Subject: [PATCH V10 06/12] of: device: Fix overflow of coherent_dma_mask

From: [email-masked] (Frank Rowand)
Date: Thu, 6 Apr 2017 00:01:11 -0700

On 04/04/17 03:18, Sricharan R wrote:
> Size of the dma-range is calculated as coherent_dma_mask + 1
> and passed to arch_setup_dma_ops further. It overflows when
> the coherent_dma_mask is set for full 64 bits 0xFFFFFFFFFFFFFFFF,
> resulting in size getting passed as 0 wrongly. Fix this by
> passsing in max(mask, mask + 1). Note that in this case
> when the mask is set to full 64bits, we will be passing the mask
> itself to arch_setup_dma_ops instead of the size. The real fix
> for this should be to make arch_setup_dma_ops receive the
> mask and handle it, to be done in the future.
> 
> Signed-off-by: Sricharan R <[email-masked]>
> ---
>  drivers/of/device.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/of/device.c b/drivers/of/device.c
> index c17c19d..c2ae6bb 100644
> --- a/drivers/of/device.c
> +++ b/drivers/of/device.c
> @@ -107,7 +107,7 @@ void of_dma_configure(struct device *dev, struct device_node *np)
>  	ret = of_dma_get_range(np, &dma_addr, &paddr, &size);
>  	if (ret < 0) {
>  		dma_addr = offset = 0;
> -		size = dev->coherent_dma_mask + 1;
> +		size = max(dev->coherent_dma_mask, dev->coherent_dma_mask + 1);
>  	} else {
>  		offset = PFN_DOWN(paddr - dma_addr);
>  		dev_dbg(dev, "dma_pfn_offset(%#08lx)\n", offset);
> 

NACK.

Passing an invalid size to arch_setup_dma_ops() is only part of the problem.
size is also used in of_dma_configure() before calling arch_setup_dma_ops():

        dev->coherent_dma_mask = min(dev->coherent_dma_mask,
                                     DMA_BIT_MASK(ilog2(dma_addr + size)));
        *dev->dma_mask = min((*dev->dma_mask),
                             DMA_BIT_MASK(ilog2(dma_addr + size)));

which would be incorrect for size == 0xffffffffffffffffULL when
dma_addr != 0.  So the proposed fix really is not papering over
the base problem very well.

I agree that the proper solution involves passing a mask instead
of a size to arch_setup_dma_ops().

-Frank