netdev - [Patch net v2 1/2] lwt: disable BH too in run_lwt

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Message-Id: <20201205075946.497763-1-xiyou.wangcong@gmail.com>
Date:   Fri,  4 Dec 2020 23:59:45 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     netdev@...r.kernel.org
Cc:     bpf@...r.kernel.org, Dongdong Wang <wangdongdong.6@...edance.com>,
        Thomas Graf <tgraf@...g.ch>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Cong Wang <cong.wang@...edance.com>
Subject: [Patch net v2 1/2] lwt: disable BH too in run_lwt_bpf()

From: Dongdong Wang <wangdongdong.6@...edance.com>

The per-cpu bpf_redirect_info is shared among all skb_do_redirect()
and BPF redirect helpers. Callers on RX path are all in BH context,
disabling preemption is not sufficient to prevent BH interruption.

In production, we observed strange packet drops because of the race
condition between LWT xmit and TC ingress, and we verified this issue
is fixed after we disable BH.

Although this bug was technically introduced from the beginning, that
is commit 3a0af8fd61f9 ("bpf: BPF for lightweight tunnel infrastructure"),
at that time call_rcu() had to be call_rcu_bh() to match the RCU context.
So this patch may not work well before RCU flavor consolidation has been
completed around v5.0.

Update the comments above the code too, as call_rcu() is now BH friendly.

Cc: Thomas Graf <tgraf@...g.ch>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>
Reviewed-by: Cong Wang <cong.wang@...edance.com>
Signed-off-by: Dongdong Wang <wangdongdong.6@...edance.com>
---
 net/core/lwt_bpf.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/net/core/lwt_bpf.c b/net/core/lwt_bpf.c
index 7d3438215f32..4f3cb7c15ddf 100644
--- a/net/core/lwt_bpf.c
+++ b/net/core/lwt_bpf.c
@@ -39,12 +39,11 @@ static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
 {
 	int ret;

-	/* Preempt disable is needed to protect per-cpu redirect_info between
-	 * BPF prog and skb_do_redirect(). The call_rcu in bpf_prog_put() and
-	 * access to maps strictly require a rcu_read_lock() for protection,
-	 * mixing with BH RCU lock doesn't work.
+	/* Preempt disable and BH disable are needed to protect per-cpu
+	 * redirect_info between BPF prog and skb_do_redirect().
 	 */
 	preempt_disable();
+	local_bh_disable();
 	bpf_compute_data_pointers(skb);
 	ret = bpf_prog_run_save_cb(lwt->prog, skb);

@@ -78,6 +77,7 @@ static int run_lwt_bpf(struct sk_buff *skb, struct bpf_lwt_prog *lwt,
 		break;
 	}

+	local_bh_enable();
 	preempt_enable();

 	return ret;
-- 
2.25.1