linux-kernel - Re: [RESEND][PATCH 1/7] powerpc/iommu: Avoid overflow at boundary

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Message-ID: <87lfht1vav.fsf@mpe.ellerman.id.au>
Date:   Tue, 01 Sep 2020 23:27:36 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Nicolin Chen <nicoleotsuka@...il.com>, benh@...nel.crashing.org,
        paulus@...ba.org, rth@...ddle.net, ink@...assic.park.msu.ru,
        mattst88@...il.com, tony.luck@...el.com, fenghua.yu@...el.com,
        schnelle@...ux.ibm.com, gerald.schaefer@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com, borntraeger@...ibm.com,
        davem@...emloft.net, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, x86@...nel.org, hpa@...or.com,
        James.Bottomley@...senPartnership.com, deller@....de
Cc:     sfr@...b.auug.org.au, hch@....de, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linux-ia64@...r.kernel.org, linux-s390@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [RESEND][PATCH 1/7] powerpc/iommu: Avoid overflow at boundary_size

Nicolin Chen <nicoleotsuka@...il.com> 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 <sfr@...b.auug.org.au>
> Signed-off-by: Nicolin Chen <nicoleotsuka@...il.com>
> Cc: Christoph Hellwig <hch@....de>
> ---
>  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