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 <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

drivers/net/pptp.c

···
 	}
 
 	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);
 
···
 		/* 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;