CVE-2026-98158

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_asyncs process_input_packet() resets a reused skbs 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: 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.

Affected Software

References

• https://git.kernel.org/stable/c/0c53eb14975f029abd6b26896a460f0d2aaefe6b
• https://git.kernel.org/stable/c/25f354c55b8435d1f51b8a0a05a3cfe429358523
• https://git.kernel.org/stable/c/717137221c7d90e7c98bda9a370c9da6cbf015e5
• https://git.kernel.org/stable/c/8dc5d98a16fa23c00999aecf10018c9f69fa5bf4
• https://git.kernel.org/stable/c/a86a17745c3e1c6fadd4d6e90c03552dfe531c8c
• https://git.kernel.org/stable/c/c4bb894362d224b699e2f95c6c26707d9654e4a3
• https://git.kernel.org/stable/c/d0fc3dabfe67caf084e7119ceb2ee23f5ad2f2da
• https://git.kernel.org/stable/c/ff035780adb0f49dc2c7ada26b4697849a68bec8