Re: [PATCH] KVM: SVM: Make sure GHCB is mapped before updating

[Tom Lendacky <thomas.lendacky@amd.com>]

On 4/7/21 4:07 PM, Sean Christopherson wrote:
> On Wed, Apr 07, 2021, Tom Lendacky wrote:
>> On 4/7/21 3:08 PM, Sean Christopherson wrote:
>>> On Wed, Apr 07, 2021, Tom Lendacky wrote:
>>>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>>>
>>>> The sev_vcpu_deliver_sipi_vector() routine will update the GHCB to inform
>>>> the caller of the AP Reset Hold NAE event that a SIPI has been delivered.
>>>> However, if a SIPI is performed without a corresponding AP Reset Hold,
>>>> then the GHCB may not be mapped, which will result in a NULL pointer
>>>> dereference.
>>>>
>>>> Check that the GHCB is mapped before attempting the update.
>>>
>>> It's tempting to say the ghcb_set_*() helpers should guard against this, but
>>> that would add a lot of pollution and the vast majority of uses are very clearly
>>> in the vmgexit path.  svm_complete_emulated_msr() is the only other case that
>>> is non-obvious; would it make sense to sanity check svm->ghcb there as well?
>>
>> Hmm... I'm not sure if we can get here without having taken the VMGEXIT
>> path to start, but it certainly couldn't hurt to add it.
> 
> Yeah, AFAICT it should be impossible to reach the callback without a valid ghcb,
> it'd be purely be a sanity check.
>  
>> I can submit a v2 with that unless you want to submit it (with one small
>> change below).
> 
> I'd say just throw it into v2.
> 
>>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>>> index 019ac836dcd0..abe9c765628f 100644
>>> --- a/arch/x86/kvm/svm/svm.c
>>> +++ b/arch/x86/kvm/svm/svm.c
>>> @@ -2728,7 +2728,8 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>>>  static int svm_complete_emulated_msr(struct kvm_vcpu *vcpu, int err)
>>>  {
>>>         struct vcpu_svm *svm = to_svm(vcpu);
>>> -       if (!sev_es_guest(vcpu->kvm) || !err)
>>> +
>>> +       if (!err || !sev_es_guest(vcpu->kvm) || !WARN_ON_ONCE(svm->ghcb))
>>
>> This should be WARN_ON_ONCE(!svm->ghcb), otherwise you'll get the right
>> result, but get a stack trace immediately.
> 
> Doh, yep.

Actually, because of the "or's", this needs to be:

if (!err || !sev_es_guest(vcpu->kvm) || (sev_es_guest(vcpu->kvm) && WARN_ON_ONCE(!svm->ghcb)))

Thanks,
Tom

>