netdev - Re: [PATCH net] r8169: work around short packet hw bug on RTL8125

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Date:   Thu, 5 Nov 2020 11:31:48 +0100
From:   Heiner Kallweit <hkallweit1@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     David Miller <davem@...emloft.net>,
        Realtek linux nic maintainers <nic_swsd@...ltek.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Maxim Plotnikov <wgh@...lan.ru>
Subject: Re: [PATCH net] r8169: work around short packet hw bug on RTL8125

On 05.11.2020 02:45, Jakub Kicinski wrote:
> On Tue, 3 Nov 2020 18:52:18 +0100 Heiner Kallweit wrote:
>> Network problems with RTL8125B have been reported [0] and with help
>> from Realtek it turned out that this chip version has a hw problem
>> with short packets (similar to RTL8168evl). Having said that activate
>> the same workaround as for RTL8168evl.
>> Realtek suggested to activate the workaround for RTL8125A too, even
>> though they're not 100% sure yet which RTL8125 versions are affected.
>>
>> [0] https://bugzilla.kernel.org/show_bug.cgi?id=209839
>>
>> Fixes: 0439297be951 ("r8169: add support for RTL8125B")
>> Reported-by: Maxim Plotnikov <wgh@...lan.ru>
>> Tested-by: Maxim Plotnikov <wgh@...lan.ru>
>> Signed-off-by: Heiner Kallweit <hkallweit1@...il.com>
> 
> Applied, thanks!
> 
>> @@ -4125,7 +4133,7 @@ static bool rtl8169_tso_csum_v2(struct rtl8169_private *tp,
>>  
>>  		opts[1] |= transport_offset << TCPHO_SHIFT;
>>  	} else {
>> -		if (unlikely(rtl_test_hw_pad_bug(tp, skb)))
>> +		if (unlikely(skb->len < ETH_ZLEN && rtl_test_hw_pad_bug(tp)))
>>  			return !eth_skb_pad(skb);
>>  	}
> 
> But looks like we may have another bug here - looks like this function
> treas skb_cow_head() and eth_skb_pad() failures the same, but former
> doesn't free the skb on error, while the latter does.
> 
Thanks for the hint, indeed we have an issue. The caller of
rtl8169_tso_csum_v2() also frees the skb if false is returned, therefore
we have a double free if eth_skb_pad() fails.

When checking eth_skb_pad() I saw that it uses kfree_skb() to free
the skb on error. Kernel documentation say about ndo_start_xmit context:

Process with BHs disabled or BH (timer),
will be called with interrupts disabled by netconsole.

Is it safe to use kfree_skb() if interrupts are disabled?
I'm asking because dev_kfree_skb_any() uses the irq path if
irqs_disabled() is true.