CVE-2026-98158: ppp_async: drop the errored frame instead of resetting its headroom

CVE-2026-98158: ppp_async: drop the errored frame instead of resetting its headroom

[Greg Kroah-Hartman]

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

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

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.

The Linux kernel CVE team has assigned CVE-2026-98158 to this issue.


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

	Issue introduced in 2.6.15 with commit 6722e78c90054101e6797d5944cdc81af9897a0a and fixed in 6.12.111 with commit d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
	Issue introduced in 2.6.15 with commit 6722e78c90054101e6797d5944cdc81af9897a0a and fixed in 6.18.53 with commit 0c53eb14975f029abd6b26896a460f0d2aaefe6b
	Issue introduced in 2.6.15 with commit 6722e78c90054101e6797d5944cdc81af9897a0a and fixed in 7.2.7 with commit 717137221c7d90e7c98bda9a370c9da6cbf015e5
	Issue introduced in 2.6.15 with commit 6722e78c90054101e6797d5944cdc81af9897a0a and fixed in 7.3-rc3 with commit 8dc5d98a16fa23c00999aecf10018c9f69fa5bf4

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-2026-98158
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:
	drivers/net/ppp/ppp_async.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/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
	https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b
	https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5
	https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4