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

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Date:   Thu, 5 Sep 2019 13:43:39 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Qian Cai <cai@....pw>
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()

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?

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,