Re: [PATCH v12 01/84] KVM: arm64: Release pfn, i.e. put page, if copying MTE tags hits ZONE_DEVICE

Steven Price
2024-08-08T09:55:13Z

On 07/08/2024 15:15, Catalin Marinas wrote:
> On Fri, Jul 26, 2024 at 04:51:10PM -0700, Sean Christopherson wrote:
>> Put the page reference acquired by gfn_to_pfn_prot() if
>> kvm_vm_ioctl_mte_copy_tags() runs into ZONE_DEVICE memory. KVM's less-
>> than-stellar heuristics for dealing with pfn-mapped memory means that KVM
>> can get a page reference to ZONE_DEVICE memory.
>>
>> Fixes: f0376edb1ddc ("KVM: arm64: Add ioctl to fetch/store tags in a guest")
>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>> ---
>> arch/arm64/kvm/guest.c | 1 +
>> 1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
>> index 11098eb7eb44..e1f0ff08836a 100644
>> --- a/arch/arm64/kvm/guest.c
>> +++ b/arch/arm64/kvm/guest.c
>> @@ -1059,6 +1059,7 @@ int kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
>> page = pfn_to_online_page(pfn);
>> if (!page) {
>> /* Reject ZONE_DEVICE memory */
>> + kvm_release_pfn_clean(pfn);
>> ret = -EFAULT;
>> goto out;
>> }
>
> This patch makes sense irrespective of whether the above pfn is a
> ZONE_DEVICE or not. gfn_to_pfn_prot() increased the page refcount via
> GUP, so it must be released before bailing out of this loop.
>
> Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
>

Yep, as Catalin says, this is an 'obviously' correct fix - the reference
needs releasing before bailing out. The comment there is perhaps
misleading - it's not just ZONE_DEVICE memory that will be rejected, but
this is the case that was in my mind when I wrote it. Although clearly I
wasn't thinking hard enough when writing the code in the first place... ;)

Reviewed-by: Steven Price <steven.price@arm.com>

Thanks,

Steve