[PATCH v2 10/11] iommu/tegra241-cmdqv: Warn on a VCMDQ base above the 48-bit hardware limit

[Nicolin Chen <nicolinc@nvidia.com>]

tegra241_vcmdq_alloc_smmu_cmdq() allocates the VCMDQ buffer via the common
arm_smmu_init_one_queue(), which uses smmu->dev and its coherent DMA mask
of DMA_BIT_MASK(smmu->oas). The architectural Q_BASE_ADDR_MASK is 52 bits,
but the Tegra241 VCMDQ_BASE holds only 48 (VCMDQ_ADDR), so the masked write
"q_base = base_dma & VCMDQ_ADDR" silently drops bits 48 and up.

A real Tegra241 never reports a 52-bit OAS alongside the 48-bit VCMDQ, so
this cannot happen on correct hardware, but a buggy or hostile hypervisor
could still advertise such an OAS to a guest. The user-VCMDQ path already
rejects a base_addr_pa with bits set outside VCMDQ_ADDR; add the same guard
to the kernel-allocated path, failing the queue init rather than silently
truncating the base. Use dev_warn_once() rather than WARN_ON(): the OAS is
hypervisor-controlled, and a WARN_ON() would let it panic a guest that is
booted with panic_on_warn.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
index f93e59b7bd2db..d27ee215032f7 100644
--- a/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
+++ b/drivers/iommu/arm/arm-smmu-v3/tegra241-cmdqv.c
@@ -674,6 +674,19 @@ static int tegra241_vcmdq_alloc_smmu_cmdq(struct tegra241_vcmdq *vcmdq)
 	if (ret)
 		return ret;
 
+	/*
+	 * The q->base_dma is bounded by DMA_BIT_MASK of SMMU's IDR5.OAS. On a
+	 * real hardware, VCMDQ_ADDR mask and IDR5.OAS are always aligned. But
+	 * a buggy VM might set a mismatched IDR5.OAS for SMMU. Spit a warning
+	 * instead of a silent truncation.
+	 */
+	if (q->base_dma & ~VCMDQ_ADDR) {
+		dev_warn_once(
+			vcmdq->cmdqv->dev,
+			"VCMDQ base %pad exceeds the 48-bit VCMDQ_ADDR limit\n",
+			&q->base_dma);
+		return -EINVAL;
+	}
 	/* ...override q_base to write VCMDQ_BASE registers */
 	q->q_base = q->base_dma & VCMDQ_ADDR;
 	q->q_base |= FIELD_PREP(VCMDQ_LOG2SIZE, q->llq.max_n_shift);
-- 
2.43.0