---
id: CVE-2026-98158
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ppp_async: drop the errored frame instead of resetting its headroom

  ppp_receive_nonmp_frame() prepends a two-byte direction tag before running
  the pass/active BPF filt…
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  ppp_async: drop the errored frame instead of resetting its headroom

  ppp_receive_nonmp_frame() prepends a two-byte direction tag before running
  the pass/active BPF filt…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 6722e78c90054101e6797d5944cdc81af9897a0a <
    d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
  - >-
    Linux >= 6722e78c90054101e6797d5944cdc81af9897a0a <
    0c53eb14975f029abd6b26896a460f0d2aaefe6b
  - >-
    Linux >= 6722e78c90054101e6797d5944cdc81af9897a0a <
    717137221c7d90e7c98bda9a370c9da6cbf015e5
  - >-
    Linux >= 6722e78c90054101e6797d5944cdc81af9897a0a <
    8dc5d98a16fa23c00999aecf10018c9f69fa5bf4
  - Linux 2.6.15
published: '2026-09-25'
updated: '2026-09-25'
sourceUpdated: '2026-09-25T11:17:47.510'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-98158'
references:
  - url: 'https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
tags:
  - nvd
  - cve.org
ingestedAt: '2026-09-25T11:06:38.804Z'
---

## Overview

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

ppp_async: drop the errored frame instead of resetting its headroom

ppp_receive_nonmp_frame() prepends a two-byte direction tag before running
the pass/active BPF filters:

	*(__be16 *)skb_push(skb, 2) = htons(PPP_FILTER_INBOUND_TAG);

Nothing on the receive path guarantees those two bytes of headroom. The
frame-error path in ppp_async's process_input_packet() resets a reused skb's
headroom to zero while claiming to restore it to a freshly allocated state -
but a fresh skb from dev_alloc_skb() carries NET_SKB_PAD:

	err:
		if (skb) {
			/* make skb appear as freshly allocated */
			skb_trim(skb, 0);
			skb_reserve(skb, - skb_headroom(skb));
		}

ap->rpkt still points at that skb, so the next frame is reassembled into it
with no headroom at all. A peer that sends a bad-FCS frame followed by one
beginning ff 03 then leaves a single byte of headroom by the time the filter
tag is pushed, which lands one byte below skb->head:

  skbuff: skb_under_panic: len:49 put:2 head:ffff888003c10000
          data:ffff888003c0ffff tail:0x30 end:0x640 dev:<NULL>
  kernel BUG at net/core/skbuff.c:214!
  RIP: 0010:skb_panic+0x13e/0x230
  Call Trace:
   skb_push+0xbd/0x100
   ppp_receive_nonmp_frame+0x48a/0x1d10
   ppp_input+0x4e9/0x2f80
   ppp_async_process+0x2a/0xe0
   tasklet_action_common+0x20f/0x8a0
   handle_softirqs+0x18e/0x590
  Kernel panic - not syncing: Fatal exception in interrupt

Zeroing the headroom violates the NET_SKB_PAD guarantee that dev_alloc_skb()
gives the rest of the receive path. Besides the filter panic above, when CCP
compression is enabled ppp_decompress_frame() hands skb->data - 2 to
->decompress()/->incomp(), which then reads out of bounds before skb->head
for the same reason.

Rather than restore the headroom, drop the errored frame - as ppp_synctty
already does on its error path - and clear ap->rpkt so the next frame is
reassembled into a fresh skb with proper headroom. This is simpler and fixes
both the filter under-panic and the CCP out-of-bounds read.

The original V1 of this patch made room in ppp_receive_nonmp_frame() with
skb_cow_head(); Eric pointed out that fixing the root cause in the transport
is the right approach.

Found by fuzzing the PPP receive path with a mutating peer on a pty; it is an
interesting (remote) DoS: root configures PPP, the peer supplies two crashing
frames. The reproducer (repro-ppp-skb.c, unchanged from v1) panics in about a
second, and returns cleanly with this applied.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.