linux-kernel - Re: [PATCH v3 4/5] iommu/s390: Fix incorrect aperture check

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Date:   Thu, 29 Sep 2022 12:58:39 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>, iommu@...ts.linux.dev,
        linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, joro@...tes.org, will@...nel.org,
        robin.murphy@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/5] iommu/s390: Fix incorrect aperture check

On Thu, Sep 29, 2022 at 05:33:01PM +0200, Niklas Schnelle wrote:
> The domain->geometry.aperture_end specifies the last valid address treat
> it as such when checking if a DMA address is valid.
> 
> Reviewed-by: Pierre Morel <pmorel@...ux.ibm.com>
> Reviewed-by: Matthew Rosato <mjrosato@...ux.ibm.com>
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>  drivers/iommu/s390-iommu.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
> index ed0e64f478cf..6d4a9c7db32c 100644
> --- a/drivers/iommu/s390-iommu.c
> +++ b/drivers/iommu/s390-iommu.c
> @@ -210,7 +210,7 @@ static int s390_iommu_update_trans(struct s390_domain *s390_domain,
>  	int rc = 0;
>  
>  	if (dma_addr < s390_domain->domain.geometry.aperture_start ||
> -	    dma_addr + size > s390_domain->domain.geometry.aperture_end)
> +	    dma_addr + size > s390_domain->domain.geometry.aperture_end + 1)

The reason the iommu layer uses 'last' (= start + size - 1) not 'end'
is to allow for the very last byte of the range to be used.

Meaning (start + size) == 0 in some cases due to the overflow.

Generally when working with lasts's I prefer people write code in a
way that doesn't trigger the overflow, because there are some
complicated C rules about integer promotion that can mean the desired
overflow silently doesn't happen in obscure cases - especially if
unsigned long != u64

So, I'd write this as:

  (dma_addr + size - 1) > s390_domain->domain.geometry.aperture_end

Jason