netdev - Re: [PATCH net 3/4] net: ethernet: cortina: Protect against oversized frames

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Date: Sat, 4 Nov 2023 15:46:09 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Linus Walleij <linus.walleij@...aro.org>
Cc: Hans Ulli Kroll <ulli.kroll@...glemail.com>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Michał Mirosław <mirq-linux@...e.qmqm.pl>,
	Vladimir Oltean <olteanv@...il.com>,
	linux-arm-kernel@...ts.infradead.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 3/4] net: ethernet: cortina: Protect against
 oversized frames

On Sat, Nov 04, 2023 at 01:43:50PM +0100, Linus Walleij wrote:
> The max size of a transfer no matter the MTU is 64KB-1 so immediately
> bail out if the skb exceeds that.
> 
> Fixes: 4d5ae32f5e1e ("net: ethernet: Add a driver for Gemini gigabit ethernet")
> Signed-off-by: Linus Walleij <linus.walleij@...aro.org>
> ---
>  drivers/net/ethernet/cortina/gemini.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/drivers/net/ethernet/cortina/gemini.c b/drivers/net/ethernet/cortina/gemini.c
> index fd08f098850b..23723c9c0f93 100644
> --- a/drivers/net/ethernet/cortina/gemini.c
> +++ b/drivers/net/ethernet/cortina/gemini.c
> @@ -1156,6 +1156,12 @@ static int gmac_map_tx_bufs(struct net_device *netdev, struct sk_buff *skb,
>  		mtu = MTU_SIZE_BIT_MASK;
>  	}
>  
> +	if (skb->len > 65535) {
> +		/* The field for length is only 16 bits */
> +		netdev_err(netdev, "%s: frame too big, max size 65535 bytes\n", __func__);
> +		return -EINVAL;
> +	}
> +

The caller of gmac_map_tx_bufs() is a but funky:

	if (gmac_map_tx_bufs(netdev, skb, txq, &w)) {
		if (skb_linearize(skb))
			goto out_drop;

		u64_stats_update_begin(&port->tx_stats_syncp);
		port->tx_frags_linearized++;
		u64_stats_update_end(&port->tx_stats_syncp);

		if (gmac_map_tx_bufs(netdev, skb, txq, &w))
			goto out_drop_free;
	}

So return -EINVAL is going to cause the skb to be linearised, and then
re-tried. Maybe you want to check the error code here, and go straight
to out_drop?

   Andrew