[PATCH net 1/3] netfilter: nft_ct: Use __refcount_inc() for per-CPU nft_ct_pcpu_template.

[Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>]

From: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>

nft_ct_pcpu_template is a per-CPU variable and relies on disabled BH for its
locking. The refcounter is read and if its value is set to one then the
refcounter is incremented and variable is used - otherwise it is already
in use and left untouched.

Without per-CPU locking in local_bh_disable() on PREEMPT_RT the
read-then-increment operation is not atomic and therefore racy.

This can be avoided by using unconditionally __refcount_inc() which will
increment counter and return the old value as an atomic operation.
In case the returned counter is not one, the variable is in use and we
need to decrement counter. Otherwise we can use it.

Use __refcount_inc() instead of read and a conditional increment.

Fixes: edee4f1e9245 ("netfilter: nft_ct: add zone id set support")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Reviewed-by: Florian Westphal <fw@xxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
---
 net/netfilter/nft_ct.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nft_ct.c b/net/netfilter/nft_ct.c
index 2e59aba681a1..d526e69a2a2b 100644
--- a/net/netfilter/nft_ct.c
+++ b/net/netfilter/nft_ct.c
@@ -230,6 +230,7 @@ static void nft_ct_set_zone_eval(const struct nft_expr *expr,
 	enum ip_conntrack_info ctinfo;
 	u16 value = nft_reg_load16(&regs->data[priv->sreg]);
 	struct nf_conn *ct;
+	int oldcnt;
 
 	ct = nf_ct_get(skb, &ctinfo);
 	if (ct) /* already tracked */
@@ -250,10 +251,11 @@ static void nft_ct_set_zone_eval(const struct nft_expr *expr,
 
 	ct = this_cpu_read(nft_ct_pcpu_template);
 
-	if (likely(refcount_read(&ct->ct_general.use) == 1)) {
-		refcount_inc(&ct->ct_general.use);
+	__refcount_inc(&ct->ct_general.use, &oldcnt);
+	if (likely(oldcnt == 1)) {
 		nf_ct_zone_add(ct, &zone);
 	} else {
+		refcount_dec(&ct->ct_general.use);
 		/* previous skb got queued to userspace, allocate temporary
 		 * one until percpu template can be reused.
 		 */
-- 
2.30.2