[PATCH v5 12/26] KVM: arm64: Support runtime sysreg visibility filtering

[Dave Martin <Dave.Martin@arm.com>]

Some optional features of the Arm architecture add new system
registers that are not present in the base architecture.

Where these features are optional for the guest, the visibility of
these registers may need to depend on some runtime configuration,
such as a flag passed to KVM_ARM_VCPU_INIT.

For example, ZCR_EL1 and ID_AA64ZFR0_EL1 need to be hidden if SVE
is not enabled for the guest, even though these registers may be
present in the hardware and visible to the host at EL2.

Adding special-case checks all over the place for individual
registers is going to get messy as the number of conditionally-
visible registers grows.

In order to help solve this problem, this patch adds a new sysreg
method restrictions() that can be used to hook in any needed
runtime visibility checks.  This method can currently return
REG_NO_USER to inhibit enumeration and ioctl access to the register
for userspace, and REG_NO_GUEST to inhibit runtime access by the
guest using MSR/MRS.

This allows a conditionally modified view of individual system
registers such as the CPU ID registers, in addition to completely
hiding register where appropriate.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>

---

Changes since v4:

 * Move from a boolean sysreg property that just suppresses register
   enumeration via KVM_GET_REG_LIST, to a multi-flag property that
   allows independent runtime control of MRS/MSR and user ioctl access.

   This allows registers to be either hidden completely, or to have
   hybrid behaviours (such as the not-enumerated, RAZ, WAZ behaviour of
   "non-present" CPU ID regs).
---
 arch/arm64/kvm/sys_regs.c | 24 +++++++++++++++++++++---
 arch/arm64/kvm/sys_regs.h | 13 +++++++++++++
 2 files changed, 34 insertions(+), 3 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index 71c5825..3f1243e 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -1863,6 +1863,12 @@ static void perform_access(struct kvm_vcpu *vcpu,
 {
 	trace_kvm_sys_access(*vcpu_pc(vcpu), params, r);
 
+	/* Check for regs disabled by runtime config */
+	if (restrictions(vcpu, r) & REG_NO_GUEST) {
+		kvm_inject_undefined(vcpu);
+		return;
+	}
+
 	/*
 	 * Not having an accessor means that we have configured a trap
 	 * that we don't know how to handle. This certainly qualifies
@@ -2370,6 +2376,10 @@ int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
 	if (!r)
 		return get_invariant_sys_reg(reg->id, uaddr);
 
+	/* Check for regs disabled by runtime config */
+	if (restrictions(vcpu, r) & REG_NO_USER)
+		return -ENOENT;
+
 	if (r->get_user)
 		return (r->get_user)(vcpu, r, reg, uaddr);
 
@@ -2391,6 +2401,10 @@ int kvm_arm_sys_reg_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
 	if (!r)
 		return set_invariant_sys_reg(reg->id, uaddr);
 
+	/* Check for regs disabled by runtime config */
+	if (restrictions(vcpu, r) & REG_NO_USER)
+		return -ENOENT;
+
 	if (r->set_user)
 		return (r->set_user)(vcpu, r, reg, uaddr);
 
@@ -2447,7 +2461,8 @@ static bool copy_reg_to_user(const struct sys_reg_desc *reg, u64 __user **uind)
 	return true;
 }
 
-static int walk_one_sys_reg(const struct sys_reg_desc *rd,
+static int walk_one_sys_reg(const struct kvm_vcpu *vcpu,
+			    const struct sys_reg_desc *rd,
 			    u64 __user **uind,
 			    unsigned int *total)
 {
@@ -2458,6 +2473,9 @@ static int walk_one_sys_reg(const struct sys_reg_desc *rd,
 	if (!(rd->reg || rd->get_user))
 		return 0;
 
+	if (restrictions(vcpu, rd) & REG_NO_USER)
+		return 0;
+
 	if (!copy_reg_to_user(rd, uind))
 		return -EFAULT;
 
@@ -2486,9 +2504,9 @@ static int walk_sys_regs(struct kvm_vcpu *vcpu, u64 __user *uind)
 		int cmp = cmp_sys_reg(i1, i2);
 		/* target-specific overrides generic entry. */
 		if (cmp <= 0)
-			err = walk_one_sys_reg(i1, &uind, &total);
+			err = walk_one_sys_reg(vcpu, i1, &uind, &total);
 		else
-			err = walk_one_sys_reg(i2, &uind, &total);
+			err = walk_one_sys_reg(vcpu, i2, &uind, &total);
 
 		if (err)
 			return err;
diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
index 174ffc0..12f196f 100644
--- a/arch/arm64/kvm/sys_regs.h
+++ b/arch/arm64/kvm/sys_regs.h
@@ -69,8 +69,15 @@ struct sys_reg_desc {
 			const struct kvm_one_reg *reg, void __user *uaddr);
 	int (*set_user)(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
 			const struct kvm_one_reg *reg, void __user *uaddr);
+
+	/* Return mask of REG_* runtime restriction flags */
+	unsigned int (*restrictions)(const struct kvm_vcpu *vcpu,
+				     const struct sys_reg_desc *rd);
 };
 
+#define REG_NO_USER	(1 << 0) /* hidden from userspace ioctl interface */
+#define REG_NO_GUEST	(1 << 1) /* hidden from guest */
+
 static inline void print_sys_reg_instr(const struct sys_reg_params *p)
 {
 	/* Look, we even formatted it for you to paste into the table! */
@@ -109,6 +116,12 @@ static inline void reset_val(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r
 	__vcpu_sys_reg(vcpu, r->reg) = r->val;
 }
 
+static inline unsigned int restrictions(const struct kvm_vcpu *vcpu,
+					const struct sys_reg_desc *r)
+{
+	return unlikely(r->restrictions) ? r->restrictions(vcpu, r) : 0;
+}
+
 static inline int cmp_sys_reg(const struct sys_reg_desc *i1,
 			      const struct sys_reg_desc *i2)
 {
-- 
2.1.4


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