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ppp_async: drop the errored frame instead of resetting its headroom

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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: 6722e78c90 ("[PPP]: handle misaligned accesses")
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Vlatko Kosturjak <kost@linux.hr>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/apkR6ZU+tqP2C3Fl@griffin.linux.hr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Vlatko Kosturjak 2026-09-03 08:21:29 +02:00 committed by Jakub Kicinski
parent f6d61fe4c1
commit 8dc5d98a16
1 changed files with 2 additions and 5 deletions
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7
drivers/net/ppp/ppp_async.c
View File

@ -742,11 +742,8 @@ process_input_packet(struct asyncppp *ap)
err:
/* frame had an error, remember that, reset SC_TOSS & SC_ESCAPE */
ap->state = SC_PREV_ERROR;
if (skb) {
/* make skb appear as freshly allocated */
skb_trim(skb, 0);
skb_reserve(skb, - skb_headroom(skb));
}
kfree_skb(skb);
ap->rpkt = NULL;
}
/* Called when the tty driver has data for us. Runs parallel with the