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.shute...@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.shute...@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.shute...@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.shute...@linux.intel.com>
> > > > > > > > Signed-off-by: Chao Peng <chao.p.p...@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
