[Patch V2 2/2] iommu/arm-smmu: Fix race condition during iommu_group creation

[Ashish Mhetre]

From: Krishna Reddy <vdumpa at nvidia.com>

iommu_group is getting created more than once during asynchronous multiple
display heads(devices) probe on Tegra194 SoC. All the display heads share
same SID and are expected to be in same iommu_group.
As arm_smmu_device_group() is not protecting group creation across devices,
it is leading to multiple groups creation across devices with same SID and
subsequent IOMMU faults.
During race, the iommu_probe_device() call for two display devices is
ending up in arm_smmu_device_group() twice and hence two groups are getting
created. Ideally after group creation for first display device, same group
should be used by second display device.
This race is leading to context faults when one display device is accessing
IOVA from other display device which shouldn't be the case for devices
sharing same SID.
Fix this by protecting group creation with smmu->stream_map_mutex.

Signed-off-by: Krishna Reddy <vdumpa at nvidia.com>
---
Changes since V1:
- Update the commit message per Will's suggestion

 drivers/iommu/arm/arm-smmu/arm-smmu.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/drivers/iommu/arm/arm-smmu/arm-smmu.c
index 6f72c4d..21af179 100644
--- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
+++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
@@ -1458,6 +1458,7 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
 	struct iommu_group *group = NULL;
 	int i, idx;
 
+	mutex_lock(&smmu->stream_map_mutex);
 	for_each_cfg_sme(cfg, fwspec, i, idx) {
 		if (group && smmu->s2crs[idx].group &&
 		    group != smmu->s2crs[idx].group)
@@ -1466,8 +1467,10 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
 		group = smmu->s2crs[idx].group;
 	}
 
-	if (group)
+	if (group) {
+		mutex_unlock(&smmu->stream_map_mutex);
 		return iommu_group_ref_get(group);
+	}
 
 	if (dev_is_pci(dev))
 		group = pci_device_group(dev);
@@ -1481,6 +1484,7 @@ static struct iommu_group *arm_smmu_device_group(struct device *dev)
 		for_each_cfg_sme(cfg, fwspec, i, idx)
 			smmu->s2crs[idx].group = group;
 
+	mutex_unlock(&smmu->stream_map_mutex);
 	return group;
 }
 
-- 
2.7.4