[for-4.15][PATCH v2 4/5] xen/iommu: x86: Don't leak the IOMMU page-tables

[Julien Grall <julien@xen.org>]

From: Julien Grall <jgrall@amazon.com>

The new IOMMU page-tables allocator will release the pages when
relinquish the domain resources. However, this is not sufficient when
the domain is dying because nothing prevents page-table to be allocated.

iommu_alloc_pgtable() is now checking if the domain is dying before
adding the page in the list. We are relying on &hd->arch.pgtables.lock
to synchronize d->is_dying.

Take the opportunity to check in arch_iommu_domain_destroy() that all
that page tables have been freed.

Signed-off-by: Julien Grall <jgrall@amazon.com>

---

There is one more bug that will be solved in the next patch as I felt
they each needed a long explanation.

Changes in v2:
    - Rework the approach
    - Move the patch earlier in the series
---
 xen/drivers/passthrough/x86/iommu.c | 36 ++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/xen/drivers/passthrough/x86/iommu.c b/xen/drivers/passthrough/x86/iommu.c
index cea1032b3d02..82d770107a47 100644
--- a/xen/drivers/passthrough/x86/iommu.c
+++ b/xen/drivers/passthrough/x86/iommu.c
@@ -149,6 +149,13 @@ int arch_iommu_domain_init(struct domain *d)
 
 void arch_iommu_domain_destroy(struct domain *d)
 {
+    /*
+     * There should be not page-tables left allocated by the time the
+     * domain is destroyed. Note that arch_iommu_domain_destroy() is
+     * called unconditionally, so pgtables may be unitialized.
+     */
+    ASSERT(dom_iommu(d)->platform_ops == NULL ||
+           page_list_empty(&dom_iommu(d)->arch.pgtables.list));
 }
 
 static bool __hwdom_init hwdom_iommu_map(const struct domain *d,
@@ -267,6 +274,12 @@ int iommu_free_pgtables(struct domain *d)
     struct page_info *pg;
     unsigned int done = 0;
 
+    if ( !is_iommu_enabled(d) )
+        return 0;
+
+    /* After this barrier no new page allocations can occur. */
+    spin_barrier(&hd->arch.pgtables.lock);
+
     while ( (pg = page_list_remove_head(&hd->arch.pgtables.list)) )
     {
         free_domheap_page(pg);
@@ -284,6 +297,7 @@ struct page_info *iommu_alloc_pgtable(struct domain *d)
     unsigned int memflags = 0;
     struct page_info *pg;
     void *p;
+    bool alive = false;
 
 #ifdef CONFIG_NUMA
     if ( hd->node != NUMA_NO_NODE )
@@ -303,9 +317,29 @@ struct page_info *iommu_alloc_pgtable(struct domain *d)
     unmap_domain_page(p);
 
     spin_lock(&hd->arch.pgtables.lock);
-    page_list_add(pg, &hd->arch.pgtables.list);
+    /*
+     * The IOMMU page-tables are freed when relinquishing the domain, but
+     * nothing prevent allocation to happen afterwards. There is no valid
+     * reasons to continue to update the IOMMU page-tables while the
+     * domain is dying.
+     *
+     * So prevent page-table allocation when the domain is dying.
+     *
+     * We relying on &hd->arch.pgtables.lock to synchronize d->is_dying.
+     */
+    if ( likely(!d->is_dying) )
+    {
+        alive = true;
+        page_list_add(pg, &hd->arch.pgtables.list);
+    }
     spin_unlock(&hd->arch.pgtables.lock);
 
+    if ( unlikely(!alive) )
+    {
+        free_domheap_page(pg);
+        pg = NULL;
+    }
+
     return pg;
 }
 
-- 
2.17.1