iommu/arm-smmu-v3: Ensure queue is read after updating prod pointer

Reading the 'prod' MMIO register in order to determine whether or not
there is valid data beyond 'cons' for a given queue does not provide
sufficient dependency ordering, as the resulting access is address
dependent only on 'cons' and can therefore be speculated ahead of time,
potentially allowing stale data to be read by the CPU.

Use readl() instead of readl_relaxed() when updating the shadow copy of
the 'prod' pointer, so that all speculated memory reads from the
corresponding queue can occur only from valid slots.

Signed-off-by: Zhou Wang <wangzhou1@hisilicon.com>
Link: https://lore.kernel.org/r/1601281922-117296-1-git-send-email-wangzhou1@hisilicon.com
[will: Use readl() instead of explicit barrier. Update 'cons' side to match.]
Signed-off-by: Will Deacon <will@kernel.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
index 3063aeb38063..1ffec08d345a 100644
--- a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
+++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
@@ -810,7 +810,7 @@ static void queue_sync_cons_out(struct arm_smmu_queue *q)
 	 * Ensure that all CPU accesses (reads and writes) to the queue
 	 * are complete before we update the cons pointer.
 	 */
-	mb();
+	__iomb();
 	writel_relaxed(q->llq.cons, q->cons_reg);
 }
 
@@ -822,8 +822,15 @@ static void queue_inc_cons(struct arm_smmu_ll_queue *q)
 
 static int queue_sync_prod_in(struct arm_smmu_queue *q)
 {
+	u32 prod;
 	int ret = 0;
-	u32 prod = readl_relaxed(q->prod_reg);
+
+	/*
+	 * We can't use the _relaxed() variant here, as we must prevent
+	 * speculative reads of the queue before we have determined that
+	 * prod has indeed moved.
+	 */
+	prod = readl(q->prod_reg);
 
 	if (Q_OVF(prod) != Q_OVF(q->llq.prod))
 		ret = -EOVERFLOW;