linux-kernel - Re: [PATCH] iommu/dma: Fix calculation overflow in __finalise

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Message-ID: <91a389be-fd76-c87f-7613-8cc972b69685@arm.com>
Date:   Mon, 1 Jul 2019 13:39:55 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>,
        Nicolin Chen <nicoleotsuka@...il.com>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/dma: Fix calculation overflow in __finalise_sg()

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@...r.kernel.org
>> Signed-off-by: Nicolin Chen <nicoleotsuka@...il.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 (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.