datapath: Backport "skbuff: Fix skb checksum flag on skb pull"

Upstream commit:

    VXLAN device can receive skb with checksum partial. But the checksum
    offset could be in outer header which is pulled on receive. This results
    in negative checksum offset for the skb. Such skb can cause the assert
    failure in skb_checksum_help(). Following patch fixes the bug by setting
    checksum-none while pulling outer header.

    Following is the kernel panic msg from old kernel hitting the bug.

    ------------[ cut here ]------------
    kernel BUG at net/core/dev.c:1906!
    RIP: 0010:[<ffffffff81518034>] skb_checksum_help+0x144/0x150
    Call Trace:
    <IRQ>
    [<ffffffffa0164c28>] queue_userspace_packet+0x408/0x470 [openvswitch]
    [<ffffffffa016614d>] ovs_dp_upcall+0x5d/0x60 [openvswitch]
    [<ffffffffa0166236>] ovs_dp_process_packet_with_key+0xe6/0x100 [openvswitch]
    [<ffffffffa016629b>] ovs_dp_process_received_packet+0x4b/0x80 [openvswitch]
    [<ffffffffa016c51a>] ovs_vport_receive+0x2a/0x30 [openvswitch]
    [<ffffffffa0171383>] vxlan_rcv+0x53/0x60 [openvswitch]
    [<ffffffffa01734cb>] vxlan_udp_encap_recv+0x8b/0xf0 [openvswitch]
    [<ffffffff8157addc>] udp_queue_rcv_skb+0x2dc/0x3b0
    [<ffffffff8157b56f>] __udp4_lib_rcv+0x1cf/0x6c0
    [<ffffffff8157ba7a>] udp_rcv+0x1a/0x20
    [<ffffffff8154fdbd>] ip_local_deliver_finish+0xdd/0x280
    [<ffffffff81550128>] ip_local_deliver+0x88/0x90
    [<ffffffff8154fa7d>] ip_rcv_finish+0x10d/0x370
    [<ffffffff81550365>] ip_rcv+0x235/0x300
    [<ffffffff8151ba1d>] __netif_receive_skb+0x55d/0x620
    [<ffffffff8151c360>] netif_receive_skb+0x80/0x90
    [<ffffffff81459935>] virtnet_poll+0x555/0x6f0
    [<ffffffff8151cd04>] net_rx_action+0x134/0x290
    [<ffffffff810683d8>] __do_softirq+0xa8/0x210
    [<ffffffff8162fe6c>] call_softirq+0x1c/0x30
    [<ffffffff810161a5>] do_softirq+0x65/0xa0
    [<ffffffff810687be>] irq_exit+0x8e/0xb0
    [<ffffffff81630733>] do_IRQ+0x63/0xe0
    [<ffffffff81625f2e>] common_interrupt+0x6e/0x6e

    Reported-by: Anupam Chanda <achanda@vmware.com>
    Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
    Acked-by: Tom Herbert <tom@herbertland.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Upstream: 6ae459bdaae ("skbuff: Fix skb checksum flag on skb pull")
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>

datapath/linux/compat/include/linux/skbuff.h

@@ -320,4 +320,28 @@ static inline void __skb_fill_page_desc(struct sk_buff *skb, int i,
 }
 #endif
 
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4,3,0)
+#define skb_postpull_rcsum rpl_skb_postpull_rcsum
+static inline void skb_postpull_rcsum(struct sk_buff *skb,
+				      const void *start, unsigned int len)
+{
+	if (skb->ip_summed == CHECKSUM_COMPLETE)
+		skb->csum = csum_sub(skb->csum, csum_partial(start, len, 0));
+	else if (skb->ip_summed == CHECKSUM_PARTIAL &&
+			skb_checksum_start_offset(skb) <= len)
+		skb->ip_summed = CHECKSUM_NONE;
+}
+
+#define skb_pull_rcsum rpl_skb_pull_rcsum
+static inline unsigned char *skb_pull_rcsum(struct sk_buff *skb, unsigned int len)
+{
+	BUG_ON(len > skb->len);
+	skb->len -= len;
+	BUG_ON(skb->len < skb->data_len);
+	skb_postpull_rcsum(skb, skb->data, len);
+	return skb->data += len;
+}
+
+#endif
+
 #endif