net: erspan: fix use-after-free

When building the erspan header for either v1 or v2, the eth_hdr()
does not point to the right inner packet's eth_hdr,
causing kasan report use-after-free and slab-out-of-bouds read.

The patch fixes the following syzkaller issues:
[1] BUG: KASAN: slab-out-of-bounds in erspan_xmit+0x22d4/0x2430 net/ipv4/ip_gre.c:735
[2] BUG: KASAN: slab-out-of-bounds in erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
[3] BUG: KASAN: use-after-free in erspan_xmit+0x22d4/0x2430 net/ipv4/ip_gre.c:735
[4] BUG: KASAN: use-after-free in erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698

[2] CPU: 0 PID: 3654 Comm: syzkaller377964 Not tainted 4.15.0-rc9+ #185
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x194/0x257 lib/dump_stack.c:53
 print_address_description+0x73/0x250 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report+0x25b/0x340 mm/kasan/report.c:409
 __asan_report_load_n_noabort+0xf/0x20 mm/kasan/report.c:440
 erspan_build_header+0x3bf/0x3d0 net/ipv4/ip_gre.c:698
 erspan_xmit+0x3b8/0x13b0 net/ipv4/ip_gre.c:740
 __netdev_start_xmit include/linux/netdevice.h:4042 [inline]
 netdev_start_xmit include/linux/netdevice.h:4051 [inline]
 packet_direct_xmit+0x315/0x6b0 net/packet/af_packet.c:266
 packet_snd net/packet/af_packet.c:2943 [inline]
 packet_sendmsg+0x3aed/0x60b0 net/packet/af_packet.c:2968
 sock_sendmsg_nosec net/socket.c:638 [inline]
 sock_sendmsg+0xca/0x110 net/socket.c:648
 SYSC_sendto+0x361/0x5c0 net/socket.c:1729
 SyS_sendto+0x40/0x50 net/socket.c:1697
 do_syscall_32_irqs_on arch/x86/entry/common.c:327 [inline]
 do_fast_syscall_32+0x3ee/0xf9d arch/x86/entry/common.c:389
 entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129
RIP: 0023:0xf7fcfc79
RSP: 002b:00000000ffc6976c EFLAGS: 00000286 ORIG_RAX: 0000000000000171
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000020011000
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000020008000
RBP: 000000000000001c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Fixes: f551c91de262 ("net: erspan: introduce erspan v2 for ip_gre")
Fixes: 84e54fe0a5ea ("gre: introduce native tunnel support for ERSPAN")
Reported-by: syzbot+9723f2d288e49b492cf0@syzkaller.appspotmail.com
Reported-by: syzbot+f0ddeb2b032a8e1d9098@syzkaller.appspotmail.com
Reported-by: syzbot+f14b3703cd8d7670203f@syzkaller.appspotmail.com
Reported-by: syzbot+eefa384efad8d7997f20@syzkaller.appspotmail.com
Signed-off-by: William Tu <u9012063@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/erspan.h

@@ -123,7 +123,7 @@ static inline void erspan_build_header(struct sk_buff *skb,
 				__be32 id, u32 index,
 				bool truncate, bool is_ipv4)
 {
-	struct ethhdr *eth = eth_hdr(skb);
+	struct ethhdr *eth = (struct ethhdr *)skb->data;
 	enum erspan_encap_type enc_type;
 	struct erspan_base_hdr *ershdr;
 	struct erspan_metadata *ersmd;
@@ -190,7 +190,7 @@ static inline void erspan_build_header_v2(struct sk_buff *skb,
 					  __be32 id, u8 direction, u16 hwid,
 					  bool truncate, bool is_ipv4)
 {
-	struct ethhdr *eth = eth_hdr(skb);
+	struct ethhdr *eth = (struct ethhdr *)skb->data;
 	struct erspan_base_hdr *ershdr;
 	struct erspan_metadata *md;
 	struct qtag_prefix {