KVM: x86: Take PIC lock on KVM_GET_IRQCHIP path

When userspace issues the KVM_SET_IRQCHIP ioctl to set the state of
the PIC, kvm_vm_ioctl_set_irqchip() grabs @kvm->arch.vpic->lock before
updating the state. However, the KVM_GET_IRQCHIP ioctl to retrieve the
same PIC state does not grab such lock, potentially causing torn reads
for userspace.

Fix this by grabbing the lock on the read path.

This issue goes all the way back. The bug was introduced with the
addition of PIC ioctl code itself in 6ceb9d7 ("KVM: Add get/
set irqchip ioctls for in-kernel PIC live migration support"). Later,
894a9c5 ("KVM: x86: missing locking in PIT/IRQCHIP/SET_BSP_CPU
ioctl paths") added the locking for kvm_vm_ioctl_set_irqchip(), but
missed kvm_vm_ioctl_get_irqchip().

Fixes: 6ceb9d7 ("KVM: Add get/set irqchip ioctls for in-kernel PIC live migration support")
Fixes: 894a9c5 ("KVM: x86: missing locking in PIT/IRQCHIP/SET_BSP_CPU ioctl paths")
Reported-by: Claude Code:claude-opus-4.6
Signed-off-by: Carlos López <clopez@suse.de>
Link: https://patch.msgid.link/20260529140013.14925-2-clopez@suse.de
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: 00xc

arch/x86/kvm/irq.c

@@ -585,12 +585,16 @@ int kvm_vm_ioctl_get_irqchip(struct kvm *kvm, struct kvm_irqchip *chip)
 	r = 0;
 	switch (chip->chip_id) {
 	case KVM_IRQCHIP_PIC_MASTER:
+		spin_lock(&pic->lock);
 		memcpy(&chip->chip.pic, &pic->pics[0],
 			sizeof(struct kvm_pic_state));
+		spin_unlock(&pic->lock);
 		break;
 	case KVM_IRQCHIP_PIC_SLAVE:
+		spin_lock(&pic->lock);
 		memcpy(&chip->chip.pic, &pic->pics[1],
 			sizeof(struct kvm_pic_state));
+		spin_unlock(&pic->lock);
 		break;
 	case KVM_IRQCHIP_IOAPIC:
 		kvm_get_ioapic(kvm, &chip->chip.ioapic);