[PATCHv2 net] net: ip6_gre: access skb data after skb_cow_head()

[William Tu <u9012063@gmail.com>]

When increases the headroom, skb's data pointer might get re-allocated.
As a result, the skb->data before the skb_cow_head becomes a dangling pointer,
and dereferences to daddr causes general protection fault at the following
line in __gre6_xmit():

  if (dev->header_ops && dev->type == ARPHRD_IP6GRE)                      
      fl6->daddr = ((struct ipv6hdr *)skb->data)->daddr;

general protection fault: 0000 [#1] SMP PTI
OE 4.15.0-43-generic #146-Ubuntu 
Hardware name: VMware, Inc. VMware Virtual Platform 440BX Desktop Reference
Platform, BIOS 6.00 07/03/2018 
RIP: 0010: __gre6_xmit+0x11f/0x2c0 [openvswitch] 
RSP: 0018:ffffb8d5c44df6a8 EFLAGS: 00010286
RAX: 00000000ffffffea RBX: ffff8b1528a0000 RCX: 0000000000000036 
RDX: ffff000000000000 RSI: 0000000000000000 RDI: ffff8db267829200
RBP: ffffb8d5c44df 700 R08: 0000000000005865 RØ9: ffffb8d5c44df724
R10: 0000000000000002 R11: 0000000000000000 R12: ffff8db267829200
R13: 0000000000000000 R14: ffffb8d5c44df 728 R15: 00000000ffffffff
FS: 00007f8744df 2700(0000) GS:ffff8db27fc0000000000) knlGS:0000000000000000 
CS: 0910 DS: 0000 ES: 9000 CRO: 0000000080050033 
CR2: 00007f893ef92148 CR3: 0000000400462003 CR4: 00000000001626f8
Call Trace: 
ip6gre_tunnel_xmit+0x1cc/0x530 [openvswitch]
? skb_clone+0x58/0xc0 
__ip6gre_tunnel_xmit+0x12/0x20 [openvswitch]
ovs_vport_send +0xd4/0x170 [openvswitch] 
do_output+0x53/0x160 [openvswitch]
do_execute_actions+0x9a1/0x1880 [openvswitch]

Fix it by moving skb_cow_head before accessing the skb->data pointer.

Fixes: 01b8d064d58b4 ("net: ip6_gre: Request headroom in __gre6_xmit()")
Reported-by: Haichao Ma <haichaom@vmware.com>
Signed-off-by: William Tu <u9012063@gmail.com>
---
v1-v2: add more details in commit message.
---
 net/ipv6/ip6_gre.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/ip6_gre.c b/net/ipv6/ip6_gre.c
index 655e46b227f9..90b2b129b105 100644
--- a/net/ipv6/ip6_gre.c
+++ b/net/ipv6/ip6_gre.c
@@ -714,6 +714,9 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 	struct ip6_tnl *tunnel = netdev_priv(dev);
 	__be16 protocol;
 
+	if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
+		return -ENOMEM;
+
 	if (dev->type == ARPHRD_ETHER)
 		IPCB(skb)->flags = 0;
 
@@ -722,9 +725,6 @@ static netdev_tx_t __gre6_xmit(struct sk_buff *skb,
 	else
 		fl6->daddr = tunnel->parms.raddr;
 
-	if (skb_cow_head(skb, dev->needed_headroom ?: tunnel->hlen))
-		return -ENOMEM;
-
 	/* Push GRE header. */
 	protocol = (dev->type == ARPHRD_ETHER) ? htons(ETH_P_TEB) : proto;
 
-- 
2.7.4