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

[Florian Fainelli <f.fainelli@gmail.com>]

On 9/23/24 14:08, Aleksander Jan Bajkowski wrote:
> 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?

There is none right now, but assuming you might add byte queue limits in 
the future, or just re-structure the code, reading from skb->len is 
error prone and is better left avoided, especially since this will 
result in few lines being changed in your case.

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


-- 
Florian