diff --git a/drivers/iommu/arm-smmu-v3-context.c b/drivers/iommu/arm-smmu-v3-context.c
index 352cba3c1a62..0e12f6804e16 100644
--- a/drivers/iommu/arm-smmu-v3-context.c
+++ b/drivers/iommu/arm-smmu-v3-context.c
@@ -65,6 +65,8 @@
 #define ARM_SMMU_TCR2CD(tcr, fld)	FIELD_PREP(CTXDESC_CD_0_TCR_##fld, \
 					FIELD_GET(ARM64_TCR_##fld, tcr))
 
+#define ARM_SMMU_NO_PASID		(-1)
+
 struct arm_smmu_cd {
 	struct iommu_pasid_entry	entry;
 
@@ -72,8 +74,14 @@ struct arm_smmu_cd {
 	u64				tcr;
 	u64				mair;
 
+	int				pasid;
+
+	/* 'refs' tracks alloc/free */
 	refcount_t			refs;
+	/* 'users' tracks attach/detach, and is only used for sanity checking */
+	unsigned int			users;
 	struct mm_struct		*mm;
+	struct arm_smmu_cd_tables	*tbl;
 };
 
 #define pasid_entry_to_cd(entry) \
@@ -105,6 +113,7 @@ struct arm_smmu_cd_tables {
 #define pasid_ops_to_tables(ops) \
 	pasid_to_cd_tables(iommu_pasid_table_ops_to_table(ops))
 
+static DEFINE_SPINLOCK(contexts_lock);
 static DEFINE_SPINLOCK(asid_lock);
 static DEFINE_IDR(asid_idr);
 
@@ -191,8 +200,8 @@ static u64 arm_smmu_cpu_tcr_to_cd(u64 tcr)
 	return val;
 }
 
-static int arm_smmu_write_ctx_desc(struct arm_smmu_cd_tables *tbl, int ssid,
-				   struct arm_smmu_cd *cd)
+static int __arm_smmu_write_ctx_desc(struct arm_smmu_cd_tables *tbl, int ssid,
+				     struct arm_smmu_cd *cd)
 {
 	u64 val;
 	bool cd_live;
@@ -262,6 +271,18 @@ static int arm_smmu_write_ctx_desc(struct arm_smmu_cd_tables *tbl, int ssid,
 	return 0;
 }
 
+static int arm_smmu_write_ctx_desc(struct arm_smmu_cd_tables *tbl, int ssid,
+				   struct arm_smmu_cd *cd)
+{
+	int ret;
+
+	spin_lock(&contexts_lock);
+	ret = __arm_smmu_write_ctx_desc(tbl, ssid, cd);
+	spin_unlock(&contexts_lock);
+
+	return ret;
+}
+
 static bool arm_smmu_free_asid(struct arm_smmu_cd *cd)
 {
 	bool free;
@@ -301,15 +322,26 @@ static struct arm_smmu_cd *arm_smmu_alloc_cd(struct arm_smmu_cd_tables *tbl)
 	if (!cd)
 		return NULL;
 
-	cd->entry.release = arm_smmu_free_cd;
+	cd->pasid		= ARM_SMMU_NO_PASID;
+	cd->tbl			= tbl;
+	cd->entry.release	= arm_smmu_free_cd;
 	refcount_set(&cd->refs, 1);
 
 	return cd;
 }
 
+/*
+ * Try to reserve this ASID in the SMMU. If it is in use, try to steal it from
+ * the private entry. Careful here, we may be modifying the context tables of
+ * another SMMU!
+ */
 static struct arm_smmu_cd *arm_smmu_share_asid(u16 asid)
 {
+	int ret;
 	struct arm_smmu_cd *cd;
+	struct arm_smmu_cd_tables *tbl;
+	struct arm_smmu_context_cfg *cfg;
+	struct iommu_pasid_entry old_entry;
 
 	cd = idr_find(&asid_idr, asid);
 	if (!cd)
@@ -319,17 +351,47 @@ static struct arm_smmu_cd *arm_smmu_share_asid(u16 asid)
 		/*
 		 * It's pretty common to find a stale CD when doing unbind-bind,
 		 * given that the release happens after a RCU grace period.
-		 * Simply reuse it.
+		 * Simply reuse it, but check that it isn't active, because it's
+		 * going to be assigned a different PASID.
 		 */
+		if (WARN_ON(cd->users))
+			return ERR_PTR(-EINVAL);
+
 		refcount_inc(&cd->refs);
 		return cd;
 	}
 
+	tbl = cd->tbl;
+	cfg = &tbl->pasid.cfg.arm_smmu;
+
+	ret = idr_alloc_cyclic(&asid_idr, cd, 0, 1 << cfg->asid_bits,
+			       GFP_ATOMIC);
+	if (ret < 0)
+		return ERR_PTR(-ENOSPC);
+
+	/* Save the previous ASID */
+	old_entry = cd->entry;
+
 	/*
-	 * Ouch, ASID is already in use for a private cd.
-	 * TODO: seize it, for the common good.
+	 * Race with unmap; TLB invalidations will start targeting the new ASID,
+	 * which isn't assigned yet. We'll do an invalidate-all on the old ASID
+	 * later, so it doesn't matter.
 	 */
-	return ERR_PTR(-EEXIST);
+	cd->entry.tag = ret;
+
+	/*
+	 * Update ASID and invalidate CD in all associated masters. There will
+	 * be some overlap between use of both ASIDs, until we invalidate the
+	 * TLB.
+	 */
+	arm_smmu_write_ctx_desc(tbl, cd->pasid, cd);
+
+	/* Invalidate TLB entries previously associated with that context */
+	iommu_pasid_flush_tlbs(&tbl->pasid, cd->pasid, &old_entry);
+
+	idr_remove(&asid_idr, asid);
+
+	return NULL;
 }
 
 static struct iommu_pasid_entry *
@@ -476,6 +538,15 @@ static int arm_smmu_set_cd(struct iommu_pasid_table_ops *ops, int pasid,
 	if (WARN_ON(pasid > (1 << tbl->pasid.cfg.order)))
 		return -EINVAL;
 
+	if (WARN_ON(cd->pasid != ARM_SMMU_NO_PASID && cd->pasid != pasid))
+		return -EEXIST;
+
+	/*
+	 * There is a single cd structure for each address space, multiple
+	 * devices may use the same in different tables.
+	 */
+	cd->users++;
+	cd->pasid = pasid;
 	return arm_smmu_write_ctx_desc(tbl, pasid, cd);
 }
 
@@ -488,6 +559,11 @@ static void arm_smmu_clear_cd(struct iommu_pasid_table_ops *ops, int pasid,
 	if (WARN_ON(pasid > (1 << tbl->pasid.cfg.order)))
 		return;
 
+	WARN_ON(cd->pasid != pasid);
+
+	if (!(--cd->users))
+		cd->pasid = ARM_SMMU_NO_PASID;
+
 	arm_smmu_write_ctx_desc(tbl, pasid, NULL);
 
 	/*
