[PATCH] netfilter: ebt_nflog: pin the NFLOG backend

nf_log_unregister() runs after the per-net teardown so its final RCU
grace period also drains readers that obtained the logger from a per-net
binding.  However, ebt_nflog passes an explicit ULOG log type to
nf_log_packet() without holding a reference on the selected logger module,
unlike the xt_NFLOG and nft_log frontends.

An ebtables nflog rule can therefore remain callable while nfnetlink_log
is unloaded.  The resulting interleaving is:

  CPU 0                               CPU 1
  nfnetlink_log_fini()
    unregister_pernet_subsys()
      kfree(nfnl_log_pernet(net))
                                      ebt_nflog_tg()
                                        nf_log_packet()
                                          nfulnl_log_packet()
                                            instance_lookup_get_rcu()

The global ULOG logger is still registered at this point, so CPU 1
dereferences the per-net state after CPU 0 has freed it.  KASAN reported:

  BUG: KASAN: slab-use-after-free in instance_lookup_get_rcu
  Read of size 8 at addr ff110001052e6210 by task poc/92
  Call Trace:
   instance_lookup_get_rcu+0x1ce/0x1f0 [nfnetlink_log]
   nfulnl_log_packet+0x248/0x2fb0 [nfnetlink_log]
   nf_log_packet+0x204/0x300
   ebt_nflog_tg+0x351/0x550
   ebt_do_table+0xedf/0x22b0
  Allocated by task 90:
   __kmalloc_noprof+0x186/0x470
   ops_init+0x6d/0x420
   register_pernet_operations+0x2f6/0x670
   register_pernet_subsys+0x23/0x40
  Freed by task 93:
   kfree+0x131/0x3c0
   ops_undo_list+0x3e3/0x700
   unregister_pernet_operations+0x232/0x490
   unregister_pernet_subsys+0x1c/0x30
   nfnetlink_log_fini+0x34/0x450 [nfnetlink_log]

Acquire the ULOG logger module reference when an ebt_nflog rule is
validated and release it when the rule is destroyed.  Request the NFLOG
backend for legacy callers when needed, matching xt_NFLOG.  This prevents
module teardown until all ebt_nflog rules have stopped using the logger.

Fixes: c83fa19603bd ("netfilter: nf_log: don't call synchronize_rcu in nf_log_unset")
Cc: stable@vger.kernel.org
Signed-off-by: Chengfeng Ye <redacted>
---
 net/bridge/netfilter/ebt_nflog.c | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/net/bridge/netfilter/ebt_nflog.c b/net/bridge/netfilter/ebt_nflog.c
index 61bf8f4465ab..426f8adc912c 100644
--- a/net/bridge/netfilter/ebt_nflog.c
+++ b/net/bridge/netfilter/ebt_nflog.c

@@ -41,11 +41,25 @@ ebt_nflog_tg(struct sk_buff *skb, const struct xt_action_param *par)
 static int ebt_nflog_tg_check(const struct xt_tgchk_param *par)
 {
 	struct ebt_nflog_info *info = par->targinfo;
+	int ret;
 
 	if (info->flags & ~EBT_NFLOG_MASK)
 		return -EINVAL;
 	info->prefix[EBT_NFLOG_PREFIX_SIZE - 1] = '\0';
-	return 0;
+
+	ret = nf_logger_find_get(par->family, NF_LOG_TYPE_ULOG);
+	if (ret != 0 && !par->nft_compat) {
+		request_module("%s", "nfnetlink_log");
+
+		ret = nf_logger_find_get(par->family, NF_LOG_TYPE_ULOG);
+	}
+
+	return ret;
+}
+
+static void ebt_nflog_tg_destroy(const struct xt_tgdtor_param *par)
+{
+	nf_logger_put(par->family, NF_LOG_TYPE_ULOG);
 }
 
 static struct xt_target ebt_nflog_tg_reg __read_mostly = {

@@ -54,6 +68,7 @@ static struct xt_target ebt_nflog_tg_reg __read_mostly = {
 	.family     = NFPROTO_BRIDGE,
 	.target     = ebt_nflog_tg,
 	.checkentry = ebt_nflog_tg_check,
+	.destroy    = ebt_nflog_tg_destroy,
 	.targetsize = sizeof(struct ebt_nflog_info),
 	.me         = THIS_MODULE,
 };

-- 
2.43.0