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

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

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.

I can submit a v2 with that unless you want to submit it (with one small
change below).

> 
> 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.

Thanks,
Tom

>                 return kvm_complete_insn_gp(vcpu, err);
> 
>         ghcb_set_sw_exit_info_1(svm->ghcb, 1);
> 
>> Fixes: 647daca25d24 ("KVM: SVM: Add support for booting APs in an SEV-ES guest")
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
> 
> Either way:
> 
> Reviewed-by: Sean Christopherson <seanjc@google.com> 
> 
>> ---
>>  arch/x86/kvm/svm/sev.c | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 83e00e524513..13758e3b106d 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -2105,5 +2105,6 @@ void sev_vcpu_deliver_sipi_vector(struct kvm_vcpu *vcpu, u8 vector)
>>  	 * the guest will set the CS and RIP. Set SW_EXIT_INFO_2 to a
>>  	 * non-zero value.
>>  	 */
>> -	ghcb_set_sw_exit_info_2(svm->ghcb, 1);
>> +	if (svm->ghcb)
>> +		ghcb_set_sw_exit_info_2(svm->ghcb, 1);
>>  }
>> -- 
>> 2.31.0
>>