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

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

pppasync: drop the errored frame instead of resetting its headroom

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

(be16 )skbpush(skb, 2) = htons(PPPFILTERINBOUNDTAG);

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

err: if (skb) { / make skb appear as freshly allocated / skbtrim(skb, 0); skbreserve(skb, - skbheadroom(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: skbunderpanic: len:49 put:2 head:ffff888003c10000 data:ffff888003c0ffff tail:0x30 end:0x640 dev:<NULL> kernel BUG at net/core/skbuff.c:214! RIP: 0010:skbpanic+0x13e/0x230 Call Trace: skbpush+0xbd/0x100 pppreceivenonmpframe+0x48a/0x1d10 pppinput+0x4e9/0x2f80 pppasyncprocess+0x2a/0xe0 taskletactioncommon+0x20f/0x8a0 handlesoftirqs+0x18e/0x590 Kernel panic - not syncing: Fatal exception in interrupt

Zeroing the headroom violates the NETSKBPAD guarantee that devallocskb() gives the rest of the receive path. Besides the filter panic above, when CCP compression is enabled pppdecompressframe() 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 pppsynctty 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 pppreceivenonmpframe() with skbcowhead(); 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.