Subject: Re: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path

From: Andre Przywara <[email-masked]>
Date: Thu, 16 Jan 2020 18:03:26 +0000

On Fri, 10 Jan 2020 17:05:45 +0000
Radhey Shyam Pandey <[email-masked]> wrote:

Hi,

> > -----Original Message-----
> > From: Andre Przywara <[email-masked]>
> > Sent: Friday, January 10, 2020 9:13 PM
> > To: Radhey Shyam Pandey <[email-masked]>
> > Cc: David S . Miller <[email-masked]>; Michal Simek
> > <[email-masked]>; Robert Hancock <[email-masked]>;
> > [email-masked]; [email-masked]; linux-
> > [email-masked]
> > Subject: Re: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
> > 
> > On Fri, 10 Jan 2020 15:14:46 +0000
> > Radhey Shyam Pandey <[email-masked]> wrote:
> > 
> > Hi Radhey,
> > 
> > thanks for having a look!
> >   
> > > > -----Original Message-----
> > > > From: Andre Przywara <[email-masked]>
> > > > Sent: Friday, January 10, 2020 5:24 PM
> > > > To: David S . Miller <[email-masked]>; Radhey Shyam Pandey
> > > > <[email-masked]>
> > > > Cc: Michal Simek <[email-masked]>; Robert Hancock
> > > > <[email-masked]>; [email-masked]; linux-arm-
> > > > [email-masked]; [email-masked]
> > > > Subject: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
> > > >
> > > > When axienet_dma_bd_init() bails out during the initialisation process,
> > > > it might do so with parts of the structure already allocated and
> > > > initialised, while other parts have not been touched yet. Before
> > > > returning in this case, we call axienet_dma_bd_release(), which does not
> > > > take care of this corner case.
> > > > This is most obvious by the first loop happily dereferencing
> > > > lp->rx_bd_v, which we actually check to be non NULL *afterwards*.
> > > >
> > > > Make sure we only unmap or free already allocated structures, by:
> > > > - directly returning with -ENOMEM if nothing has been allocated at all
> > > > - checking for lp->rx_bd_v to be non-NULL *before* using it
> > > > - only unmapping allocated DMA RX regions
> > > >
> > > > This avoids NULL pointer dereferences when initialisation fails.
> > > >
> > > > Signed-off-by: Andre Przywara <[email-masked]>
> > > > ---
> > > >  .../net/ethernet/xilinx/xilinx_axienet_main.c | 43 ++++++++++++-------
> > > >  1 file changed, 28 insertions(+), 15 deletions(-)
> > > >
> > > > diff --git a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > > > b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > > > index 97482cf093ce..7e90044cf2d9 100644
> > > > --- a/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > > > +++ b/drivers/net/ethernet/xilinx/xilinx_axienet_main.c
> > > > @@ -160,24 +160,37 @@ static void axienet_dma_bd_release(struct
> > > > net_device *ndev)
> > > >  	int i;
> > > >  	struct axienet_local *lp = netdev_priv(ndev);
> > > >
> > > > +	/* If we end up here, tx_bd_v must have been DMA allocated. */
> > > > +	dma_free_coherent(ndev->dev.parent,
> > > > +			  sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
> > > > +			  lp->tx_bd_v,
> > > > +			  lp->tx_bd_p);
> > > > +
> > > > +	if (!lp->rx_bd_v)
> > > > +		return;
> > > > +
> > > >  	for (i = 0; i < lp->rx_bd_num; i++) {
> > > > -		dma_unmap_single(ndev->dev.parent, lp->rx_bd_v[i].phys,
> > > > -				 lp->max_frm_size, DMA_FROM_DEVICE);
> > > > +		/* A NULL skb means this descriptor has not been initialised
> > > > +		 * at all.
> > > > +		 */
> > > > +		if (!lp->rx_bd_v[i].skb)
> > > > +			break;
> > > > +
> > > >  		dev_kfree_skb(lp->rx_bd_v[i].skb);
> > > > -	}
> > > >
> > > > -	if (lp->rx_bd_v) {
> > > > -		dma_free_coherent(ndev->dev.parent,
> > > > -				  sizeof(*lp->rx_bd_v) * lp->rx_bd_num,
> > > > -				  lp->rx_bd_v,
> > > > -				  lp->rx_bd_p);
> > > > -	}
> > > > -	if (lp->tx_bd_v) {
> > > > -		dma_free_coherent(ndev->dev.parent,
> > > > -				  sizeof(*lp->tx_bd_v) * lp->tx_bd_num,
> > > > -				  lp->tx_bd_v,
> > > > -				  lp->tx_bd_p);
> > > > +		/* For each descriptor, we programmed cntrl with the (non-
> > > > zero)
> > > > +		 * descriptor size, after it had been successfully allocated.
> > > > +		 * So a non-zero value in there means we need to unmap it.
> > > > +		 */  
> > >  
> > > > +		if (lp->rx_bd_v[i].cntrl)  
> > >
> > > I think it should ok to unmap w/o any check?  
> > 
> > Do you mean because .phys would be 0 if not initialised? AFAIK 0 can be a
> > valid DMA address, so there is no special check for that, and unmapping
> > DMA address 0 will probably go wrong at some point. So it's unlike
> > kfree(NULL).  
> 
> I mean if skb allocation is successful in _dma_bd_init then in release path
> we can assume .phys is always a valid address and skip rx_bd_v[i].cntrl
> check.

I don't think we can assume this. If the skb allocation succeeded, but then the dma_map_single failed (which we check with dma_mapping_error()), we would end up with a valid skb, but an uninitialised phys DMA address in the registers. That's why I set .cntrl only after having checked the dma_map_single() result.

Or am I missing something?

Cheers,
Andre
 
> > > > +			dma_unmap_single(ndev->dev.parent, lp-  
> > > > >rx_bd_v[i].phys,  
> > > > +					 lp->max_frm_size,
> > > > DMA_FROM_DEVICE);
> > > >  	}
> > > > +
> > > > +	dma_free_coherent(ndev->dev.parent,
> > > > +			  sizeof(*lp->rx_bd_v) * lp->rx_bd_num,
> > > > +			  lp->rx_bd_v,
> > > > +			  lp->rx_bd_p);
> > > >  }
> > > >
> > > >  /**
> > > > @@ -207,7 +220,7 @@ static int axienet_dma_bd_init(struct net_device
> > > > *ndev)
> > > >  					 sizeof(*lp->tx_bd_v) * lp-  
> > > > >tx_bd_num,  
> > > >  					 &lp->tx_bd_p, GFP_KERNEL);
> > > >  	if (!lp->tx_bd_v)
> > > > -		goto out;
> > > > +		return -ENOMEM;
> > > >
> > > >  	lp->rx_bd_v = dma_alloc_coherent(ndev->dev.parent,
> > > >  					 sizeof(*lp->rx_bd_v) * lp-  
> > > > >rx_bd_num,  
> > > > --
> > > > 2.17.1  
> > >  
> 

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