Re: [PATCH net v2 1/1] net: ethernet: lantiq_etop: fix memory disclosure

[Aleksander Jan Bajkowski <olek2@wp.pl>]

Hi Florian,


On 23.09.2024 20:21, Florian Fainelli wrote:
> On 9/21/24 03:58, Aleksander Jan Bajkowski wrote:
>> When applying padding, the buffer is not zeroed, which results in memory
>> disclosure. The mentioned data is observed on the wire. This patch uses
>> skb_put_padto() to pad Ethernet frames properly. The mentioned function
>> zeroes the expanded buffer.
>>
>> In case the packet cannot be padded it is silently dropped. Statistics
>> are also not incremented. This driver does not support statistics in the
>> old 32-bit format or the new 64-bit format. These will be added in the
>> future. In its current form, the patch should be easily backported to
>> stable versions.
>>
>> Ethernet MACs on Amazon-SE and Danube cannot do padding of the packets
>> in hardware, so software padding must be applied.
>>
>> Fixes: 504d4721ee8e ("MIPS: Lantiq: Add ethernet driver")
>> Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
>> ---
>>   drivers/net/ethernet/lantiq_etop.c | 11 ++++++-----
>>   1 file changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/lantiq_etop.c 
>> b/drivers/net/ethernet/lantiq_etop.c
>> index 3c289bfe0a09..36f1e3c93ca5 100644
>> --- a/drivers/net/ethernet/lantiq_etop.c
>> +++ b/drivers/net/ethernet/lantiq_etop.c
>> @@ -477,11 +477,11 @@ ltq_etop_tx(struct sk_buff *skb, struct 
>> net_device *dev)
>>       struct ltq_etop_priv *priv = netdev_priv(dev);
>>       struct ltq_etop_chan *ch = &priv->ch[(queue << 1) | 1];
>>       struct ltq_dma_desc *desc = &ch->dma.desc_base[ch->dma.desc];
>> -    int len;
>>       unsigned long flags;
>>       u32 byte_offset;
>>   -    len = skb->len < ETH_ZLEN ? ETH_ZLEN : skb->len;
>> +    if (skb_put_padto(skb, ETH_ZLEN))
>> +        return NETDEV_TX_OK;
>
> You should consider continuing to use the temporary variable 'len' 
> here, and just re-assign it after the call to skb_put_padto() and 
> avoid introducing potential user-after-free near the point where you 
> program the buffer length into the HW. This also minimizes the amount 
> of lines to review.

To the best of my knowledge, the skb is not released until the DMA 
finishes the transfer.
Then the ltq_etop_poll_tx() function is called, which releases the skb. 
Can you explain
what sequence of events can lead to a use-after-free error?

-->ltq_etop_tx()
      |
      | (dma irq fires)
      |
      -->ltq_etop_dma_irq()
           |
           | (napi task schedule)
           |
           -->ltq_etop_poll_tx()
                |
                |
                |
                -->dev_kfree_skb_any()

Regards