iommu/arm-smmu-v3-sva: Fix mm use-after-free

We currently call arm64_mm_context_put() without holding a reference to
the mm, which can result in use-after-free. Call mmgrab()/mmdrop() to
ensure the mm only gets freed after we unpinned the ASID.

Fixes: 32784a9 ("iommu/arm-smmu-v3: Implement iommu_sva_bind/unbind()")
Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Tested-by: Zhangfei Gao <zhangfei.gao@linaro.org>
Link: https://lore.kernel.org/r/20220426130444.300556-1-jean-philippe@linaro.org
Signed-off-by: Will Deacon <will@kernel.org>

Author: Jean-Philippe Brucker

drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-sva.c

@@ -6,6 +6,7 @@
 #include <linux/mm.h>
 #include <linux/mmu_context.h>
 #include <linux/mmu_notifier.h>
+#include <linux/sched/mm.h>
 #include <linux/slab.h>
 
 #include "arm-smmu-v3.h"
@@ -96,9 +97,14 @@ static struct arm_smmu_ctx_desc *arm_smmu_alloc_shared_cd(struct mm_struct *mm)
 	struct arm_smmu_ctx_desc *cd;
 	struct arm_smmu_ctx_desc *ret = NULL;
 
+	/* Don't free the mm until we release the ASID */
+	mmgrab(mm);
+
 	asid = arm64_mm_context_get(mm);
-	if (!asid)
-		return ERR_PTR(-ESRCH);
+	if (!asid) {
+		err = -ESRCH;
+		goto out_drop_mm;
+	}
 
 	cd = kzalloc(sizeof(*cd), GFP_KERNEL);
 	if (!cd) {
@@ -165,6 +171,8 @@ static struct arm_smmu_ctx_desc *arm_smmu_alloc_shared_cd(struct mm_struct *mm)
 	kfree(cd);
 out_put_context:
 	arm64_mm_context_put(mm);
+out_drop_mm:
+	mmdrop(mm);
 	return err < 0 ? ERR_PTR(err) : ret;
 }
 
@@ -173,6 +181,7 @@ static void arm_smmu_free_shared_cd(struct arm_smmu_ctx_desc *cd)
 	if (arm_smmu_free_asid(cd)) {
 		/* Unpin ASID */
 		arm64_mm_context_put(cd->mm);
+		mmdrop(cd->mm);
 		kfree(cd);
 	}
 }