Re: [PATCH net v2] ppp_async: drop the errored frame instead of resetting its headroom

[Qingfang Deng <qingfang.deng@linux.dev>]

Hi,

On 2026/9/3 14:21, Vlatko Kosturjak wrote:
> 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")

Nit: the skb->len == 0 alignment check introduced by said commit is now 
redundant and can be moved back into the allocation branch.

> Suggested-by: Eric Dumazet <edumazet@google.com>
> Signed-off-by: Vlatko Kosturjak <kost@linux.hr>
Kind regards,