[RFC PATCH] iommu/sva: Fix PASID use-after-free issue

[Fenghua Yu]

A PASID might be still used even though it is freed on mm exit.

process A:
	sva_bind();
	ioasid_alloc() = N; // Get PASID N for the mm
	fork(): // spawn process B
	exit();
	ioasid_free(N);

process B:
	device uses PASID N -> failure
	sva_unbind();

Dave Hansen suggests to take a refcount on the mm whenever binding the
PASID to a device and drop the refcount on unbinding. The mm won't be
dropped if the PASID is still bound to it.

Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit")

Reported-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx>
Suggested-by: Dave Hansen" <dave.hansen@xxxxxxxxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---
 drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c | 6 ++++++
 drivers/iommu/intel/svm.c                       | 4 ++++
 2 files changed, 10 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
index 22ddd05bbdcd..3fcb842a0df0 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c
@@ -7,6 +7,7 @@
 #include <linux/mmu_context.h>
 #include <linux/mmu_notifier.h>
 #include <linux/slab.h>
+#include <linux/sched/mm.h>
 
 #include "arm-smmu-v3.h"
 #include "../../iommu-sva-lib.h"
@@ -363,6 +364,9 @@ arm_smmu_sva_bind(struct device *dev, struct mm_struct *mm, void *drvdata)
 
 	mutex_lock(&sva_lock);
 	handle = __arm_smmu_sva_bind(dev, mm);
+	/* Take an mm refcount on a successful bind. */
+	if (!IS_ERR(handle))
+		mmget(mm);
 	mutex_unlock(&sva_lock);
 	return handle;
 }
@@ -372,6 +376,8 @@ void arm_smmu_sva_unbind(struct iommu_sva *handle)
 	struct arm_smmu_bond *bond = sva_to_bond(handle);
 
 	mutex_lock(&sva_lock);
+	/* Drop an mm refcount. */
+	mmput(bond->mm);
 	if (refcount_dec_and_test(&bond->refs)) {
 		list_del(&bond->list);
 		arm_smmu_mmu_notifier_put(bond->smmu_mn);
diff --git a/drivers/iommu/intel/svm.c b/drivers/iommu/intel/svm.c
index 23a38763c1d1..345a0d5d7922 100644
--- a/drivers/iommu/intel/svm.c
+++ b/drivers/iommu/intel/svm.c
@@ -403,6 +403,8 @@ static struct iommu_sva *intel_svm_bind_mm(struct intel_iommu *iommu,
 		goto free_sdev;
 
 	list_add_rcu(&sdev->list, &svm->devs);
+	/* Take an mm refcount on binding mm. */
+	mmget(mm);
 success:
 	return &sdev->sva;
 
@@ -465,6 +467,8 @@ static int intel_svm_unbind_mm(struct device *dev, u32 pasid)
 				kfree(svm);
 			}
 		}
+		/* Drop an mm reference on unbinding mm. */
+		mmput(mm);
 	}
 out:
 	return ret;
-- 
2.32.0