c811ca372d53c2be7d01a1614e71fae1054836
	https://git.kernel.org/stable/c/abc5bc84e0f2edc7ea2d437afa6ef3fe1fc43200
	https://git.kernel.org/stable/c/18ae9eacfc95cc715c0606b2c86e8aa8a86cf3e3
	https://git.kernel.org/stable/c/db104b0d8a7856397c0469d83a4289adf7c54863
	https://git.kernel.org/stable/c/6044392d9cace3a3672b02c8bc7d38b502e51734
	https://git.kernel.org/stable/c/0d00b9015069712944934bab09eaa6c542143049
	https://git.kernel.org/stable/c/7c93f353eab4ea911e394630f07d72e040a729d8
	https://git.kernel.org/stable/c/d6c19b31a3c1d519fabdcf0aa239e6b6109b9473