Re: [PATCH v2] LoongArch: KVM: Fix TOCTOU race on pv_features

[Tao Cui <cui.tao@linux.dev>]

Hi Huacai,

在 2026/8/14 16:24, Huacai Chen 写道:
> Hi, Tao,
> 
> On Fri, Aug 14, 2026 at 9:09 AM Bibo Mao <maobibo@loongson.cn> wrote:
>>
>>
>>
>> On 2026/8/14 上午7:25, Tao Cui wrote:
>>> From: Tao Cui <cuitao@kylinos.cn>
>>>
>>> The check-then-set on kvm->arch.pv_features in
>>> kvm_loongarch_cpucfg_set_attr() is lockless, so two vCPUs can race
>>> past the validation and set different values.  Add a spinlock to
>>> serialize it.
>>>
>>> Signed-off-by: Tao Cui <cuitao@kylinos.cn>
>>> ---
>>>
>>> Changes in v2: use a spinlock instead of a cmpxchg loop (Bibo Mao).
>>> Link: https://lore.kernel.org/all/20260810081321.157258-1-cui.tao@linux.dev/
>>>
>>>   arch/loongarch/include/asm/kvm_host.h | 2 ++
>>>   arch/loongarch/kvm/vcpu.c             | 6 +++++-
>>>   arch/loongarch/kvm/vm.c               | 1 +
>>>   3 files changed, 8 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/loongarch/include/asm/kvm_host.h b/arch/loongarch/include/asm/kvm_host.h
>>> index 23cfbecebbd7..cdbbd90e0584 100644
>>> --- a/arch/loongarch/include/asm/kvm_host.h
>>> +++ b/arch/loongarch/include/asm/kvm_host.h
>>> @@ -128,6 +128,8 @@ struct kvm_arch {
>>>       struct kvm_phyid_map  *phyid_map;
>>>       /* Enabled PV features */
>>>       unsigned long pv_features;
>>> +     /* Serializes pv_features updates */
>>> +     spinlock_t pv_features_lock;
> I think it is better to rename it to pv_setting_lock, and put it near
> phyid_map_lock.
> 
Got it, v3 on the way with the rename and the lock moved.

Thanks,
Tao

> Huacai
> 
>>>       /* Supported KVM features */
>>>       unsigned long kvm_features;
>>>
>>> diff --git a/arch/loongarch/kvm/vcpu.c b/arch/loongarch/kvm/vcpu.c
>>> index 20c207d80e31..551ed39b2d1f 100644
>>> --- a/arch/loongarch/kvm/vcpu.c
>>> +++ b/arch/loongarch/kvm/vcpu.c
>>> @@ -1165,10 +1165,14 @@ static int kvm_loongarch_cpucfg_set_attr(struct kvm_vcpu *vcpu,
>>>                       return -EINVAL;
>>>
>>>               /* All vCPUs need set the same PV features */
>>> +             spin_lock(&kvm->arch.pv_features_lock);
>>>               if ((kvm->arch.pv_features & LOONGARCH_PV_FEAT_UPDATED)
>>> -                             && ((kvm->arch.pv_features & valid) != val))
>>> +                             && ((kvm->arch.pv_features & valid) != val)) {
>>> +                     spin_unlock(&kvm->arch.pv_features_lock);
>>>                       return -EINVAL;
>>> +             }
>>>               kvm->arch.pv_features = val | LOONGARCH_PV_FEAT_UPDATED;
>>> +             spin_unlock(&kvm->arch.pv_features_lock);
>>>               return 0;
>>>       default:
>>>               return -ENXIO;
>>> diff --git a/arch/loongarch/kvm/vm.c b/arch/loongarch/kvm/vm.c
>>> index 1317c718f896..b984cc54f305 100644
>>> --- a/arch/loongarch/kvm/vm.c
>>> +++ b/arch/loongarch/kvm/vm.c
>>> @@ -76,6 +76,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>>>               return -ENOMEM;
>>>       }
>>>       spin_lock_init(&kvm->arch.phyid_map_lock);
>>> +     spin_lock_init(&kvm->arch.pv_features_lock);
>>>
>>>       kvm_init_vmcs(kvm);
>>>       kvm_vm_init_features(kvm);
>>>
>> Reviewed-by: Bibo Mao <maobibo@loongson.cn>
>>