xfrm: add the missing verify_sec_ctx_len check in xfrm_add_acquire

Without doing verify_sec_ctx_len() check in xfrm_add_acquire(), it may be
out-of-bounds to access uctx->ctx_str with uctx->ctx_len, as noticed by
syz:

  BUG: KASAN: slab-out-of-bounds in selinux_xfrm_alloc_user+0x237/0x430
  Read of size 768 at addr ffff8880123be9b4 by task syz-executor.1/11650

  Call Trace:
   dump_stack+0xe8/0x16e
   print_address_description.cold.3+0x9/0x23b
   kasan_report.cold.4+0x64/0x95
   memcpy+0x1f/0x50
   selinux_xfrm_alloc_user+0x237/0x430
   security_xfrm_policy_alloc+0x5c/0xb0
   xfrm_policy_construct+0x2b1/0x650
   xfrm_add_acquire+0x21d/0xa10
   xfrm_user_rcv_msg+0x431/0x6f0
   netlink_rcv_skb+0x15a/0x410
   xfrm_netlink_rcv+0x6d/0x90
   netlink_unicast+0x50e/0x6a0
   netlink_sendmsg+0x8ae/0xd40
   sock_sendmsg+0x133/0x170
   ___sys_sendmsg+0x834/0x9a0
   __sys_sendmsg+0x100/0x1e0
   do_syscall_64+0xe5/0x660
   entry_SYSCALL_64_after_hwframe+0x6a/0xdf

So fix it by adding the missing verify_sec_ctx_len check there.

Fixes: 980ebd2579 ("[IPSEC]: Sync series - acquire insert")
Reported-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_user.c

@@ -2274,6 +2274,9 @@ static int xfrm_add_acquire(struct sk_buff *skb, struct nlmsghdr *nlh,
 	xfrm_mark_get(attrs, &mark);
 
 	err = verify_newpolicy_info(&ua->policy);
+	if (err)
+		goto free_state;
+	err = verify_sec_ctx_len(attrs);
 	if (err)
 		goto free_state;