[PATCH v9 20/22] RISC-V: KVM: Fix race-condition in kvm_riscv_vcpu_sync_interrupts()

[Anup Patel]

Currently, we sync-up Guest VSIP and VSIE CSRs with HW state upon
VM-exit. This helps us track enable/disable state of interrupts
and VSIP.SSIP bit updates by Guest.

Unfortunately, the implementation of kvm_riscv_vcpu_sync_interrupts()
is racey when running SMP Guest on SMP Host because it can happen
that IRQ_S_SOFT is already queued from other Host CPU and we might
accidentally clear a valid pending IRQ_S_SOFT.

To fix this, we use test_and_set_bit() to update irqs_pending_mask
in kvm_riscv_vcpu_sync_interrupts() instead of directly calling
kvm_riscv_vcpu_set/unset_interrupt() functions.

Signed-off-by: Anup Patel <anup.patel@xxxxxxx>
---
 arch/riscv/kvm/vcpu.c | 23 +++++++++++++++--------
 1 file changed, 15 insertions(+), 8 deletions(-)

diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
index f1a218d3a8cf..844542dd13e4 100644
--- a/arch/riscv/kvm/vcpu.c
+++ b/arch/riscv/kvm/vcpu.c
@@ -662,15 +662,22 @@ void kvm_riscv_vcpu_flush_interrupts(struct kvm_vcpu *vcpu)
 
 void kvm_riscv_vcpu_sync_interrupts(struct kvm_vcpu *vcpu)
 {
-	vcpu->arch.guest_csr.vsip = csr_read(CSR_VSIP);
-	vcpu->arch.guest_csr.vsie = csr_read(CSR_VSIE);
+	unsigned long vsip;
+	struct kvm_vcpu_arch *v = &vcpu->arch;
+	struct kvm_vcpu_csr *csr = &vcpu->arch.guest_csr;
 
-	/* Guest can directly update VSIP software interrupt bits */
-	if (vcpu->arch.guest_csr.vsip ^ READ_ONCE(vcpu->arch.irqs_pending)) {
-		if (vcpu->arch.guest_csr.vsip & (1UL << IRQ_S_SOFT))
-			kvm_riscv_vcpu_set_interrupt(vcpu, IRQ_S_SOFT);
-		else
-			kvm_riscv_vcpu_unset_interrupt(vcpu, IRQ_S_SOFT);
+	/* Read current VSIP and VSIE CSRs */
+	vsip = csr_read(CSR_VSIP);
+	csr->vsie = csr_read(CSR_VSIE);
+
+	/* Sync-up VSIP.SSIP bit changes does by Guest */
+	if ((csr->vsip ^ vsip) & (1UL << IRQ_S_SOFT)) {
+		if (!test_and_set_bit(IRQ_S_SOFT, &v->irqs_pending_mask)) {
+			if (vsip & (1UL << IRQ_S_SOFT))
+				set_bit(IRQ_S_SOFT, &v->irqs_pending);
+			else
+				clear_bit(IRQ_S_SOFT, &v->irqs_pending);
+		}
 	}
 }
 
-- 
2.17.1