ppp: fix BUG on non-linear SKB (multilink receive)

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 could safely be read.

Signed-off-by: Ben McKeegan <ben@netservers.co.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/net/ppp_generic.c b/drivers/net/ppp_generic.c
index 9bf2a6be9031..965adb6174c3 100644
--- a/drivers/net/ppp_generic.c
+++ b/drivers/net/ppp_generic.c
@@ -1944,8 +1944,15 @@ ppp_receive_mp_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
         }
 
         /* Pull completed packets off the queue and receive them. */
-        while ((skb = ppp_mp_reconstruct(ppp)))
-                ppp_receive_nonmp_frame(ppp, skb);
+        while ((skb = ppp_mp_reconstruct(ppp))) {
+                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;