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Subject: Re: [PATCH] ethernet/realtek/r8169: Fix a double free in
 rtl8169_start_xmit
To:     Lv Yunlong <lyl2019@mail.ustc.edu.cn>, nic_swsd@realtek.com,
        kuba@kernel.org
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210329090248.4209-1-lyl2019@mail.ustc.edu.cn>
From:   Heiner Kallweit <hkallweit1@gmail.com>
Message-ID: <8d40cd7a-c47e-199d-dccb-e242ec93e143@gmail.com>
Date:   Mon, 29 Mar 2021 11:51:30 +0200
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On 29.03.2021 11:02, Lv Yunlong wrote:
> In rtl8169_start_xmit, it calls rtl8169_tso_csum_v2(tp, skb, opts) and
> rtl8169_tso_csum_v2() calls __skb_put_padto(skb, padto, false). If
> __skb_put_padto() failed, it will free the skb in the first time and
> return error. Then rtl8169_start_xmit will goto err_dma_0.
> 

No, the skb isn't freed here in case of error. Have a look at the
implementation of __skb_put_padto() and see also cc6528bc9a0c
("r8169: fix potential skb double free in an error path").


> But in err_dma_0 label, the skb is freed by dev_kfree_skb_any(skb) in
> the second time.
> 
> My patch adds a new label inside the old err_dma_0 label to avoid the
> double free and renames the error labels to keep the origin function
> unchanged.
> 
> Fixes: b8447abc4c8fb ("r8169: factor out rtl8169_tx_map")
> Signed-off-by: Lv Yunlong <lyl2019@mail.ustc.edu.cn>
> ---
>  drivers/net/ethernet/realtek/r8169_main.c | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/realtek/r8169_main.c b/drivers/net/ethernet/realtek/r8169_main.c
> index f704da3f214c..91c43399712b 100644
> --- a/drivers/net/ethernet/realtek/r8169_main.c
> +++ b/drivers/net/ethernet/realtek/r8169_main.c
> @@ -4217,13 +4217,13 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
>  
>  	if (unlikely(rtl8169_tx_map(tp, opts, skb_headlen(skb), skb->data,
>  				    entry, false)))
> -		goto err_dma_0;
> +		goto err_dma_1;
>  
>  	txd_first = tp->TxDescArray + entry;
>  
>  	if (frags) {
>  		if (rtl8169_xmit_frags(tp, skb, opts, entry))
> -			goto err_dma_1;
> +			goto err_dma_2;
>  		entry = (entry + frags) % NUM_TX_DESC;
>  	}
>  
> @@ -4270,10 +4270,11 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
>  
>  	return NETDEV_TX_OK;
>  
> -err_dma_1:
> +err_dma_2:
>  	rtl8169_unmap_tx_skb(tp, entry);
> -err_dma_0:
> +err_dma_1:
>  	dev_kfree_skb_any(skb);
> +err_dma_0:
>  	dev->stats.tx_dropped++;
>  	return NETDEV_TX_OK;
>  
>