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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 14 Jun 2022 13:59:41 -0700
Message-ID: <CALCETrUdGoZ2yUnNGbxJ-Xr3KD7QhTi-ddhS8AUMjFyJM5pDfA@mail.gmail.com>
Subject: Re: [PATCH v6 0/8] KVM: mm: fd-based approach for supporting KVM
 guest private memory
To:     Sean Christopherson <seanjc@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Chao Peng <chao.p.peng@linux.intel.com>,
        Vishal Annapurve <vannapurve@google.com>,
        Marc Orr <marcorr@google.com>, kvm list <kvm@vger.kernel.org>,
        LKML <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>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        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 <x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>,
        Hugh Dickins <hughd@google.com>,
        Jeff Layton <jlayton@kernel.org>,
        "J . Bruce Fields" <bfields@fieldses.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mike Rapoport <rppt@kernel.org>,
        Steven Price <steven.price@arm.com>,
        "Maciej S . Szmigiero" <mail@maciej.szmigiero.name>,
        Vlastimil Babka <vbabka@suse.cz>,
        Yu Zhang <yu.c.zhang@linux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Jun Nakajima <jun.nakajima@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        David Hildenbrand <david@redhat.com>, aarcange@redhat.com,
        ddutile@redhat.com, dhildenb@redhat.com,
        Quentin Perret <qperret@google.com>,
        Michael Roth <michael.roth@amd.com>, mhocko@suse.com
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On Tue, Jun 14, 2022 at 12:09 PM Sean Christopherson <seanjc@google.com> wrote:
>
> On Tue, Jun 14, 2022, Andy Lutomirski wrote:
> > On Tue, Jun 14, 2022 at 12:32 AM Chao Peng <chao.p.peng@linux.intel.com> wrote:
> > >
> > > On Thu, Jun 09, 2022 at 08:29:06PM +0000, Sean Christopherson wrote:
> > > > On Wed, Jun 08, 2022, Vishal Annapurve wrote:
> > > >
> > > > One argument is that userspace can simply rely on cgroups to detect misbehaving
> > > > guests, but (a) those types of OOMs will be a nightmare to debug and (b) an OOM
> > > > kill from the host is typically considered a _host_ issue and will be treated as
> > > > a missed SLO.
> > > >
> > > > An idea for handling this in the kernel without too much complexity would be to
> > > > add F_SEAL_FAULT_ALLOCATIONS (terrible name) that would prevent page faults from
> > > > allocating pages, i.e. holes can only be filled by an explicit fallocate().  Minor
> > > > faults, e.g. due to NUMA balancing stupidity, and major faults due to swap would
> > > > still work, but writes to previously unreserved/unallocated memory would get a
> > > > SIGSEGV on something it has mapped.  That would allow the userspace VMM to prevent
> > > > unintentional allocations without having to coordinate unmapping/remapping across
> > > > multiple processes.
> > >
> > > Since this is mainly for shared memory and the motivation is catching
> > > misbehaved access, can we use mprotect(PROT_NONE) for this? We can mark
> > > those range backed by private fd as PROT_NONE during the conversion so
> > > subsequence misbehaved accesses will be blocked instead of causing double
> > > allocation silently.
>
> PROT_NONE, a.k.a. mprotect(), has the same vma downsides as munmap().
>
> > This patch series is fairly close to implementing a rather more
> > efficient solution.  I'm not familiar enough with hypervisor userspace
> > to really know if this would work, but:
> >
> > What if shared guest memory could also be file-backed, either in the
> > same fd or with a second fd covering the shared portion of a memslot?
> > This would allow changes to the backing store (punching holes, etc) to
> > be some without mmap_lock or host-userspace TLB flushes?  Depending on
> > what the guest is doing with its shared memory, userspace might need
> > the memory mapped or it might not.
>
> That's what I'm angling for with the F_SEAL_FAULT_ALLOCATIONS idea.  The issue,
> unless I'm misreading code, is that punching a hole in the shared memory backing
> store doesn't prevent reallocating that hole on fault, i.e. a helper process that
> keeps a valid mapping of guest shared memory can silently fill the hole.
>
> What we're hoping to achieve is a way to prevent allocating memory without a very
> explicit action from userspace, e.g. fallocate().

Ah, I misunderstood.  I thought your goal was to mmap it and prevent
page faults from allocating.

It is indeed the case (and has been since before quite a few of us
were born) that a hole in a sparse file is logically just a bunch of
zeros.  A way to make a file for which a hole is an actual hole seems
like it would solve this problem nicely.  It could also be solved more
specifically for KVM by making sure that the private/shared mode that
userspace programs is strict enough to prevent accidental allocations
-- if a GPA is definitively private, shared, neither, or (potentially,
on TDX only) both, then a page that *isn't* shared will never be
accidentally allocated by KVM.  If the shared backing is not mmapped,
it also won't be accidentally allocated by host userspace on a stray
or careless write.


--Andy