linux-kernel - Re: [RFC PATCH] iommu/amd: fix a race in increase_address

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Date:   Thu, 05 Sep 2019 16:57:21 -0400
From:   Qian Cai <cai@....pw>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     hch@....de, iommu@...ts.linux-foundation.org,
        don.brace@...rosemi.com, esc.storagedev@...rosemi.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] iommu/amd: fix a race in increase_address_space()

On Thu, 2019-09-05 at 13:43 +0200, Joerg Roedel wrote:
> Hi Qian,
> 
> On Wed, Sep 04, 2019 at 05:24:22PM -0400, Qian Cai wrote:
> > 	if (domain->mode == PAGE_MODE_6_LEVEL)
> > 		/* address space already 64 bit large */
> > 		return false;
> > 
> > This gives a clue that there must be a race between multiple concurrent
> > threads in increase_address_space().
> 
> Thanks for tracking this down, there is a race indeed.
> 
> > +	mutex_lock(&domain->api_lock);
> >  	*dma_addr = __map_single(dev, dma_dom, page_to_phys(page),
> >  				 size, DMA_BIDIRECTIONAL, dma_mask);
> > +	mutex_unlock(&domain->api_lock);
> >  
> >  	if (*dma_addr == DMA_MAPPING_ERROR)
> >  		goto out_free;
> > @@ -2696,7 +2698,9 @@ static void free_coherent(struct device *dev, size_t size,
> >  
> >  	dma_dom = to_dma_ops_domain(domain);
> >  
> > +	mutex_lock(&domain->api_lock);
> >  	__unmap_single(dma_dom, dma_addr, size, DMA_BIDIRECTIONAL);
> > +	mutex_unlock(&domain->api_lock);
> 
> But I think the right fix is to lock the operation in
> increase_address_space() directly, and not the calls around it, like in
> the diff below. It is untested, so can you please try it and report back
> if it fixes your issue?

Yes, it works great so far.

> 
> diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
> index b607a92791d3..1ff705f16239 100644
> --- a/drivers/iommu/amd_iommu.c
> +++ b/drivers/iommu/amd_iommu.c
> @@ -1424,18 +1424,21 @@ static void free_pagetable(struct protection_domain *domain)
>   * another level increases the size of the address space by 9 bits to a size up
>   * to 64 bits.
>   */
> -static bool increase_address_space(struct protection_domain *domain,
> +static void increase_address_space(struct protection_domain *domain,
>  				   gfp_t gfp)
>  {
> +	unsigned long flags;
>  	u64 *pte;
>  
> -	if (domain->mode == PAGE_MODE_6_LEVEL)
> +	spin_lock_irqsave(&domain->lock, flags);
> +
> +	if (WARN_ON_ONCE(domain->mode == PAGE_MODE_6_LEVEL))
>  		/* address space already 64 bit large */
> -		return false;
> +		goto out;
>  
>  	pte = (void *)get_zeroed_page(gfp);
>  	if (!pte)
> -		return false;
> +		goto out;
>  
>  	*pte             = PM_LEVEL_PDE(domain->mode,
>  					iommu_virt_to_phys(domain->pt_root));
> @@ -1443,7 +1446,10 @@ static bool increase_address_space(struct protection_domain *domain,
>  	domain->mode    += 1;
>  	domain->updated  = true;
>  
> -	return true;
> +out:
> +	spin_unlock_irqrestore(&domain->lock, flags);
> +
> +	return;
>  }
>  
>  static u64 *alloc_pte(struct protection_domain *domain,