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.

Fixes 6722e78c9005 ("[PPP]: handle misaligned accesses")

Suggested-by Eric Dumazet <[email protected]>

Signed-off-by Vlatko Kosturjak <[email protected]>

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: