[Patch 1/2] x86/vtd: Fix suspected data race condition in iommu_set_root_entry()

[Andrew Cooper <andrew.cooper3@citrix.com>]

Coverity ID: 1054967

Coverity spotted that iommu->root_maddr was optionally allocated within the
protection of the iommu->lock, but was referenced with the protection of the
iommu->register_lock, and freed without any lock.

Luckily, the code as-is is not vulnerable to the potential risks identified.

However, the alloc_pgtable_maddr() is far more appropriately done in
iommu_alloc(), removing a set of spinlock calls, and a possibility for the
iommu setup to fail later than iommu_alloc() with an -ENOMEM.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Keir Fraser <keir@xen.org>
CC: Jan Beulich <JBeulich@suse.com>
CC: Xiantao Zhang <xiantao.zhang@intel.com>
---
 xen/drivers/passthrough/vtd/iommu.c |   19 +++----------------
 1 file changed, 3 insertions(+), 16 deletions(-)

diff --git a/xen/drivers/passthrough/vtd/iommu.c b/xen/drivers/passthrough/vtd/iommu.c
index 12e0165..2dbe97a 100644
--- a/xen/drivers/passthrough/vtd/iommu.c
+++ b/xen/drivers/passthrough/vtd/iommu.c
@@ -696,25 +696,9 @@ static void iommu_free_pagetable(u64 pt_maddr, int level)
 
 static int iommu_set_root_entry(struct iommu *iommu)
 {
-    struct acpi_drhd_unit *drhd;
     u32 sts;
     unsigned long flags;
 
-    spin_lock(&iommu->lock);
-
-    if ( iommu->root_maddr == 0 )
-    {
-        drhd = iommu_to_drhd(iommu);
-        iommu->root_maddr = alloc_pgtable_maddr(drhd, 1);
-    }
-
-    if ( iommu->root_maddr == 0 )
-    {
-        spin_unlock(&iommu->lock);
-        return -ENOMEM;
-    }
-
-    spin_unlock(&iommu->lock);
     spin_lock_irqsave(&iommu->register_lock, flags);
     dmar_writeq(iommu->reg, DMAR_RTADDR_REG, iommu->root_maddr);
 
@@ -1144,6 +1128,9 @@ int __init iommu_alloc(struct acpi_drhd_unit *drhd)
     iommu->intel->drhd = drhd;
     drhd->iommu = iommu;
 
+    if ( !(iommu->root_maddr = alloc_pgtable_maddr(drhd, 1)) )
+        return -ENOMEM;
+
     iommu->reg = ioremap(drhd->address, PAGE_SIZE);
     if ( !iommu->reg )
         return -ENOMEM;
-- 
1.7.10.4