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SubjectRE: [PATCH 03/14] net: axienet: Fix DMA descriptor cleanup path
Date
> -----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 = 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.
>
> 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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