On Tue, May 23, 2017 at 10:05:13AM +0100, Marc Zyngier wrote:
> On 23/05/17 09:43, Christoffer Dall wrote:
> > On Mon, May 22, 2017 at 04:30:22PM +0100, Marc Zyngier wrote:
> >> On 16/05/17 11:04, Christoffer Dall wrote:
> >>> We don't need to stop a specific VCPU when changing the active state,
> >>> because private IRQs can only be modified by a running VCPU for the
> >>> VCPU itself and it is therefore already stopped.
> >>>
> >>> However, it is also possible for two VCPUs to be modifying the active
> >>> state of SPIs at the same time, which can cause the thread being stuck
> >>> in the loop that checks other VCPU threads for a potentially very long
> >>> time, or to modify the active state of a running VCPU. Fix this by
> >>> serializing all accesses to setting and clearing the active state of
> >>> interrupts using the KVM mutex.
> >>>
> >>> Reported-by: Andrew Jones <drjones@xxxxxxxxxx>
> >>> Signed-off-by: Christoffer Dall <cdall@xxxxxxxxxx>
> >>> ---
> >>> arch/arm/include/asm/kvm_host.h | 2 --
> >>> arch/arm64/include/asm/kvm_host.h | 2 --
> >>> virt/kvm/arm/arm.c | 20 ++++----------------
> >>> virt/kvm/arm/vgic/vgic-mmio.c | 18 ++++++++++--------
> >>> virt/kvm/arm/vgic/vgic.c | 11 ++++++-----
> >>> 5 files changed, 20 insertions(+), 33 deletions(-)
> >>>
> >>> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> >>> index f0e6657..12274d4 100644
> >>> --- a/arch/arm/include/asm/kvm_host.h
> >>> +++ b/arch/arm/include/asm/kvm_host.h
> >>> @@ -233,8 +233,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> >>> struct kvm_vcpu __percpu **kvm_get_running_vcpus(void);
> >>> void kvm_arm_halt_guest(struct kvm *kvm);
> >>> void kvm_arm_resume_guest(struct kvm *kvm);
> >>> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> >>> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> >>>
> >>> int kvm_arm_copy_coproc_indices(struct kvm_vcpu *vcpu, u64 __user *uindices);
> >>> unsigned long kvm_arm_num_coproc_regs(struct kvm_vcpu *vcpu);
> >>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >>> index 5e19165..32cbe8a 100644
> >>> --- a/arch/arm64/include/asm/kvm_host.h
> >>> +++ b/arch/arm64/include/asm/kvm_host.h
> >>> @@ -333,8 +333,6 @@ struct kvm_vcpu *kvm_arm_get_running_vcpu(void);
> >>> struct kvm_vcpu * __percpu *kvm_get_running_vcpus(void);
> >>> void kvm_arm_halt_guest(struct kvm *kvm);
> >>> void kvm_arm_resume_guest(struct kvm *kvm);
> >>> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu);
> >>> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu);
> >>>
> >>> u64 __kvm_call_hyp(void *hypfn, ...);
> >>> #define kvm_call_hyp(f, ...) __kvm_call_hyp(kvm_ksym_ref(f), ##__VA_ARGS__)
> >>> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> >>> index 3417e18..3c387fd 100644
> >>> --- a/virt/kvm/arm/arm.c
> >>> +++ b/virt/kvm/arm/arm.c
> >>> @@ -539,27 +539,15 @@ void kvm_arm_halt_guest(struct kvm *kvm)
> >>> kvm_make_all_cpus_request(kvm, KVM_REQ_VCPU_EXIT);
> >>> }
> >>>
> >>> -void kvm_arm_halt_vcpu(struct kvm_vcpu *vcpu)
> >>> -{
> >>> - vcpu->arch.pause = true;
> >>> - kvm_vcpu_kick(vcpu);
> >>> -}
> >>> -
> >>> -void kvm_arm_resume_vcpu(struct kvm_vcpu *vcpu)
> >>> -{
> >>> - struct swait_queue_head *wq = kvm_arch_vcpu_wq(vcpu);
> >>> -
> >>> - vcpu->arch.pause = false;
> >>> - swake_up(wq);
> >>> -}
> >>> -
> >>> void kvm_arm_resume_guest(struct kvm *kvm)
> >>> {
> >>> int i;
> >>> struct kvm_vcpu *vcpu;
> >>>
> >>> - kvm_for_each_vcpu(i, vcpu, kvm)
> >>> - kvm_arm_resume_vcpu(vcpu);
> >>> + kvm_for_each_vcpu(i, vcpu, kvm) {
> >>> + vcpu->arch.pause = false;
> >>> + swake_up(kvm_arch_vcpu_wq(vcpu));
> >>> + }
> >>> }
> >>>
> >>> static void vcpu_sleep(struct kvm_vcpu *vcpu)
> >>> diff --git a/virt/kvm/arm/vgic/vgic-mmio.c b/virt/kvm/arm/vgic/vgic-mmio.c
> >>> index 64cbcb4..c1e4bdd 100644
> >>> --- a/virt/kvm/arm/vgic/vgic-mmio.c
> >>> +++ b/virt/kvm/arm/vgic/vgic-mmio.c
> >>> @@ -231,23 +231,21 @@ static void vgic_mmio_change_active(struct kvm_vcpu *vcpu, struct vgic_irq *irq,
> >>> * be migrated while we don't hold the IRQ locks and we don't want to be
> >>> * chasing moving targets.
> >>> *
> >>> - * For private interrupts, we only have to make sure the single and only VCPU
> >>> - * that can potentially queue the IRQ is stopped.
> >>> + * For private interrupts we don't have to do anything because userspace
> >>> + * accesses to the VGIC state already require all VCPUs to be stopped, and
> >>> + * only the VCPU itself can modify its private interrupts active state, which
> >>> + * guarantees that the VCPU is not running.
> >>> */
> >>> static void vgic_change_active_prepare(struct kvm_vcpu *vcpu, u32 intid)
> >>> {
> >>> - if (intid < VGIC_NR_PRIVATE_IRQS)
> >>> - kvm_arm_halt_vcpu(vcpu);
> >>> - else
> >>> + if (intid > VGIC_NR_PRIVATE_IRQS)
> >>> kvm_arm_halt_guest(vcpu->kvm);
> >>> }
> >>>
> >>> /* See vgic_change_active_prepare */
> >>> static void vgic_change_active_finish(struct kvm_vcpu *vcpu, u32 intid)
> >>> {
> >>> - if (intid < VGIC_NR_PRIVATE_IRQS)
> >>> - kvm_arm_resume_vcpu(vcpu);
> >>> - else
> >>> + if (intid > VGIC_NR_PRIVATE_IRQS)
> >>> kvm_arm_resume_guest(vcpu->kvm);
> >>> }
> >>>
> >>> @@ -271,11 +269,13 @@ void vgic_mmio_write_cactive(struct kvm_vcpu *vcpu,
> >>> {
> >>> u32 intid = VGIC_ADDR_TO_INTID(addr, 1);
> >>>
> >>> + mutex_lock(&vcpu->kvm->lock);
> >>> vgic_change_active_prepare(vcpu, intid);
> >>>
> >>> __vgic_mmio_write_cactive(vcpu, addr, len, val);
> >>>
> >>> vgic_change_active_finish(vcpu, intid);
> >>> + mutex_unlock(&vcpu->kvm->lock);
> >>
> >> Any reason not to move the lock/unlock calls to prepare/finish? Also, do
> >> we need to take that mutex if intid is a PPI?
> >
> > I guess we strictly don't need to take the mutex if it's a PPI, no.
> >
> > But I actually preferred this symmetry because you can easily tell we
> > don't have a bug (famous last words) by locking and unlocking the mutex
> > in the same function.
> >
> > I don't feel strongly about it though, so I can move it if you prefer
> > it.
>
> No, that's fine, I just wanted to check whether my understanding was
> correct.
>
> Reviewed-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>
Thanks.
So assuming Drew's patches will go on top of these, should we merge this
as fixes to -rcX, or queue them for v4.13 ?
I'm leaning towards the latter because I don't think we've seen these
races do something bad in the wild, and they're probably not going to be
backportable to stable anyway. Thoughts?
-Christoffer
