PPP does not correctly call pskb_may_pull() on all necessary receive paths
before reading the PPP protocol, thus causing PPP to report seemingly
random 'unsupported protocols' and eventually trigger BUG_ON(skb->len <
skb->data_len) in skb_pull_rcsum() when receiving multilink protocol in
non-linear skbs.
ppp_receive_nonmp_frame() does not call pskb_may_pull() before reading the
protocol number. For the non-mp receive path this is not a problem, as
this check is done in ppp_receive_frame(). For the mp receive path,
ppp_mp_reconstruct() usually copies the data into a new linear skb.
However, in the case where the frame is made up of a single mp fragment,
the mp header is pulled and the existing skb used. This skb was then
passed to ppp_receive_nonmp_frame() without checking if the encapsulated
protocol header can safely be read.
Signed-off-by: Ben McKeegan <ben@xxxxxxxxxxxxxxxx>
---
This a long standing bug we have observed with PPP over L2TP received on
an e1000e interface, although it may arise with other PPP encapsulations
or devices.
diff -uprN linux-2.6.31.6/drivers/net/ppp_generic.c linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c
--- linux-2.6.31.6/drivers/net/ppp_generic.c 2009-11-10 00:32:31.000000000 +0000
+++ linux-2.6.31.6-ppp-non-linear-skb/drivers/net/ppp_generic.c 2009-11-12 11:58:40.000000000 +0000
@@ -1944,7 +1944,13 @@ ppp_receive_mp_frame(struct ppp *ppp, st
/* Pull completed packets off the queue and receive them. */
while ((skb = ppp_mp_reconstruct(ppp)))
- ppp_receive_nonmp_frame(ppp, skb);
+ if (pskb_may_pull(skb, 2))
+ ppp_receive_nonmp_frame(ppp, skb);
+ else {
+ ++ppp->dev->stats.rx_length_errors;
+ kfree_skb(skb);
+ ppp_receive_error(ppp);
+ }
return;
--
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