[PPP]: L2TP: Fix oops in transmit and receive paths

Changes made on 18-sep to fix skb handling in the pppol2tp driver
broke the transmit and receive paths. Users are only running into this
now because distros are now using 2.6.23 and I must have messed up
when I tested the change.

For receive, we now do our own calculation of how much to pull from
the skb (variable length L2TP header) rather than using
skb_transport_offset(). Also, if the skb isn't a data packet, it must
be passed back to UDP with skb->data pointing to the UDP header.

For transmit, make sure skb->sk is set up because ip_queue_xmit()
needs it.

Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 18 insertions, 7 deletions

diff --git a/drivers/net/pppol2tp.c b/drivers/net/pppol2tp.c
index f8904fd92369..a7556cd2df79 100644
--- a/drivers/net/pppol2tp.c
+++ b/drivers/net/pppol2tp.c
@@ -488,7 +488,7 @@ static int pppol2tp_recv_core(struct sock *sock, struct sk_buff *skb)
 {
         struct pppol2tp_session *session = NULL;
         struct pppol2tp_tunnel *tunnel;
-        unsigned char *ptr;
+        unsigned char *ptr, *optr;
         u16 hdrflags;
         u16 tunnel_id, session_id;
         int length;
@@ -496,7 +496,7 @@ static int pppol2tp_recv_core(struct sock *sock, struct sk_buff *skb)
 
         tunnel = pppol2tp_sock_to_tunnel(sock);
         if (tunnel == NULL)
-                goto error;
+                goto no_tunnel;
 
         /* UDP always verifies the packet length. */
         __skb_pull(skb, sizeof(struct udphdr));
@@ -509,7 +509,7 @@ static int pppol2tp_recv_core(struct sock *sock, struct sk_buff *skb)
         }
 
         /* Point to L2TP header */
-        ptr = skb->data;
+        optr = ptr = skb->data;
 
         /* Get L2TP header flags */
         hdrflags = ntohs(*(__be16*)ptr);
@@ -637,12 +637,14 @@ static int pppol2tp_recv_core(struct sock *sock, struct sk_buff *skb)
         /* If offset bit set, skip it. */
         if (hdrflags & L2TP_HDRFLAG_O) {
                 offset = ntohs(*(__be16 *)ptr);
-                skb->transport_header += 2 + offset;
-                if (!pskb_may_pull(skb, skb_transport_offset(skb) + 2))
-                        goto discard;
+                ptr += 2 + offset;
         }
 
-        __skb_pull(skb, skb_transport_offset(skb));
+        offset = ptr - optr;
+        if (!pskb_may_pull(skb, offset))
+                goto discard;
+
+        __skb_pull(skb, offset);
 
         /* Skip PPP header, if present.  In testing, Microsoft L2TP clients
          * don't send the PPP header (PPP header compression enabled), but
@@ -652,6 +654,9 @@ static int pppol2tp_recv_core(struct sock *sock, struct sk_buff *skb)
          * Note that skb->data[] isn't dereferenced from a u16 ptr here since
          * the field may be unaligned.
          */
+        if (!pskb_may_pull(skb, 2))
+                goto discard;
+
         if ((skb->data[0] == 0xff) && (skb->data[1] == 0x03))
                 skb_pull(skb, 2);
 
@@ -709,6 +714,10 @@ discard:
         return 0;
 
 error:
+        /* Put UDP header back */
+        __skb_push(skb, sizeof(struct udphdr));
+
+no_tunnel:
         return 1;
 }
 
@@ -1050,6 +1059,8 @@ static int pppol2tp_xmit(struct ppp_channel *chan, struct sk_buff *skb)
         /* Get routing info from the tunnel socket */
         dst_release(skb->dst);
         skb->dst = sk_dst_get(sk_tun);
+        skb_orphan(skb);
+        skb->sk = sk_tun;
 
         /* Queue the packet to IP for output */
         len = skb->len;