From: Chao Peng <chao.p.peng@linux.intel.com>
To: Fuad Tabba <tabba@google.com>
Cc: Sean Christopherson <seanjc@google.com>,
David Hildenbrand <david@redhat.com>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
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"Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
Vlastimil Babka <vbabka@suse.cz>,
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Yu Zhang <yu.c.zhang@linux.intel.com>,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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dhildenb@redhat.com, Quentin Perret <qperret@google.com>,
Michael Roth <michael.roth@amd.com>,
mhocko@suse.com, Muchun Song <songmuchun@bytedance.com>,
wei.w.wang@intel.com, Will Deacon <will@kernel.org>,
Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
Date: Mon, 26 Sep 2022 22:23:30 +0800 [thread overview]
Message-ID: <20220926142330.GC2658254@chaop.bj.intel.com> (raw)
In-Reply-To: <CA+EHjTzy4iOxLF=5UX=s5v6HSB3Nb1LkwmGqoKhp_PAnFeVPSQ@mail.gmail.com>
On Fri, Sep 23, 2022 at 04:19:46PM +0100, Fuad Tabba wrote:
> > Regarding pKVM's use case, with the shim approach I believe this can be done by
> > allowing userspace mmap() the "hidden" memfd, but with a ton of restrictions
> > piled on top.
> >
> > My first thought was to make the uAPI a set of KVM ioctls so that KVM could tightly
> > tightly control usage without taking on too much complexity in the kernel, but
> > working through things, routing the behavior through the shim itself might not be
> > all that horrific.
> >
> > IIRC, we discarded the idea of allowing userspace to map the "private" fd because
> > things got too complex, but with the shim it doesn't seem _that_ bad.
> >
> > E.g. on the memfd side:
> >
> > 1. The entire memfd must be mapped, and at most one mapping is allowed, i.e.
> > mapping is all or nothing.
> >
> > 2. Acquiring a reference via get_pfn() is disallowed if there's a mapping for
> > the restricted memfd.
> >
> > 3. Add notifier hooks to allow downstream users to further restrict things.
> >
> > 4. Disallow splitting VMAs, e.g. to force userspace to munmap() everything in
> > one shot.
> >
> > 5. Require that there are no outstanding references at munmap(). Or if this
> > can't be guaranteed by userspace, maybe add some way for userspace to wait
> > until it's ok to convert to private? E.g. so that get_pfn() doesn't need
> > to do an expensive check every time.
> >
> > static int memfd_restricted_mmap(struct file *file, struct vm_area_struct *vma)
> > {
> > if (vma->vm_pgoff)
> > return -EINVAL;
> >
> > if ((vma->vm_end - vma->vm_start) != <file size>)
> > return -EINVAL;
> >
> > mutex_lock(&data->lock);
> >
> > if (data->has_mapping) {
> > r = -EINVAL;
> > goto err;
> > }
> > list_for_each_entry(notifier, &data->notifiers, list) {
> > r = notifier->ops->mmap_start(notifier, ...);
> > if (r)
> > goto abort;
> > }
> >
> > notifier->ops->mmap_end(notifier, ...);
> > mutex_unlock(&data->lock);
> > return 0;
> >
> > abort:
> > list_for_each_entry_continue_reverse(notifier &data->notifiers, list)
> > notifier->ops->mmap_abort(notifier, ...);
> > err:
> > mutex_unlock(&data->lock);
> > return r;
> > }
> >
> > static void memfd_restricted_close(struct vm_area_struct *vma)
> > {
> > mutex_lock(...);
> >
> > /*
> > * Destroy the memfd and disable all future accesses if there are
> > * outstanding refcounts (or other unsatisfied restrictions?).
> > */
> > if (<outstanding references> || ???)
> > memfd_restricted_destroy(...);
> > else
> > data->has_mapping = false;
> >
> > mutex_unlock(...);
> > }
> >
> > static int memfd_restricted_may_split(struct vm_area_struct *area, unsigned long addr)
> > {
> > return -EINVAL;
> > }
> >
> > static int memfd_restricted_mapping_mremap(struct vm_area_struct *new_vma)
> > {
> > return -EINVAL;
> > }
> >
> > Then on the KVM side, its mmap_start() + mmap_end() sequence would:
> >
> > 1. Not be supported for TDX or SEV-SNP because they don't allow adding non-zero
> > memory into the guest (after pre-boot phase).
> >
> > 2. Be mutually exclusive with shared<=>private conversions, and is allowed if
> > and only if the entire gfn range of the associated memslot is shared.
>
> In general I think that this would work with pKVM. However, limiting
> private<->shared conversions to the granularity of a whole memslot
> might be difficult to handle in pKVM, since the guest doesn't have the
> concept of memslots. For example, in pKVM right now, when a guest
> shares back its restricted DMA pool with the host it does so at the
> page-level. pKVM would also need a way to make an fd accessible again
> when shared back, which I think isn't possible with this patch.
But does pKVM really want to mmap/munmap a new region at the page-level,
that can cause VMA fragmentation if the conversion is frequent as I see.
Even with a KVM ioctl for mapping as mentioned below, I think there will
be the same issue.
>
> You were initially considering a KVM ioctl for mapping, which might be
> better suited for this since KVM knows which pages are shared and
> which ones are private. So routing things through KVM might simplify
> things and allow it to enforce all the necessary restrictions (e.g.,
> private memory cannot be mapped). What do you think?
>
> Thanks,
> /fuad
next prev parent reply other threads:[~2022-09-26 15:29 UTC|newest]
Thread overview: 97+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-15 14:29 [PATCH v8 0/8] KVM: mm: fd-based approach for supporting KVM Chao Peng
2022-09-15 14:29 ` [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd Chao Peng
2022-09-19 9:12 ` David Hildenbrand
2022-09-19 19:10 ` Sean Christopherson
2022-09-21 21:10 ` Andy Lutomirski
2022-09-22 13:23 ` Wang, Wei W
2022-09-23 15:20 ` Fuad Tabba
2022-09-23 15:19 ` Fuad Tabba
2022-09-26 14:23 ` Chao Peng [this message]
2022-09-26 15:51 ` Fuad Tabba
2022-09-27 22:47 ` Sean Christopherson
2022-09-30 16:19 ` Fuad Tabba
2022-10-13 13:34 ` Chao Peng
2022-10-17 10:31 ` Fuad Tabba
2022-10-17 14:58 ` Chao Peng
2022-10-17 19:05 ` Fuad Tabba
2022-10-19 13:30 ` Chao Peng
2022-10-18 0:33 ` Sean Christopherson
2022-10-19 15:04 ` Fuad Tabba
2022-09-23 0:58 ` Kirill A . Shutemov
2022-09-26 10:35 ` David Hildenbrand
2022-09-26 14:48 ` Kirill A. Shutemov
2022-09-26 14:53 ` David Hildenbrand
2022-09-27 23:23 ` Sean Christopherson
2022-09-28 13:36 ` Kirill A. Shutemov
2022-09-22 13:26 ` Wang, Wei W
2022-09-22 19:49 ` Sean Christopherson
2022-09-23 0:53 ` Kirill A . Shutemov
2022-09-23 15:20 ` Fuad Tabba
2022-09-30 16:14 ` Fuad Tabba
2022-09-30 16:23 ` Kirill A . Shutemov
2022-10-03 7:33 ` Fuad Tabba
2022-10-03 11:01 ` Kirill A. Shutemov
2022-10-04 15:39 ` Fuad Tabba
2022-10-06 8:50 ` Fuad Tabba
2022-10-06 13:04 ` Kirill A. Shutemov
2022-10-17 13:00 ` Vlastimil Babka
2022-10-17 16:19 ` Kirill A . Shutemov
2022-10-17 16:39 ` Gupta, Pankaj
2022-10-17 21:56 ` Kirill A . Shutemov
2022-10-18 13:42 ` Vishal Annapurve
2022-10-19 15:32 ` Kirill A . Shutemov
2022-10-20 10:50 ` Vishal Annapurve
2022-10-21 13:54 ` Chao Peng
2022-10-21 16:53 ` Sean Christopherson
2022-10-19 12:23 ` Vishal Annapurve
2022-10-21 13:47 ` Chao Peng
2022-10-21 16:18 ` Sean Christopherson
2022-10-24 14:59 ` Kirill A . Shutemov
2022-10-24 15:26 ` David Hildenbrand
2022-11-03 16:27 ` Vishal Annapurve
2022-09-15 14:29 ` [PATCH v8 2/8] KVM: Extend the memslot to support fd-based private memory Chao Peng
2022-09-16 9:14 ` Bagas Sanjaya
2022-09-16 9:53 ` Chao Peng
2022-09-26 10:26 ` Fuad Tabba
2022-09-26 14:04 ` Chao Peng
2022-09-29 22:45 ` Isaku Yamahata
2022-09-29 23:22 ` Sean Christopherson
2022-10-05 13:04 ` Jarkko Sakkinen
2022-10-05 22:05 ` Jarkko Sakkinen
2022-10-06 9:00 ` Fuad Tabba
2022-10-06 14:58 ` Jarkko Sakkinen
2022-10-06 15:07 ` Jarkko Sakkinen
2022-10-06 15:34 ` Sean Christopherson
2022-10-07 11:14 ` Jarkko Sakkinen
2022-10-07 14:58 ` Sean Christopherson
2022-10-07 21:54 ` Jarkko Sakkinen
2022-10-08 16:15 ` Jarkko Sakkinen
2022-10-08 17:35 ` Jarkko Sakkinen
2022-10-10 8:25 ` Chao Peng
2022-10-12 8:14 ` Jarkko Sakkinen
2022-09-15 14:29 ` [PATCH v8 3/8] KVM: Add KVM_EXIT_MEMORY_FAULT exit Chao Peng
2022-09-16 9:17 ` Bagas Sanjaya
2022-09-16 9:54 ` Chao Peng
2022-09-15 14:29 ` [PATCH v8 4/8] KVM: Use gfn instead of hva for mmu_notifier_retry Chao Peng
2022-09-15 14:29 ` [PATCH v8 5/8] KVM: Register/unregister the guest private memory regions Chao Peng
2022-09-26 10:36 ` Fuad Tabba
2022-09-26 14:07 ` Chao Peng
2022-10-11 9:48 ` Fuad Tabba
2022-10-12 2:35 ` Chao Peng
2022-10-17 10:15 ` Fuad Tabba
2022-10-17 22:17 ` Sean Christopherson
2022-10-19 13:23 ` Chao Peng
2022-10-19 15:02 ` Fuad Tabba
2022-10-19 16:09 ` Sean Christopherson
2022-10-19 18:32 ` Fuad Tabba
2022-09-15 14:29 ` [PATCH v8 6/8] KVM: Update lpage info when private/shared memory are mixed Chao Peng
2022-09-29 16:52 ` Isaku Yamahata
2022-09-30 8:59 ` Chao Peng
2022-09-15 14:29 ` [PATCH v8 7/8] KVM: Handle page fault for private memory Chao Peng
2022-10-14 18:57 ` Sean Christopherson
2022-10-17 14:48 ` Chao Peng
2022-09-15 14:29 ` [PATCH v8 8/8] KVM: Enable and expose KVM_MEM_PRIVATE Chao Peng
2022-10-04 14:55 ` Jarkko Sakkinen
2022-10-10 8:31 ` Chao Peng
2022-10-06 8:55 ` Fuad Tabba
2022-10-10 8:33 ` Chao Peng
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