[PATCH v2 07/13] KVM: arm64: Use a maple tree to represent the SMCCC filter

Oliver Upton
2023-03-30T15:50:25Z

Maple tree is an efficient B-tree implementation that is intended for
storing non-overlapping intervals. Such a data structure is a good fit
for the SMCCC filter as it is desirable to sparsely allocate the 32 bit
function ID space.

To that end, add a maple tree to kvm_arch and correctly init/teardown
along with the VM. Wire in a test against the hypercall filter for HVCs
which does nothing until the controls are exposed to userspace.

Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
---
arch/arm64/include/asm/kvm_host.h | 5 ++-
arch/arm64/kvm/arm.c | 2 ++
arch/arm64/kvm/hypercalls.c | 57 +++++++++++++++++++++++++++++++
include/kvm/arm_hypercalls.h | 1 +
4 files changed, 64 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index d091d1c9890b..2682b3fd0881 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -16,6 +16,7 @@
#include <linux/types.h>
#include <linux/jump_label.h>
#include <linux/kvm_types.h>
+#include <linux/maple_tree.h>
#include <linux/percpu.h>
#include <linux/psci.h>
#include <asm/arch_gicv3.h>
@@ -221,7 +222,8 @@ struct kvm_arch {
#define KVM_ARCH_FLAG_EL1_32BIT 4
/* PSCI SYSTEM_SUSPEND enabled for the guest */
#define KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED 5
-
+ /* SMCCC filter initialized for the VM */
+#define KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED 6
unsigned long flags;

/*
@@ -242,6 +244,7 @@ struct kvm_arch {

/* Hypercall features firmware registers' descriptor */
struct kvm_smccc_features smccc_feat;
+ struct maple_tree smccc_filter;

/*
* For an untrusted host VM, 'pkvm.handle' is used to lookup
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index b6e26c0e65e5..1202ac03bee0 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -192,6 +192,8 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
kvm_destroy_vcpus(kvm);

kvm_unshare_hyp(kvm, kvm + 1);
+
+ kvm_arm_teardown_hypercalls(kvm);
}

int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
diff --git a/arch/arm64/kvm/hypercalls.c b/arch/arm64/kvm/hypercalls.c
index 194c65d9ca05..73b218ddd1a5 100644
--- a/arch/arm64/kvm/hypercalls.c
+++ b/arch/arm64/kvm/hypercalls.c
@@ -121,8 +121,58 @@ static bool kvm_smccc_test_fw_bmap(struct kvm_vcpu *vcpu, u32 func_id)
}
}

+#define SMCCC_ARCH_RANGE_BEGIN ARM_SMCCC_VERSION_FUNC_ID
+#define SMCCC_ARCH_RANGE_END \
+ ARM_SMCCC_CALL_VAL(ARM_SMCCC_FAST_CALL, \
+ ARM_SMCCC_SMC_32, \
+ 0, ARM_SMCCC_FUNC_MASK)
+
+static void init_smccc_filter(struct kvm *kvm)
+{
+ int r;
+
+ mt_init(&kvm->arch.smccc_filter);
+
+ /*
+ * Prevent userspace from handling any SMCCC calls in the architecture
+ * range, avoiding the risk of misrepresenting Spectre mitigation status
+ * to the guest.
+ */
+ r = mtree_insert_range(&kvm->arch.smccc_filter,
+ SMCCC_ARCH_RANGE_BEGIN, SMCCC_ARCH_RANGE_END,
+ xa_mk_value(KVM_SMCCC_FILTER_ALLOW),
+ GFP_KERNEL_ACCOUNT);
+ WARN_ON_ONCE(r);
+}
+
+static u8 kvm_smccc_filter_get_action(struct kvm *kvm, u32 func_id)
+{
+ unsigned long idx = func_id;
+ void *val;
+
+ if (!test_bit(KVM_ARCH_FLAG_SMCCC_FILTER_CONFIGURED, &kvm->arch.flags))
+ return KVM_SMCCC_FILTER_ALLOW;
+
+ /*
+ * But where's the error handling, you say?
+ *
+ * mt_find() returns NULL if no entry was found, which just so happens
+ * to match KVM_SMCCC_FILTER_ALLOW.
+ */
+ val = mt_find(&kvm->arch.smccc_filter, &idx, idx);
+ return xa_to_value(val);
+}
+
static u8 kvm_smccc_get_action(struct kvm_vcpu *vcpu, u32 func_id)
{
+ /*
+ * Intervening actions in the SMCCC filter take precedence over the
+ * pseudo-firmware register bitmaps.
+ */
+ u8 action = kvm_smccc_filter_get_action(vcpu->kvm, func_id);
+ if (action != KVM_SMCCC_FILTER_ALLOW)
+ return action;
+
if (kvm_smccc_test_fw_bmap(vcpu, func_id) ||
kvm_smccc_default_allowed(func_id))
return KVM_SMCCC_FILTER_ALLOW;
@@ -256,6 +306,13 @@ void kvm_arm_init_hypercalls(struct kvm *kvm)
smccc_feat->std_bmap = KVM_ARM_SMCCC_STD_FEATURES;
smccc_feat->std_hyp_bmap = KVM_ARM_SMCCC_STD_HYP_FEATURES;
smccc_feat->vendor_hyp_bmap = KVM_ARM_SMCCC_VENDOR_HYP_FEATURES;
+
+ init_smccc_filter(kvm);
+}
+
+void kvm_arm_teardown_hypercalls(struct kvm *kvm)
+{
+ mtree_destroy(&kvm->arch.smccc_filter);
}

int kvm_arm_get_fw_num_regs(struct kvm_vcpu *vcpu)
diff --git a/include/kvm/arm_hypercalls.h b/include/kvm/arm_hypercalls.h
index 8f4e33bc43e8..fe6c31575b05 100644
--- a/include/kvm/arm_hypercalls.h
+++ b/include/kvm/arm_hypercalls.h
@@ -43,6 +43,7 @@ static inline void smccc_set_retval(struct kvm_vcpu *vcpu,
struct kvm_one_reg;

void kvm_arm_init_hypercalls(struct kvm *kvm);
+void kvm_arm_teardown_hypercalls(struct kvm *kvm);
int kvm_arm_get_fw_num_regs(struct kvm_vcpu *vcpu);
int kvm_arm_copy_fw_reg_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
int kvm_arm_get_fw_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg);
--
2.40.0.348.gf938b09366-goog