ppp_generic: pull 2 bytes so that PPP_PROTO(skb) is valid

In ppp_input(), PPP_PROTO(skb) may refer to invalid data in the skb.

If this happens and (proto >= 0xc000 || proto == PPP_CCPFRAG) then
the packet is passed directly to pppd.

This occurs frequently when using PPPoE with an interface MTU
greater than 1500 because the skb is more likely to be non-linear.

The next 2 bytes need to be pulled in ppp_input(). The pull of 2
bytes in ppp_receive_frame() has been removed as it is no longer
required.

Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 11 deletions

diff --git a/drivers/net/ppp_generic.c b/drivers/net/ppp_generic.c
index 6e281bc825e5..75e8903c3754 100644
--- a/drivers/net/ppp_generic.c
+++ b/drivers/net/ppp_generic.c
@@ -1567,13 +1567,22 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
         struct channel *pch = chan->ppp;
         int proto;
 
-        if (!pch || skb->len == 0) {
+        if (!pch) {
                 kfree_skb(skb);
                 return;
         }
 
-        proto = PPP_PROTO(skb);
         read_lock_bh(&pch->upl);
+        if (!pskb_may_pull(skb, 2)) {
+                kfree_skb(skb);
+                if (pch->ppp) {
+                        ++pch->ppp->dev->stats.rx_length_errors;
+                        ppp_receive_error(pch->ppp);
+                }
+                goto done;
+        }
+
+        proto = PPP_PROTO(skb);
         if (!pch->ppp || proto >= 0xc000 || proto == PPP_CCPFRAG) {
                 /* put it on the channel queue */
                 skb_queue_tail(&pch->file.rq, skb);
@@ -1585,6 +1594,8 @@ ppp_input(struct ppp_channel *chan, struct sk_buff *skb)
         } else {
                 ppp_do_recv(pch->ppp, skb, pch);
         }
+
+done:
         read_unlock_bh(&pch->upl);
 }
 
@@ -1617,7 +1628,8 @@ ppp_input_error(struct ppp_channel *chan, int code)
 static void
 ppp_receive_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
 {
-        if (pskb_may_pull(skb, 2)) {
+        /* note: a 0-length skb is used as an error indication */
+        if (skb->len > 0) {
 #ifdef CONFIG_PPP_MULTILINK
                 /* XXX do channel-level decompression here */
                 if (PPP_PROTO(skb) == PPP_MP)
@@ -1625,15 +1637,10 @@ ppp_receive_frame(struct ppp *ppp, struct sk_buff *skb, struct channel *pch)
                 else
 #endif /* CONFIG_PPP_MULTILINK */
                         ppp_receive_nonmp_frame(ppp, skb);
-                return;
+        } else {
+                kfree_skb(skb);
+                ppp_receive_error(ppp);
         }
-
-        if (skb->len > 0)
-                /* note: a 0-length skb is used as an error indication */
-                ++ppp->dev->stats.rx_length_errors;
-
-        kfree_skb(skb);
-        ppp_receive_error(ppp);
 }
 
 static void