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From:   Radhey Shyam Pandey <radheys@xilinx.com>
To:     Andre Przywara <andre.przywara@arm.com>
CC:     "David S . Miller" <davem@davemloft.net>,
        Michal Simek <michals@xilinx.com>,
        Robert Hancock <hancock@sedsystems.ca>,
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        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
Thread-Topic: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
Thread-Index: AQHVx6y2ShwELFcgQk2RmkO2ceD3dafj/VgAgAAMtgCAABRgIIAJgLgAgAZRIvA=
Date:   Mon, 20 Jan 2020 18:32:09 +0000
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 -----Original Message-----
> From: Andre Przywara <andre.przywara@arm.com>
> Sent: Thursday, January 16, 2020 11:33 PM
> To: Radhey Shyam Pandey <radheys@xilinx.com>
> Cc: David S . Miller <davem@davemloft.net>; Michal Simek
> <michals@xilinx.com>; Robert Hancock <hancock@sedsystems.ca>;
> netdev@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
> kernel@vger.kernel.org
> Subject: Re: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
>=20
> On Fri, 10 Jan 2020 17:05:45 +0000
> Radhey Shyam Pandey <radheys@xilinx.com> wrote:
>=20
> Hi,
>=20
> > > -----Original Message-----
> > > From: Andre Przywara <andre.przywara@arm.com>
> > > Sent: Friday, January 10, 2020 9:13 PM
> > > To: Radhey Shyam Pandey <radheys@xilinx.com>
> > > Cc: David S . Miller <davem@davemloft.net>; Michal Simek
> > > <michals@xilinx.com>; Robert Hancock <hancock@sedsystems.ca>;
> > > netdev@vger.kernel.org; linux-arm-kernel@lists.infradead.org; linux-
> > > kernel@vger.kernel.org
> > > Subject: Re: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup
> > > path
> > >
> > > On Fri, 10 Jan 2020 15:14:46 +0000
> > > Radhey Shyam Pandey <radheys@xilinx.com> wrote:
> > >
> > > Hi Radhey,
> > >
> > > thanks for having a look!
> > >
> > > > > -----Original Message-----
> > > > > From: Andre Przywara <andre.przywara@arm.com>
> > > > > Sent: Friday, January 10, 2020 5:24 PM
> > > > > To: David S . Miller <davem@davemloft.net>; Radhey Shyam Pandey
> > > > > <radheys@xilinx.com>
> > > > > Cc: Michal Simek <michals@xilinx.com>; Robert Hancock
> > > > > <hancock@sedsystems.ca>; netdev@vger.kernel.org; linux-arm-
> > > > > kernel@lists.infradead.org; linux-kernel@vger.kernel.org
> > > > > 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 <andre.przywara@arm.com>
> > > > > ---
> > > > >  .../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 =3D 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 =3D 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.
>=20
> I don't think we can assume this. If the skb allocation succeeded, but th=
en 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 reg=
isters.
> That's why I set .cntrl only after having checked the dma_map_single() re=
sult.
>=20
> Or am I missing something?

Got it. Looks fine then.
>=20
> Cheers,
> Andre
>=20
> > > > > +			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 =3D dma_alloc_coherent(ndev->dev.parent,
> > > > >  					 sizeof(*lp->rx_bd_v) * lp-
> > > > > >rx_bd_num,
> > > > > --
> > > > > 2.17.1
> > > >
> >