Subject: Re: [RESEND][PATCH 1/7] powerpc/iommu: Avoid overflow at boundary_size

From: Michael Ellerman <[email-masked]>
Date: Tue, 01 Sep 2020 23:27:36 +1000

Nicolin Chen <[email-masked]> writes:
> The boundary_size might be as large as ULONG_MAX, which means
> that a device has no specific boundary limit. So either "+ 1"
> or passing it to ALIGN() would potentially overflow.
>
> According to kernel defines:
>     #define ALIGN_MASK(x, mask) (((x) + (mask)) & ~(mask))
>     #define ALIGN(x, a)	ALIGN_MASK(x, (typeof(x))(a) - 1)
>
> We can simplify the logic here:
>   ALIGN(boundary + 1, 1 << shift) >> shift
> = ALIGN_MASK(b + 1, (1 << s) - 1) >> s
> = {[b + 1 + (1 << s) - 1] & ~[(1 << s) - 1]} >> s
> = [b + 1 + (1 << s) - 1] >> s
> = [b + (1 << s)] >> s
> = (b >> s) + 1
>
> So fixing a potential overflow with the safer shortcut.
>
> Reported-by: Stephen Rothwell <[email-masked]>
> Signed-off-by: Nicolin Chen <[email-masked]>
> Cc: Christoph Hellwig <[email-masked]>
> ---
>  arch/powerpc/kernel/iommu.c | 11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)

Are you asking for acks, or for maintainers to merge the patches
individually?

> diff --git a/arch/powerpc/kernel/iommu.c b/arch/powerpc/kernel/iommu.c
> index 9704f3f76e63..c01ccbf8afdd 100644
> --- a/arch/powerpc/kernel/iommu.c
> +++ b/arch/powerpc/kernel/iommu.c
> @@ -236,15 +236,14 @@ static unsigned long iommu_range_alloc(struct device *dev,
>  		}
>  	}
>  
> -	if (dev)
> -		boundary_size = ALIGN(dma_get_seg_boundary(dev) + 1,
> -				      1 << tbl->it_page_shift);
> -	else
> -		boundary_size = ALIGN(1UL << 32, 1 << tbl->it_page_shift);
>  	/* 4GB boundary for iseries_hv_alloc and iseries_hv_map */
> +	boundary_size = dev ? dma_get_seg_boundary(dev) : U32_MAX;

Is there any path that passes a NULL dev anymore?

Both iseries_hv_alloc() and iseries_hv_map() were removed years ago.
See:
  8ee3e0d69623 ("powerpc: Remove the main legacy iSerie platform code")

So maybe we should do a lead-up patch that drops the NULL dev support,
which will then make this patch simpler.

cheers

> +	/* Overflow-free shortcut for: ALIGN(b + 1, 1 << s) >> s */
> +	boundary_size = (boundary_size >> tbl->it_page_shift) + 1;
>  
>  	n = iommu_area_alloc(tbl->it_map, limit, start, npages, tbl->it_offset,
> -			     boundary_size >> tbl->it_page_shift, align_mask);
> +			     boundary_size, align_mask);
>  	if (n == -1) {
>  		if (likely(pass == 0)) {
>  			/* First try the pool from the start */
> -- 
> 2.17.1