Re: [PATCH] iommu/dma: Fix calculation overflow in __finalise_sg()

[Robin Murphy]

On 01/07/2019 13:21, Joerg Roedel wrote:
> On Fri, Jun 21, 2019 at 09:38:14PM -0700, Nicolin Chen wrote:
> > The max_len is a u32 type variable so the calculation on the
> > left hand of the last if-condition will potentially overflow
> > when a cur_len gets closer to UINT_MAX -- note that there're
> > drivers setting max_seg_size to UINT_MAX:
> >    drivers/dma/dw-edma/dw-edma-core.c:745:
> >      dma_set_max_seg_size(dma->dev, U32_MAX);
> >    drivers/dma/dma-axi-dmac.c:871:
> >      dma_set_max_seg_size(&pdev->dev, UINT_MAX);
> >    drivers/mmc/host/renesas_sdhi_internal_dmac.c:338:
> >      dma_set_max_seg_size(dev, 0xffffffff);
> >    drivers/nvme/host/pci.c:2520:
> >      dma_set_max_seg_size(dev->dev, 0xffffffff);
> >
> > So this patch just casts the cur_len in the calculation to a
> > size_t type to fix the overflow issue, as it's not necessary
> > to change the type of cur_len after all.
> >
> > Fixes: 809eac54cdd6 ("iommu/dma: Implement scatterlist segment merging")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Signed-off-by: Nicolin Chen <nicoleotsuka@xxxxxxxxx>
>
> Looks good to me, but I let Robin take a look too before I apply it,
> Robin?
I'll need to take a closer look at how exactly an overflow would happen 
here (just got back off some holiday), but my immediate thought is that 
if this is a real problem, then what about 32-bit builds where size_t 
would still overflow?

Robin.