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SubjectRe: [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,
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