linux-kernel - Re: [PATCH v6 1/5] KVM: arm64: Block cacheable PFNMAP mapping

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20250609122402.GM19710@nvidia.com>
Date: Mon, 9 Jun 2025 09:24:02 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: ankita@...dia.com, maz@...nel.org, oliver.upton@...ux.dev,
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
	catalin.marinas@....com, will@...nel.org, ryan.roberts@....com,
	shahuang@...hat.com, lpieralisi@...nel.org, david@...hat.com,
	aniketa@...dia.com, cjia@...dia.com, kwankhede@...dia.com,
	kjaju@...dia.com, targupta@...dia.com, vsethi@...dia.com,
	acurrid@...dia.com, apopple@...dia.com, jhubbard@...dia.com,
	danw@...dia.com, zhiw@...dia.com, mochs@...dia.com,
	udhoke@...dia.com, dnigam@...dia.com, alex.williamson@...hat.com,
	sebastianene@...gle.com, coltonlewis@...gle.com,
	kevin.tian@...el.com, yi.l.liu@...el.com, ardb@...nel.org,
	akpm@...ux-foundation.org, gshan@...hat.com, linux-mm@...ck.org,
	ddutile@...hat.com, tabba@...gle.com, qperret@...gle.com,
	kvmarm@...ts.linux.dev, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, maobibo@...ngson.cn
Subject: Re: [PATCH v6 1/5] KVM: arm64: Block cacheable PFNMAP mapping

On Fri, Jun 06, 2025 at 11:11:56AM -0700, Sean Christopherson wrote:
> > @@ -1612,6 +1624,10 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >  
> >  	vfio_allow_any_uc = vma->vm_flags & VM_ALLOW_ANY_UNCACHED;
> >  
> > +	if ((vma->vm_flags & VM_PFNMAP) &&
> > +	    !mapping_type_noncacheable(vma->vm_page_prot))
> 
> I don't think this is correct, and there's a very real chance this will break
> existing setups.  PFNMAP memory isn't strictly device memory, and IIUC, KVM
> force DEVICE/NORMAL_NC based on kvm_is_device_pfn(), not based on VM_PFNMAP.

kvm_is_device_pfn() effecitvely means KVM can't use CMOs on that
PFN. It doesn't really mean anything more..

PFNMAP says the same thing, or at least from a mm perspective we don't
want drivers taking PFNMAP memory and then trying to guess if there
are struct pages/KVAs for it. PFNMAP memory is supposed to be fully
opaque.

Though that confusion seems to be a separate issue from this patch.

> 	if (kvm_is_device_pfn(pfn)) {
> 		/*
> 		 * If the page was identified as device early by looking at
> 		 * the VMA flags, vma_pagesize is already representing the
> 		 * largest quantity we can map.  If instead it was mapped
> 		 * via __kvm_faultin_pfn(), vma_pagesize is set to PAGE_SIZE
> 		 * and must not be upgraded.
> 		 *
> 		 * In both cases, we don't let transparent_hugepage_adjust()
> 		 * change things at the last minute.
> 		 */
> 		device = true;

"device" here is sort of a mis-nomer, it is really just trying to
setup the S2 so that CMOs are not going go to be done.

Calling it 'disable_cmo' would sure make this code clearer..

> @@ -1639,6 +1653,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>                 return -EFAULT;
>  
>         if (kvm_is_device_pfn(pfn)) {
> +               if (is_vma_cacheable)
> +                       return -EINVAL;
> +

eg

if (!kvm_can_use_cmo_pfn(pfn)) {
               if (is_vma_cacheable)
                       return -EINVAL;

>                  * If the page was identified as device early by looking at
>                  * the VMA flags, vma_pagesize is already representing the
> @@ -1722,6 +1739,11 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
>                 prot |= KVM_PGTABLE_PROT_X;
>  
>         if (device) {
> +               if (is_vma_cacheable) {
> +                       ret = -EINVAL;
> +                       goto out;
> +               }

if (disable_cmo) {
               if (is_vma_cacheable)
                       return -EINVAL;

Makes alot more sense, right? If KVM can't do CMOs then it should not
attempt to use memory mapped into the VMA as cachable.

>                 if (vfio_allow_any_uc)
>                         prot |= KVM_PGTABLE_PROT_NORMAL_NC;
>                 else
> 

Regardless, this seems good for this patch at least.

Jason