netdev - [PATCH net 3/4] mlxsw: spectrum: Do not modify cloned SKBs during xmit

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Message-Id: <20200112160641.282108-4-idosch@idosch.org>
Date:   Sun, 12 Jan 2020 18:06:40 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     netdev@...r.kernel.org
Cc:     davem@...emloft.net, jiri@...lanox.com, mlxsw@...lanox.com,
        Shalom Toledo <shalomt@...lanox.com>,
        Ido Schimmel <idosch@...lanox.com>
Subject: [PATCH net 3/4] mlxsw: spectrum: Do not modify cloned SKBs during xmit

From: Shalom Toledo <shalomt@...lanox.com>

The driver needs to prepend a Tx header to each packet it is transmitting.
The header includes information such as the egress port and traffic class.

The addition of the header requires the driver to modify the SKB's data
buffer and therefore the SKB must be unshared first. Otherwise, we risk
hitting various race conditions with cloned SKBs.

For example, when a packet is flooded (cloned) by the bridge driver to two
switch ports swp1 and swp2:

t0 - mlxsw_sp_port_xmit() is called for swp1. Tx header is prepended with
     swp1's port number
t1 - mlxsw_sp_port_xmit() is called for swp2. Tx header is prepended with
     swp2's port number, overwriting swp1's port number
t2 - The device processes data buffer from t0. Packet is transmitted via
     swp2
t3 - The device processes data buffer from t1. Packet is transmitted via
     swp2

Usually, the device is fast enough and transmits the packet before its
Tx header is overwritten, but this is not the case in emulated
environments.

Fix this by unsharing the SKB.

Fixes: 56ade8fe3fe1 ("mlxsw: spectrum: Add initial support for Spectrum ASIC")
Signed-off-by: Shalom Toledo <shalomt@...lanox.com>
Acked-by: Jiri Pirko <jiri@...lanox.com>
Signed-off-by: Ido Schimmel <idosch@...lanox.com>
---
 drivers/net/ethernet/mellanox/mlxsw/spectrum.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/net/ethernet/mellanox/mlxsw/spectrum.c b/drivers/net/ethernet/mellanox/mlxsw/spectrum.c
index 5408a964bd10..6c7bf93dd804 100644
--- a/drivers/net/ethernet/mellanox/mlxsw/spectrum.c
+++ b/drivers/net/ethernet/mellanox/mlxsw/spectrum.c
@@ -860,6 +860,10 @@ static netdev_tx_t mlxsw_sp_port_xmit(struct sk_buff *skb,
 	u64 len;
 	int err;
 
+	skb = skb_unshare(skb, GFP_ATOMIC);
+	if (unlikely(!skb))
+		return NETDEV_TX_BUSY;
+
 	memset(skb->cb, 0, sizeof(struct mlxsw_skb_cb));
 
 	if (mlxsw_core_skb_transmit_busy(mlxsw_sp->core, &tx_info))
-- 
2.24.1