[PATCH v3 26/32] KVM: arm64: Introduce PROT_NONE mappings for stage 2

[Quentin Perret]

Once we start unmapping portions of memory from the host stage 2 (such
as e.g. the hypervisor memory sections, or pages that belong to
protected guests), we will need a way to track page ownership. And
given that all mappings in the host stage 2 will be identity-mapped, we
can use the host stage 2 page-table itself as a simplistic rmap.

As a first step towards this, introduce a new protection attribute
in the stage 2 page table code, called KVM_PGTABLE_PROT_NONE, which
allows to annotate portions of the IPA space as inaccessible. For
simplicity, PROT_NONE mappings are created as invalid mappings with a
software bit set.

Signed-off-by: Quentin Perret <qperret at google.com>
---
 arch/arm64/include/asm/kvm_pgtable.h |  2 ++
 arch/arm64/kvm/hyp/pgtable.c         | 26 ++++++++++++++++++++++++--
 2 files changed, 26 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 9935dbae2cc1..c9f6ed76e0ad 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -80,6 +80,7 @@ struct kvm_pgtable {
  * @KVM_PGTABLE_PROT_W:		Write permission.
  * @KVM_PGTABLE_PROT_R:		Read permission.
  * @KVM_PGTABLE_PROT_DEVICE:	Device attributes.
+ * @KVM_PGTABLE_PROT_NONE:	No permission.
  */
 enum kvm_pgtable_prot {
 	KVM_PGTABLE_PROT_X			= BIT(0),
@@ -87,6 +88,7 @@ enum kvm_pgtable_prot {
 	KVM_PGTABLE_PROT_R			= BIT(2),
 
 	KVM_PGTABLE_PROT_DEVICE			= BIT(3),
+	KVM_PGTABLE_PROT_NONE			= BIT(4),
 };
 
 #define PAGE_HYP		(KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
index bdd6e3d4eeb6..8e7059fcfd40 100644
--- a/arch/arm64/kvm/hyp/pgtable.c
+++ b/arch/arm64/kvm/hyp/pgtable.c
@@ -48,6 +48,8 @@
 					 KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
 					 KVM_PTE_LEAF_ATTR_HI_S2_XN)
 
+#define KVM_PTE_LEAF_SW_BIT_PROT_NONE	BIT(55)
+
 struct kvm_pgtable_walk_data {
 	struct kvm_pgtable		*pgt;
 	struct kvm_pgtable_walker	*walker;
@@ -120,6 +122,16 @@ static bool kvm_pte_valid(kvm_pte_t pte)
 	return pte & KVM_PTE_VALID;
 }
 
+static bool kvm_pte_prot_none(kvm_pte_t pte)
+{
+	return pte & KVM_PTE_LEAF_SW_BIT_PROT_NONE;
+}
+
+static inline bool stage2_is_permanent_mapping(kvm_pte_t pte)
+{
+	return kvm_pte_prot_none(pte);
+}
+
 static bool kvm_pte_table(kvm_pte_t pte, u32 level)
 {
 	if (level == KVM_PGTABLE_MAX_LEVELS - 1)
@@ -182,7 +194,8 @@ static kvm_pte_t kvm_init_valid_leaf_pte(u64 pa, kvm_pte_t attr, u32 level)
 
 	pte |= attr & (KVM_PTE_LEAF_ATTR_LO | KVM_PTE_LEAF_ATTR_HI);
 	pte |= FIELD_PREP(KVM_PTE_TYPE, type);
-	pte |= KVM_PTE_VALID;
+	if (!kvm_pte_prot_none(pte))
+		pte |= KVM_PTE_VALID;
 
 	return pte;
 }
@@ -317,7 +330,7 @@ static int hyp_map_set_prot_attr(enum kvm_pgtable_prot prot,
 	u32 ap = (prot & KVM_PGTABLE_PROT_W) ? KVM_PTE_LEAF_ATTR_LO_S1_AP_RW :
 					       KVM_PTE_LEAF_ATTR_LO_S1_AP_RO;
 
-	if (!(prot & KVM_PGTABLE_PROT_R))
+	if (!(prot & KVM_PGTABLE_PROT_R) || (prot & KVM_PGTABLE_PROT_NONE))
 		return -EINVAL;
 
 	if (prot & KVM_PGTABLE_PROT_X) {
@@ -489,6 +502,13 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
 			    PAGE_S2_MEMATTR(NORMAL);
 	u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
 
+	if (prot & KVM_PGTABLE_PROT_NONE) {
+		if (prot != KVM_PGTABLE_PROT_NONE)
+			return -EINVAL;
+		attr |= KVM_PTE_LEAF_SW_BIT_PROT_NONE;
+		goto out;
+	}
+
 	if (!(prot & KVM_PGTABLE_PROT_X))
 		attr |= KVM_PTE_LEAF_ATTR_HI_S2_XN;
 	else if (device)
@@ -502,6 +522,8 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
 
 	attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
 	attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
+
+out:
 	data->attr = attr;
 	return 0;
 }
-- 
2.30.1.766.gb4fecdf3b7-goog