linux-kernel - Re: [PATCH] iommu/dma: limit iova free size to unmmaped iova

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Message-ID: <7aaa8dcc-6a47-f256-431d-2a1b034b4076@arm.com>
Date:   Thu, 21 May 2020 21:16:30 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Prakash Gupta <guptap@...eaurora.org>, akpm@...ux-foundation.org,
        mhocko@...e.com, joro@...tes.org
Cc:     linux-mm@...ck.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/dma: limit iova free size to unmmaped iova

On 2020-05-21 12:30, Prakash Gupta wrote:
> Limit the iova size while freeing based on unmapped size. In absence of
> this even with unmap failure, invalid iova is pushed to iova rcache and
> subsequently can cause panic while rcache magazine is freed.

Can you elaborate on that panic?

> Signed-off-by: Prakash Gupta <guptap@...eaurora.org>
> 
> :100644 100644 4959f5df21bd 098f7d377e04 M	drivers/iommu/dma-iommu.c
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 4959f5df21bd..098f7d377e04 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -472,7 +472,8 @@ static void __iommu_dma_unmap(struct device *dev, dma_addr_t dma_addr,
>   
>   	if (!cookie->fq_domain)
>   		iommu_tlb_sync(domain, &iotlb_gather);
> -	iommu_dma_free_iova(cookie, dma_addr, size);
> +	if (unmapped)
> +		iommu_dma_free_iova(cookie, dma_addr, unmapped);

Frankly, if any part of the unmap fails then things have gone 
catastrophically wrong already, but either way this isn't right. The 
IOVA API doesn't support partial freeing - an IOVA *must* be freed with 
its original size, or not freed at all, otherwise it will corrupt the 
state of the rcaches and risk a cascade of further misbehaviour for 
future callers.

TBH my gut feeling here is that you're really just trying to treat a 
symptom of another bug elsewhere, namely some driver calling dma_unmap_* 
or dma_free_* with the wrong address or size in the first place.

Robin.

>   }
>   
>   static dma_addr_t __iommu_dma_map(struct device *dev, phys_addr_t phys,
>