linux-kernel - [PATCH AUTOSEL 5.12 20/39] net/packet: annotate data race in packet

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Message-Id: <20210621175156.735062-20-sashal@kernel.org>
Date:   Mon, 21 Jun 2021 13:51:36 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Sasha Levin <sashal@...nel.org>, netdev@...r.kernel.org
Subject: [PATCH AUTOSEL 5.12 20/39] net/packet: annotate data race in packet_sendmsg()

From: Eric Dumazet <edumazet@...gle.com>

[ Upstream commit d1b5bee4c8be01585033be9b3a8878789285285f ]

There is a known race in packet_sendmsg(), addressed
in commit 32d3182cd2cd ("net/packet: fix race in tpacket_snd()")

Now we have data_race(), we can use it to avoid a future KCSAN warning,
as syzbot loves stressing af_packet sockets :)

Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 net/packet/af_packet.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index c52557ec7fb3..84d8921391c3 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -3034,10 +3034,13 @@ static int packet_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 	struct sock *sk = sock->sk;
 	struct packet_sock *po = pkt_sk(sk);
 
-	if (po->tx_ring.pg_vec)
+	/* Reading tx_ring.pg_vec without holding pg_vec_lock is racy.
+	 * tpacket_snd() will redo the check safely.
+	 */
+	if (data_race(po->tx_ring.pg_vec))
 		return tpacket_snd(po, msg);
-	else
-		return packet_snd(sock, msg, len);
+
+	return packet_snd(sock, msg, len);
 }
 
 /*
-- 
2.30.2