net/xfrm: fix out-of-bounds packet access

BUG: KASAN: slab-out-of-bounds in _decode_session6+0x1331/0x14e0
net/ipv6/xfrm6_policy.c:161
Read of size 1 at addr ffff8801d882eec7 by task syz-executor1/6667
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
  print_address_description+0x6c/0x20b mm/kasan/report.c:256
  kasan_report_error mm/kasan/report.c:354 [inline]
  kasan_report.cold.7+0x242/0x30d mm/kasan/report.c:412
  __asan_report_load1_noabort+0x14/0x20 mm/kasan/report.c:430
  _decode_session6+0x1331/0x14e0 net/ipv6/xfrm6_policy.c:161
  __xfrm_decode_session+0x71/0x140 net/xfrm/xfrm_policy.c:2299
  xfrm_decode_session include/net/xfrm.h:1232 [inline]
  vti6_tnl_xmit+0x3c3/0x1bc1 net/ipv6/ip6_vti.c:542
  __netdev_start_xmit include/linux/netdevice.h:4313 [inline]
  netdev_start_xmit include/linux/netdevice.h:4322 [inline]
  xmit_one net/core/dev.c:3217 [inline]
  dev_hard_start_xmit+0x272/0xc10 net/core/dev.c:3233
  __dev_queue_xmit+0x2ab2/0x3870 net/core/dev.c:3803
  dev_queue_xmit+0x17/0x20 net/core/dev.c:3836

Reported-by: syzbot+acffccec848dc13fe459@syzkaller.appspotmail.com
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/ipv6/xfrm6_policy.c

@@ -146,8 +146,8 @@ _decode_session6(struct sk_buff *skb, struct flowi *fl, int reverse)
 	fl6->daddr = reverse ? hdr->saddr : hdr->daddr;
 	fl6->saddr = reverse ? hdr->daddr : hdr->saddr;
 
-	while (nh + offset + 1 < skb->data ||
-	       pskb_may_pull(skb, nh + offset + 1 - skb->data)) {
+	while (nh + offset + sizeof(*exthdr) < skb->data ||
+	       pskb_may_pull(skb, nh + offset + sizeof(*exthdr) - skb->data)) {
 		nh = skb_network_header(skb);
 		exthdr = (struct ipv6_opt_hdr *)(nh + offset);