[PATCH v2 08/28] KVM: arm64: nv: Treat DBM as writable at stage-2

[Oliver Upton <oupton@kernel.org>]

When using direct permissions, the DBM bit has the effect of granting
the write permission. And, depending on the access, it could grant write
permission without a dirty state update.

Reviewed-by: Leonardo Bras <leo.bras@arm.com>
Signed-off-by: Oliver Upton <oupton@kernel.org>
---
 arch/arm64/include/asm/kvm_pgtable.h |  1 +
 arch/arm64/kvm/nested.c              | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index edfae3a6f362..23e93f14af7b 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -102,6 +102,7 @@ typedef u64 kvm_pte_t;
 #define KVM_PTE_LEAF_ATTR_HI_S1_PXN	BIT(53)
 
 #define KVM_PTE_LEAF_ATTR_HI_S2_XN	GENMASK(54, 53)
+#define KVM_PTE_LEAF_ATTR_HI_S2_DBM	BIT(51)
 
 #define KVM_PTE_LEAF_ATTR_HI_S1_GP	BIT(50)
 
diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
index 1d581d3bd557..e388cefad9e5 100644
--- a/arch/arm64/kvm/nested.c
+++ b/arch/arm64/kvm/nested.c
@@ -328,6 +328,17 @@ static void compute_s2_permissions(struct kvm_vcpu *vcpu, struct s2_walk_info *w
 		break;
 	}
 
+	/*
+	 * Descriptors with the DBM bit set while hardware dirty state are
+	 * considered writable, even though certain accesses (like AT instructions)
+	 * don't actually update the dirty state.
+	 *
+	 * Assume that walk_nestd_s2_pgd() made the necessary descriptor updates
+	 * for the access and just treat DBM as writable here.
+	 */
+	if (wi->hd && ws->desc & KVM_PTE_LEAF_ATTR_HI_S2_DBM)
+		s2ap |= BIT(1);
+
 	trans->readable = s2ap & BIT(0);
 	trans->writable = s2ap & BIT(1);
 
-- 
2.47.3