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

[Nicolin Chen]

Hi Robin,

On Mon, Jul 01, 2019 at 01:39:55PM +0100, Robin Murphy 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

It was triggered by a test case that was trying to map a 4GB
dma_buf (1000+ nents in the scatterlist). This function then
seemed to reduce the nents by merging most of them, probably
because they were contiguous.

> (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?

I think most of callers are also using size_t type for their
size parameters like dma_buf, so the cur_len + s_length will
unlikely go higher than UINT_MAX. But just in case that some
driver allocates a large sg with a size parameter defined in
64-bit and uses this map() function, so it might be safer to
change to "size_t" here to "u64"?

Thank you
Nicolin