Re: [PATCH 3.2 108/221] spi: dw-mid: avoid potential NULL dereference

Ben Hutchings
2015-05-06T15:45:26Z

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On Wed, 2015-05-06 at 11:25 +0100, Luis Henriques wrote:
> On Tue, May 05, 2015 at 02:16:39AM +0100, Ben Hutchings wrote:
> > 3.2.69-rc1 review patch. If anyone has any objections, please let me know.
> >
> > ------------------
> >
> > From: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> >
> > commit c9dafb27c84412fe4b17c3b94cc4ffeef5df1833 upstream.
> >
> > When DMA descriptor allocation fails we should not try to assign any fields in
> > the bad descriptor. The patch adds the necessary checks for that.
> >
> > Fixes: 7063c0d942a1 (spi/dw_spi: add DMA support)
> > Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Signed-off-by: Mark Brown <broonie@kernel.org>
> > [bwh: Backported to 3.2: adjust context]
> > Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
> > ---
> > drivers/spi/spi-dw-mid.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > --- a/drivers/spi/spi-dw-mid.c
> > +++ b/drivers/spi/spi-dw-mid.c
> > @@ -155,6 +155,9 @@ static int mid_spi_dma_transfer(struct d
> > 1,
> > DMA_TO_DEVICE,
> > DMA_PREP_INTERRUPT | DMA_COMPL_SKIP_DEST_UNMAP);
> > + if (!txdesc)
> > + return NULL;
> > +
>
> This backport doesn't look right to me as the function returns an int,
> not a pointer. OTOH, it seems like this return value is not checked
> anywhere, which is why I dropped this patch in the 3.16 kernel.

In mainline the DMA descriptor setup is split out into the
dw_spi_dma_prepare_tx() and dw_spi_dma_prepare_rx() functions. If they
return NULL then mid_spi_dma_transfer() doesn't attempt to submit
adescriptor to the respective channel, but it still returns 0.

So not only is this backport pretty useless, but so is the upstream
'fix'.

Ben.

> Cheers,
> --
> Luís
>
> > txdesc->callback = dw_spi_dma_done;
> > txdesc->callback_param = dws;
> >
> > @@ -177,6 +180,9 @@ static int mid_spi_dma_transfer(struct d
> > 1,
> > DMA_FROM_DEVICE,
> > DMA_PREP_INTERRUPT | DMA_COMPL_SKIP_DEST_UNMAP);
> > + if (!rxdesc)
> > + return NULL;
> > +
> > rxdesc->callback = dw_spi_dma_done;
> > rxdesc->callback_param = dws;
> >
> >
> > --
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--
Ben Hutchings
If at first you don't succeed, you're doing about average.

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