From: Chao Peng <chao.p.peng@linux.intel.com>
To: Vishal Annapurve <vannapurve@google.com>
Cc: "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
"Gupta, Pankaj" <pankaj.gupta@amd.com>,
Vlastimil Babka <vbabka@suse.cz>,
kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
linux-api@vger.kernel.org, linux-doc@vger.kernel.org,
qemu-devel@nongnu.org, Paolo Bonzini <pbonzini@redhat.com>,
Jonathan Corbet <corbet@lwn.net>,
Sean Christopherson <seanjc@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
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Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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"Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
Yu Zhang <yu.c.zhang@linux.intel.com>,
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ak@linux.intel.com, david@redhat.com, aarcange@redhat.com,
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Quentin Perret <qperret@google.com>,
Michael Roth <michael.roth@amd.com>,
mhocko@suse.com, Muchun Song <songmuchun@bytedance.com>,
wei.w.wang@intel.com
Subject: Re: [PATCH v8 1/8] mm/memfd: Introduce userspace inaccessible memfd
Date: Fri, 21 Oct 2022 21:54:34 +0800 [thread overview]
Message-ID: <20221021135434.GB3607894@chaop.bj.intel.com> (raw)
In-Reply-To: <CAGtprH-8y9iTyVZ+EYW2t=zGqz7fVgPu-3wVm0Wgv5134NU6WQ@mail.gmail.com>
On Thu, Oct 20, 2022 at 04:20:58PM +0530, Vishal Annapurve wrote:
> On Wed, Oct 19, 2022 at 9:02 PM Kirill A . Shutemov
> <kirill.shutemov@linux.intel.com> wrote:
> >
> > On Tue, Oct 18, 2022 at 07:12:10PM +0530, Vishal Annapurve wrote:
> > > On Tue, Oct 18, 2022 at 3:27 AM Kirill A . Shutemov
> > > <kirill.shutemov@linux.intel.com> wrote:
> > > >
> > > > On Mon, Oct 17, 2022 at 06:39:06PM +0200, Gupta, Pankaj wrote:
> > > > > On 10/17/2022 6:19 PM, Kirill A . Shutemov wrote:
> > > > > > On Mon, Oct 17, 2022 at 03:00:21PM +0200, Vlastimil Babka wrote:
> > > > > > > On 9/15/22 16:29, Chao Peng wrote:
> > > > > > > > From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> > > > > > > >
> > > > > > > > KVM can use memfd-provided memory for guest memory. For normal userspace
> > > > > > > > accessible memory, KVM userspace (e.g. QEMU) mmaps the memfd into its
> > > > > > > > virtual address space and then tells KVM to use the virtual address to
> > > > > > > > setup the mapping in the secondary page table (e.g. EPT).
> > > > > > > >
> > > > > > > > With confidential computing technologies like Intel TDX, the
> > > > > > > > memfd-provided memory may be encrypted with special key for special
> > > > > > > > software domain (e.g. KVM guest) and is not expected to be directly
> > > > > > > > accessed by userspace. Precisely, userspace access to such encrypted
> > > > > > > > memory may lead to host crash so it should be prevented.
> > > > > > > >
> > > > > > > > This patch introduces userspace inaccessible memfd (created with
> > > > > > > > MFD_INACCESSIBLE). Its memory is inaccessible from userspace through
> > > > > > > > ordinary MMU access (e.g. read/write/mmap) but can be accessed via
> > > > > > > > in-kernel interface so KVM can directly interact with core-mm without
> > > > > > > > the need to map the memory into KVM userspace.
> > > > > > > >
> > > > > > > > It provides semantics required for KVM guest private(encrypted) memory
> > > > > > > > support that a file descriptor with this flag set is going to be used as
> > > > > > > > the source of guest memory in confidential computing environments such
> > > > > > > > as Intel TDX/AMD SEV.
> > > > > > > >
> > > > > > > > KVM userspace is still in charge of the lifecycle of the memfd. It
> > > > > > > > should pass the opened fd to KVM. KVM uses the kernel APIs newly added
> > > > > > > > in this patch to obtain the physical memory address and then populate
> > > > > > > > the secondary page table entries.
> > > > > > > >
> > > > > > > > The userspace inaccessible memfd can be fallocate-ed and hole-punched
> > > > > > > > from userspace. When hole-punching happens, KVM can get notified through
> > > > > > > > inaccessible_notifier it then gets chance to remove any mapped entries
> > > > > > > > of the range in the secondary page tables.
> > > > > > > >
> > > > > > > > The userspace inaccessible memfd itself is implemented as a shim layer
> > > > > > > > on top of real memory file systems like tmpfs/hugetlbfs but this patch
> > > > > > > > only implemented tmpfs. The allocated memory is currently marked as
> > > > > > > > unmovable and unevictable, this is required for current confidential
> > > > > > > > usage. But in future this might be changed.
> > > > > > > >
> > > > > > > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > > > > > > Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> > > > > > > > ---
> > > > > > >
> > > > > > > ...
> > > > > > >
> > > > > > > > +static long inaccessible_fallocate(struct file *file, int mode,
> > > > > > > > + loff_t offset, loff_t len)
> > > > > > > > +{
> > > > > > > > + struct inaccessible_data *data = file->f_mapping->private_data;
> > > > > > > > + struct file *memfd = data->memfd;
> > > > > > > > + int ret;
> > > > > > > > +
> > > > > > > > + if (mode & FALLOC_FL_PUNCH_HOLE) {
> > > > > > > > + if (!PAGE_ALIGNED(offset) || !PAGE_ALIGNED(len))
> > > > > > > > + return -EINVAL;
> > > > > > > > + }
> > > > > > > > +
> > > > > > > > + ret = memfd->f_op->fallocate(memfd, mode, offset, len);
> > > > > > > > + inaccessible_notifier_invalidate(data, offset, offset + len);
> > > > > > >
> > > > > > > Wonder if invalidate should precede the actual hole punch, otherwise we open
> > > > > > > a window where the page tables point to memory no longer valid?
> > > > > >
> > > > > > Yes, you are right. Thanks for catching this.
> > > > >
> > > > > I also noticed this. But then thought the memory would be anyways zeroed
> > > > > (hole punched) before this call?
> > > >
> > > > Hole punching can free pages, given that offset/len covers full page.
> > > >
> > > > --
> > > > Kiryl Shutsemau / Kirill A. Shutemov
> > >
> > > I think moving this notifier_invalidate before fallocate may not solve
> > > the problem completely. Is it possible that between invalidate and
> > > fallocate, KVM tries to handle the page fault for the guest VM from
> > > another vcpu and uses the pages to be freed to back gpa ranges? Should
> > > hole punching here also update mem_attr first to say that KVM should
> > > consider the corresponding gpa ranges to be no more backed by
> > > inaccessible memfd?
> >
> > We rely on external synchronization to prevent this. See code around
> > mmu_invalidate_retry_hva().
> >
> > --
> > Kiryl Shutsemau / Kirill A. Shutemov
>
> IIUC, mmu_invalidate_retry_hva/gfn ensures that page faults on gfn
> ranges that are being invalidated are retried till invalidation is
> complete. In this case, is it possible that KVM tries to serve the
> page fault after inaccessible_notifier_invalidate is complete but
> before fallocate could punch hole into the files?
> e.g.
> inaccessible_notifier_invalidate(...)
> ... (system event preempting this control flow, giving a window for
> the guest to retry accessing the gfn range which was invalidated)
> fallocate(.., PUNCH_HOLE..)
Looks this is something can happen. And sounds to me the solution needs
just follow the mmu_notifier's way of using a invalidate_start/end pair.
invalidate_start() --> kvm->mmu_invalidate_in_progress++;
zap KVM page table entries;
fallocate()
invalidate_end() --> kvm->mmu_invalidate_in_progress--;
Then during invalidate_start/end time window mmu_invalidate_retry_gfn
checks 'mmu_invalidate_in_progress' and prevent repopulating the same
page in KVM page table.
if(kvm->mmu_invalidate_in_progress)
return 1; /* retry */
Thanks,
Chao
next prev parent reply other threads:[~2022-10-21 13:59 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
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 [this message]
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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