netfilter: Fix use-after-free in get_info()

netfilter: Fix use-after-free in get_info()

ip6table_nat module unload has refcnt warning for UAF. call trace is:

WARNING: CPU: 1 PID: 379 at kernel/module/main.c:853 module_put+0x6f/0x80
Modules linked in: ip6table_nat(-)
CPU: 1 UID: 0 PID: 379 Comm: ip6tables Not tainted 6.12.0-rc4-00047-gc2ee9f594da8-dirty #205
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
RIP: 0010:module_put+0x6f/0x80
Call Trace:
 <TASK>
 get_info+0x128/0x180
 do_ip6t_get_ctl+0x6a/0x430
 nf_getsockopt+0x46/0x80
 ipv6_getsockopt+0xb9/0x100
 rawv6_getsockopt+0x42/0x190
 do_sock_getsockopt+0xaa/0x180
 __sys_getsockopt+0x70/0xc0
 __x64_sys_getsockopt+0x20/0x30
 do_syscall_64+0xa2/0x1a0
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Concurrent execution of module unload and get_info() trigered the warning.
The root cause is as follows:

cpu0				      cpu1
module_exit
//mod->state = MODULE_STATE_GOING
  ip6table_nat_exit
    xt_unregister_template
	kfree(t)
	//removed from templ_list
				      getinfo()
					  t = xt_find_table_lock
						list_for_each_entry(tmpl, &xt_templates[af]...)
							if (strcmp(tmpl->name, name))
								continue;  //table not found
							try_module_get
						list_for_each_entry(t, &xt_net->tables[af]...)
							return t;  //not get refcnt
					  module_put(t->me) //uaf
    unregister_pernet_subsys
    //remove table from xt_net list

While xt_table module was going away and has been removed from
xt_templates list, we couldnt get refcnt of xt_table->me. Check
module in xt_net->tables list re-traversal to fix it.

Fixes: fdacd57c79b7 ("netfilter: x_tables: never register tables by default")
Signed-off-by: Dong Chenchen <dongchenchen2@huawei.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>