netdev - [PATCH net 1/3] netfilter: nft_counter: Disable BH in nft_counter_offload

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Message-Id: <20240822001707.2116-2-pablo@netfilter.org>
Date: Thu, 22 Aug 2024 02:17:05 +0200
From: Pablo Neira Ayuso <pablo@...filter.org>
To: netfilter-devel@...r.kernel.org
Cc: davem@...emloft.net,
	netdev@...r.kernel.org,
	kuba@...nel.org,
	pabeni@...hat.com,
	edumazet@...gle.com,
	fw@...len.de
Subject: [PATCH net 1/3] netfilter: nft_counter: Disable BH in nft_counter_offload_stats().

From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>

The sequence counter nft_counter_seq is a per-CPU counter. There is no
lock associated with it. nft_counter_do_eval() is using the same counter
and disables BH which suggest that it can be invoked from a softirq.
This in turn means that nft_counter_offload_stats(), which disables only
preemption, can be interrupted by nft_counter_do_eval() leading to two
writer for one seqcount_t.
This can lead to loosing stats or reading statistics while they are
updated.

Disable BH during stats update in nft_counter_offload_stats() to ensure
one writer at a time.

Fixes: b72920f6e4a9d ("netfilter: nftables: counter hardware offload support")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Reviewed-by: Florian Westphal <fw@...len.de>
Signed-off-by: Pablo Neira Ayuso <pablo@...filter.org>
---
 net/netfilter/nft_counter.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/netfilter/nft_counter.c b/net/netfilter/nft_counter.c
index 291ed2026367..16f40b503d37 100644
--- a/net/netfilter/nft_counter.c
+++ b/net/netfilter/nft_counter.c
@@ -265,7 +265,7 @@ static void nft_counter_offload_stats(struct nft_expr *expr,
 	struct nft_counter *this_cpu;
 	seqcount_t *myseq;
 
-	preempt_disable();
+	local_bh_disable();
 	this_cpu = this_cpu_ptr(priv->counter);
 	myseq = this_cpu_ptr(&nft_counter_seq);
 
@@ -273,7 +273,7 @@ static void nft_counter_offload_stats(struct nft_expr *expr,
 	this_cpu->packets += stats->pkts;
 	this_cpu->bytes += stats->bytes;
 	write_seqcount_end(myseq);
-	preempt_enable();
+	local_bh_enable();
 }
 
 void nft_counter_init_seqcount(void)
-- 
2.30.2