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

[Aleksander Jan Bajkowski]

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@xxxxx>
> > ---
> >   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