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

[Florian Fainelli]

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@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?

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