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Date: Tue, 6 Feb 2024 20:10:10 -0800
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Subject: Re: [PATCH v12 11/20] KVM: xen: allow shared_info to be mapped by
 fixed HVA
From: Sean Christopherson <seanjc@google.com>
To: Paul Durrant <paul@xen.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>, Jonathan Corbet <corbet@lwn.net>, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org, 
	"H. Peter Anvin" <hpa@zytor.com>, David Woodhouse <dwmw2@infradead.org>, Shuah Khan <shuah@kernel.org>, 
	kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-kselftest@vger.kernel.org
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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.

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.

> +				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);
> +			}