net/sched: act_tunnel_key: fix OOB write in case of IPv6 ERSPAN tunnels

the following command

 # tc action add action tunnel_key \
 > set src_ip 2001:db8::1 dst_ip 2001:db8::2 id 10 erspan_opts 1:6789:0:0

generates the following splat:

 BUG: KASAN: slab-out-of-bounds in tunnel_key_copy_opts+0xcc9/0x1010 [act_tunnel_key]
 Write of size 4 at addr ffff88813f5f1cc8 by task tc/873

 CPU: 2 PID: 873 Comm: tc Not tainted 5.9.0+ #282
 Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014
 Call Trace:
  dump_stack+0x99/0xcb
  print_address_description.constprop.7+0x1e/0x230
  kasan_report.cold.13+0x37/0x7c
  tunnel_key_copy_opts+0xcc9/0x1010 [act_tunnel_key]
  tunnel_key_init+0x160c/0x1f40 [act_tunnel_key]
  tcf_action_init_1+0x5b5/0x850
  tcf_action_init+0x15d/0x370
  tcf_action_add+0xd9/0x2f0
  tc_ctl_action+0x29b/0x3a0
  rtnetlink_rcv_msg+0x341/0x8d0
  netlink_rcv_skb+0x120/0x380
  netlink_unicast+0x439/0x630
  netlink_sendmsg+0x719/0xbf0
  sock_sendmsg+0xe2/0x110
  ____sys_sendmsg+0x5ba/0x890
  ___sys_sendmsg+0xe9/0x160
  __sys_sendmsg+0xd3/0x170
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7f872a96b338
 Code: 89 02 48 c7 c0 ff ff ff ff eb b5 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 25 43 2c 00 8b 00 85 c0 75 17 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 41 54 41 89 d4 55
 RSP: 002b:00007ffffe367518 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
 RAX: ffffffffffffffda RBX: 000000005f8f5aed RCX: 00007f872a96b338
 RDX: 0000000000000000 RSI: 00007ffffe367580 RDI: 0000000000000003
 RBP: 0000000000000000 R08: 0000000000000001 R09: 000000000000001c
 R10: 000000000000000b R11: 0000000000000246 R12: 0000000000000001
 R13: 0000000000686760 R14: 0000000000000601 R15: 0000000000000000

 Allocated by task 873:
  kasan_save_stack+0x19/0x40
  __kasan_kmalloc.constprop.7+0xc1/0xd0
  __kmalloc+0x151/0x310
  metadata_dst_alloc+0x20/0x40
  tunnel_key_init+0xfff/0x1f40 [act_tunnel_key]
  tcf_action_init_1+0x5b5/0x850
  tcf_action_init+0x15d/0x370
  tcf_action_add+0xd9/0x2f0
  tc_ctl_action+0x29b/0x3a0
  rtnetlink_rcv_msg+0x341/0x8d0
  netlink_rcv_skb+0x120/0x380
  netlink_unicast+0x439/0x630
  netlink_sendmsg+0x719/0xbf0
  sock_sendmsg+0xe2/0x110
  ____sys_sendmsg+0x5ba/0x890
  ___sys_sendmsg+0xe9/0x160
  __sys_sendmsg+0xd3/0x170
  do_syscall_64+0x33/0x40
  entry_SYSCALL_64_after_hwframe+0x44/0xa9

 The buggy address belongs to the object at ffff88813f5f1c00
  which belongs to the cache kmalloc-256 of size 256
 The buggy address is located 200 bytes inside of
  256-byte region [ffff88813f5f1c00, ffff88813f5f1d00)
 The buggy address belongs to the page:
 page:0000000011b48a19 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x13f5f0
 head:0000000011b48a19 order:1 compound_mapcount:0
 flags: 0x17ffffc0010200(slab|head)
 raw: 0017ffffc0010200 0000000000000000 0000000d00000001 ffff888107c43400
 raw: 0000000000000000 0000000080100010 00000001ffffffff 0000000000000000
 page dumped because: kasan: bad access detected

 Memory state around the buggy address:
  ffff88813f5f1b80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff88813f5f1c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 >ffff88813f5f1c80: 00 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc
                                               ^
  ffff88813f5f1d00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
  ffff88813f5f1d80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

using IPv6 tunnels, act_tunnel_key allocates a fixed amount of memory for
the tunnel metadata, but then it expects additional bytes to store tunnel
specific metadata with tunnel_key_copy_opts().

Fix the arguments of __ipv6_tun_set_dst(), so that 'md_size' contains the
size previously computed by tunnel_key_get_opts_len(), like it's done for
IPv4 tunnels.

Fixes: 0ed5269f9e ("net/sched: add tunnel option support to act_tunnel_key")
Reported-by: Shuang Li <shuali@redhat.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://lore.kernel.org/r/36ebe969f6d13ff59912d6464a4356fe6f103766.1603231100.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/sched/act_tunnel_key.c

@@ -459,7 +459,7 @@ static int tunnel_key_init(struct net *net, struct nlattr *nla,
 
 			metadata = __ipv6_tun_set_dst(&saddr, &daddr, tos, ttl, dst_port,
 						      0, flags,
-						      key_id, 0);
+						      key_id, opts_len);
 		} else {
 			NL_SET_ERR_MSG(extack, "Missing either ipv4 or ipv6 src and dst");
 			ret = -EINVAL;