[PATCH 6.15.y 3/5] KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN

[Sasha Levin <sashal@kernel.org>]

From: Marc Zyngier <maz@kernel.org>

[ Upstream commit efff9dd2fee7a5969b5b2a04995e638c3ba15826 ]

We don't really pay attention to what gets written to MDCR_EL2.HPMN,
and funky guests could play ugly games on us.

Restrict what gets written there, and limit the number of counters
to what the PMU is allowed to have.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Stable-dep-of: c6e35dff58d3 ("KVM: arm64: Check for SYSREGS_ON_CPU before accessing the CPU state")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/arm64/kvm/sys_regs.c | 29 +++++++++++++++++++++++------
 1 file changed, 23 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
index c1e900a66d35..2ef68b66b950 100644
--- a/arch/arm64/kvm/sys_regs.c
+++ b/arch/arm64/kvm/sys_regs.c
@@ -2576,16 +2576,33 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
 			struct sys_reg_params *p,
 			const struct sys_reg_desc *r)
 {
-	u64 old = __vcpu_sys_reg(vcpu, MDCR_EL2);
+	u64 hpmn, val, old = __vcpu_sys_reg(vcpu, MDCR_EL2);
 
-	if (!access_rw(vcpu, p, r))
-		return false;
+	if (!p->is_write) {
+		p->regval = old;
+		return true;
+	}
+
+	val = p->regval;
+	hpmn = FIELD_GET(MDCR_EL2_HPMN, val);
+
+	/*
+	 * If HPMN is out of bounds, limit it to what we actually
+	 * support. This matches the UNKNOWN definition of the field
+	 * in that case, and keeps the emulation simple. Sort of.
+	 */
+	if (hpmn > vcpu->kvm->arch.nr_pmu_counters) {
+		hpmn = vcpu->kvm->arch.nr_pmu_counters;
+		u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
+	}
+
+	__vcpu_sys_reg(vcpu, MDCR_EL2) = val;
 
 	/*
-	 * Request a reload of the PMU to enable/disable the counters affected
-	 * by HPME.
+	 * Request a reload of the PMU to enable/disable the counters
+	 * affected by HPME.
 	 */
-	if ((old ^ __vcpu_sys_reg(vcpu, MDCR_EL2)) & MDCR_EL2_HPME)
+	if ((old ^ val) & MDCR_EL2_HPME)
 		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
 
 	return true;
-- 
2.39.5