Subject: [PATCH] netfiler: protect nft_ct_pcpu_template_refcnt with mutex

From: Pavel Skripkin <[email-masked]>
Date: Tue, 10 Aug 2021 15:55:23 +0300

Syzbot hit use-after-free in nf_tables_dump_sets. The problem was in
missing lock protection for nft_ct_pcpu_template_refcnt.

Before commit f102d66b335a ("netfilter: nf_tables: use dedicated
mutex to guard transactions") all transactions were serialized by global
mutex, but then global mutex was changed to local per netnamespace
commit_mutex.

This change causes use-after-free bug, when 2 netnamespaces concurently
changing nft_ct_pcpu_template_refcnt without proper locking. Fix it by
adding nft_ct_pcpu_mutex and protect all nft_ct_pcpu_template_refcnt
changes with it.

Fixes: f102d66b335a ("netfilter: nf_tables: use dedicated mutex to guard transactions")
Reported-and-tested-by: [email-masked]
Signed-off-by: Pavel Skripkin <[email-masked]>
---
 net/netfilter/nft_ct.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
index 337e22d8b40b..99b1de14ff7e 100644
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -41,6 +41,7 @@ struct nft_ct_helper_obj  {
 #ifdef CONFIG_NF_CONNTRACK_ZONES
 static DEFINE_PER_CPU(struct nf_conn *, nft_ct_pcpu_template);
 static unsigned int nft_ct_pcpu_template_refcnt __read_mostly;
+static DEFINE_MUTEX(nft_ct_pcpu_mutex);
 #endif
 
 static u64 nft_ct_get_eval_counter(const struct nf_conn_counter *c,
@@ -525,8 +526,10 @@ static void __nft_ct_set_destroy(const struct nft_ctx *ctx, struct nft_ct *priv)
 #endif
 #ifdef CONFIG_NF_CONNTRACK_ZONES
 	case NFT_CT_ZONE:
+		mutex_lock(&nft_ct_pcpu_mutex);
 		if (--nft_ct_pcpu_template_refcnt == 0)
 			nft_ct_tmpl_put_pcpu();
+		mutex_unlock(&nft_ct_pcpu_mutex);
 		break;
 #endif
 	default:
@@ -564,9 +567,13 @@ static int nft_ct_set_init(const struct nft_ctx *ctx,
 #endif
 #ifdef CONFIG_NF_CONNTRACK_ZONES
 	case NFT_CT_ZONE:
-		if (!nft_ct_tmpl_alloc_pcpu())
+		mutex_lock(&nft_ct_pcpu_mutex);
+		if (!nft_ct_tmpl_alloc_pcpu()) {
+			mutex_unlock(&nft_ct_pcpu_mutex);
 			return -ENOMEM;
+		}
 		nft_ct_pcpu_template_refcnt++;
+		mutex_unlock(&nft_ct_pcpu_mutex);
 		len = sizeof(u16);
 		break;
 #endif
-- 
2.32.0