linux-kernel - Re: [PATCH v12 11/20] KVM: xen: allow shared

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Message-ID: <96e25eed-8487-40a4-b56c-b5ca42667645@xen.org>
Date: Wed, 7 Feb 2024 08:53:37 +0000
From: Paul Durrant <xadimgnik@...il.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 David Woodhouse <dwmw2@...radead.org>, Shuah Khan <shuah@...nel.org>,
 kvm@...r.kernel.org, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v12 11/20] KVM: xen: allow shared_info to be mapped by
 fixed HVA

On 07/02/2024 04:10, Sean Christopherson wrote:
> On Mon, Jan 15, 2024, Paul Durrant wrote:
>> @@ -638,20 +637,32 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
>>   		}
>>   		break;
>>   
>> -	case KVM_XEN_ATTR_TYPE_SHARED_INFO: {
>> +	case KVM_XEN_ATTR_TYPE_SHARED_INFO:
>> +	case KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA: {
>>   		int idx;
>>   
>>   		mutex_lock(&kvm->arch.xen.xen_lock);
>>   
>>   		idx = srcu_read_lock(&kvm->srcu);
>>   
>> -		if (data->u.shared_info.gfn == KVM_XEN_INVALID_GFN) {
>> -			kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
>> -			r = 0;
>> +		if (data->type == KVM_XEN_ATTR_TYPE_SHARED_INFO) {
>> +			if (data->u.shared_info.gfn == KVM_XEN_INVALID_GFN) {
>> +				kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
>> +				r = 0;
>> +			} else {
>> +				r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
>> +						     gfn_to_gpa(data->u.shared_info.gfn),
>> +						     PAGE_SIZE);
>> +			}
>>   		} else {
>> -			r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
>> -					     gfn_to_gpa(data->u.shared_info.gfn),
>> -					     PAGE_SIZE);
>> +			if (data->u.shared_info.hva == 0) {
> 
> I know I said I don't care about the KVM Xen ABI, but I still think using '0' as
> "invalid" is ridiculous.
> 

I don't have any massive preference; we could define a 
KVM_XEN_INVALID_HVA too.

> More importantly, this code needs to check that HVA is a userspace pointer.
> Because __kvm_set_memory_region() performs the address checks, KVM assumes any
> hva that it gets out of a memslot, i.e. from a gfn, is a safe userspace address.
> 
> kvm_is_error_hva() will catch most addresses, but I'm pretty sure there's still
> a small window where userspace could use this to write kernel memory.

Ok. Good point. I'll add some appropriate checks.

   Paul

> 
>> +				kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
>> +				r = 0;
>> +			} else {
>> +				r = kvm_gpc_activate_hva(&kvm->arch.xen.shinfo_cache,
>> +							 data->u.shared_info.hva,
>> +							 PAGE_SIZE);
>> +			}