Messages in this thread | | | Date | Mon, 1 Jul 2019 14:50:17 -0700 | From | Nicolin Chen <> | Subject | Re: [PATCH] iommu/dma: Fix calculation overflow in __finalise_sg() |
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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@vger.kernel.org > > > Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com> > > > > 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
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