Re: [PATCH] pxa2xx_spi: fix memory corruption

[Russell King - ARM Linux <linux@arm.linux.org.uk>]

On Sun, Jul 10, 2011 at 01:05:48AM +0200, Marek Vasut wrote:
> On Saturday, July 09, 2011 11:14:58 PM Vasily Khoruzhick wrote:
> > pxa2xx_spi_probe allocates struct driver_data and null_dma_buf
> > at same time via spi_alloc_master(), but then calculates
> > null_dma_buf pointer incorrectly, and it causes memory corruption
> > later if DMA usage is enabled.
> > 
> > Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
> > ---
> >  drivers/spi/pxa2xx_spi.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/spi/pxa2xx_spi.c b/drivers/spi/pxa2xx_spi.c
> > index dc25bee..ef38fbf 100644
> > --- a/drivers/spi/pxa2xx_spi.c
> > +++ b/drivers/spi/pxa2xx_spi.c
> > @@ -1569,7 +1569,7 @@ static int __devinit pxa2xx_spi_probe(struct
> > platform_device *pdev) master->transfer = transfer;
> > 
> >  	drv_data->ssp_type = ssp->type;
> > -	drv_data->null_dma_buf = (u32 *)ALIGN((u32)(drv_data +
> > +	drv_data->null_dma_buf = (u32 *)ALIGN(((u32)drv_data +
> >  						sizeof(struct driver_data)), 8);
> 
> This thing looks a bit disturbing in itself. Like, where the heck is that thing 
> pointing in the end ? Since some data are written to address in "null_dma_buf" 
> ... isn't this just changing the corruption impact ?

        /* Allocate master with space for drv_data and null dma buffer */
        master = spi_alloc_master(dev, sizeof(struct driver_data) + 16);

So there's 16 bytes at the end of driver_data.

However:

	(u32)(drv_data + sizeof(struct driver_data))

is pointer arithmetic.  drv_data points at an object of sizeof(struct
driver_data).  Adding one to this increments the pointer by
sizeof(struct driver_data) bytes.  So the above expression increments
the pointer by sizeof(struct driver_data)*sizeof(struct driver_data)
bytes, which is obviously complete rubbish.

	((u32)drv_data + sizeof(struct driver_data))

casts drv_data to a u32 first, then adds the sizeof(struct driver_data)
which moves us into the 16 bytes allocated off the end of the struct.