Re: [PATCH v2] KVM: SVM: Make sure GHCB is mapped before updating

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

On 4/8/21 12:10 PM, Sean Christopherson wrote:
> On Thu, Apr 08, 2021, Tom Lendacky wrote:
>> From: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> Access to the GHCB is mainly in the VMGEXIT path and it is known that the
>> GHCB will be mapped. But there are two paths where it is possible the GHCB
>> might not be mapped.
>>
>> 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 might not be mapped (depending on the previous VMEXIT),
>> which will result in a NULL pointer dereference.
>>
>> The svm_complete_emulated_msr() routine will update the GHCB to inform
>> the caller of a RDMSR/WRMSR operation about any errors. While it is likely
>> that the GHCB will be mapped in this situation, add a safe guard
>> in this path to be certain a NULL pointer dereference is not encountered.
>>
>> Fixes: f1c6366e3043 ("KVM: SVM: Add required changes to support intercepts under SEV-ES")
>> Fixes: 647daca25d24 ("KVM: SVM: Add support for booting APs in an SEV-ES guest")
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>>
>> ---
>>
>> Changes from v1:
>> - Added the svm_complete_emulated_msr() path as suggested by Sean
>>   Christopherson
>> - Add a WARN_ON_ONCE() to the sev_vcpu_deliver_sipi_vector() path
>> ---
>>  arch/x86/kvm/svm/sev.c | 3 +++
>>  arch/x86/kvm/svm/svm.c | 2 +-
>>  2 files changed, 4 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>> index 83e00e524513..7ac67615c070 100644
>> --- a/arch/x86/kvm/svm/sev.c
>> +++ b/arch/x86/kvm/svm/sev.c
>> @@ -2105,5 +2105,8 @@ 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.
>>  	 */
>> +	if (WARN_ON_ONCE(!svm->ghcb))
> 
> Isn't this guest triggerable?  I.e. send a SIPI without doing the reset hold?
> If so, this should not WARN.

Yes, it is a guest triggerable event. But a guest shouldn't be doing that,
so I thought adding the WARN_ON_ONCE() just to detect it wasn't bad.
Definitely wouldn't want a WARN_ON().

Thanks,
Tom

> 
>> +		return;
>> +
>>  	ghcb_set_sw_exit_info_2(svm->ghcb, 1);
>>  }
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index 271196400495..534e52ba6045 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -2759,7 +2759,7 @@ 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))
>>  		return kvm_complete_insn_gp(vcpu, err);
>>  
>>  	ghcb_set_sw_exit_info_1(svm->ghcb, 1);
>> -- 
>> 2.31.0
>>