pptp: pptp_rcv_core() misses pskb_may_pull() call

e1000e uses paged frags, so any layer incorrectly pulling bytes from skb
can trigger a BUG in skb_pull()

[951.142737]  [<ffffffff813d2f36>] skb_pull+0x15/0x17
[951.142737]  [<ffffffffa0286824>] pptp_rcv_core+0x126/0x19a [pptp]
[951.152725]  [<ffffffff813d17c4>] sk_receive_skb+0x69/0x105
[951.163558]  [<ffffffffa0286993>] pptp_rcv+0xc8/0xdc [pptp]
[951.165092]  [<ffffffffa02800a3>] gre_rcv+0x62/0x75 [gre]
[951.165092]  [<ffffffff81410784>] ip_local_deliver_finish+0x150/0x1c1
[951.177599]  [<ffffffff81410634>] ? ip_local_deliver_finish+0x0/0x1c1
[951.177599]  [<ffffffff81410846>] NF_HOOK.clone.7+0x51/0x58
[951.177599]  [<ffffffff81410996>] ip_local_deliver+0x51/0x55
[951.177599]  [<ffffffff814105b9>] ip_rcv_finish+0x31a/0x33e
[951.177599]  [<ffffffff8141029f>] ? ip_rcv_finish+0x0/0x33e
[951.204898]  [<ffffffff81410846>] NF_HOOK.clone.7+0x51/0x58
[951.214651]  [<ffffffff81410bb5>] ip_rcv+0x21b/0x246

pptp_rcv_core() is a nice example of a function assuming everything it
needs is available in skb head.

Reported-by: Bradley Peterson <despite@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 8 deletions

diff --git a/drivers/net/pptp.c b/drivers/net/pptp.c
index 9c0403d0107c..89f829f5f725 100644
--- a/drivers/net/pptp.c
+++ b/drivers/net/pptp.c
@@ -307,11 +307,18 @@ static int pptp_rcv_core(struct sock *sk, struct sk_buff *skb)
         }
 
         header = (struct pptp_gre_header *)(skb->data);
+        headersize  = sizeof(*header);
 
         /* test if acknowledgement present */
         if (PPTP_GRE_IS_A(header->ver)) {
-                __u32 ack = (PPTP_GRE_IS_S(header->flags)) ?
-                                header->ack : header->seq; /* ack in different place if S = 0 */
+                __u32 ack;
+
+                if (!pskb_may_pull(skb, headersize))
+                        goto drop;
+                header = (struct pptp_gre_header *)(skb->data);
+
+                /* ack in different place if S = 0 */
+                ack = PPTP_GRE_IS_S(header->flags) ? header->ack : header->seq;
 
                 ack = ntohl(ack);
 
@@ -320,21 +327,18 @@ static int pptp_rcv_core(struct sock *sk, struct sk_buff *skb)
                 /* also handle sequence number wrap-around  */
                 if (WRAPPED(ack, opt->ack_recv))
                         opt->ack_recv = ack;
+        } else {
+                headersize -= sizeof(header->ack);
         }
-
         /* test if payload present */
         if (!PPTP_GRE_IS_S(header->flags))
                 goto drop;
 
-        headersize  = sizeof(*header);
         payload_len = ntohs(header->payload_len);
         seq         = ntohl(header->seq);
 
-        /* no ack present? */
-        if (!PPTP_GRE_IS_A(header->ver))
-                headersize -= sizeof(header->ack);
         /* check for incomplete packet (length smaller than expected) */
-        if (skb->len - headersize < payload_len)
+        if (!pskb_may_pull(skb, headersize + payload_len))
                 goto drop;
 
         payload = skb->data + headersize;