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SubjectRe: [PATCH v12 11/20] KVM: xen: allow shared_info to be mapped by fixed HVA
On 08/02/2024 16:48, Sean Christopherson wrote:
> On Thu, Feb 08, 2024, Paul Durrant wrote:
>> 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.
>>>
>>
>> With the benefit of some sleep, I'm wondering why 0 is a 'ridiculous'
>> invalid value for a *virtual* address? Surely it's essentially a numerical
>> cast of the canonically invalid NULL pointer?
>
> It's legal to mmap() virtual address '0', albeit not by default:
>
> config DEFAULT_MMAP_MIN_ADDR
> int "Low address space to protect from user allocation"
> depends on MMU
> default 4096
> help
> This is the portion of low virtual memory which should be protected
> from userspace allocation. Keeping a user from writing to low pages
> can help reduce the impact of kernel NULL pointer bugs.
>
> For most ppc64 and x86 users with lots of address space
> a value of 65536 is reasonable and should cause no problems.
> On arm and other archs it should not be higher than 32768.
> Programs which use vm86 functionality or have some need to map
> this low address space will need CAP_SYS_RAWIO or disable this
> protection by setting the value to 0.
>
> This value can be changed after boot using the
> /proc/sys/vm/mmap_min_addr tunable.
>
>
> Obviously it's equally ridiculous that userspace would ever mmap() '0' and pass
> that as the shared_info, but given that this is x86-only, there are architecturally
> illegal addresses that can be used, at least until Intel adds LA64 ;-)

Ok. Thanks for the reference.


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