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From: Sean Christopherson <seanjc@google.com>
To: Marc Orr <marcorr@google.com>
Cc: Peter Gonda <pgonda@google.com>, kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	David Rientjes <rientjes@google.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Brijesh Singh <brijesh.singh@amd.com>,
	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>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration
Date: Fri, 20 Aug 2021 14:50:39 +0000	[thread overview]
Message-ID: <YR/BPwnj6Nudgu1r@google.com> (raw)
In-Reply-To: <CAA03e5FAXDVSwMAQO57gztYmB2K8K8fNrHwsX_N3Hbgwch8pBw@mail.gmail.com>

On Thu, Aug 19, 2021, Marc Orr wrote:
> On Thu, Aug 19, 2021 at 3:58 PM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Thu, Aug 19, 2021, Peter Gonda wrote:
> > > Marc I think that only having the spin lock could result in
> > > deadlocking. If userspace double migrated 2 VMs, A and B for
> > > discussion, A could grab VM_A.spin_lock then VM_A.kvm_mutex. Meanwhile
> > > B could grab VM_B.spin_lock and VM_B.kvm_mutex. Then A attempts to
> > > grab VM_B.spin_lock and we have a deadlock. If the same happens with
> > > the proposed scheme when A attempts to lock B, VM_B.spin_lock will be
> > > open but the bool will mark the VM under migration so A will unlock
> > > and bail. Sean originally proposed a global spin lock but I thought a
> > > per kvm_sev_info struct would also be safe.
> >
> > Close.  The issue is taking kvm->lock from both VM_A and VM_B.  If userspace
> > double migrates we'll end up with lock ordering A->B and B-A, so we need a way
> > to guarantee one of those wins.  My proposed solution is to use a flag as a sort
> > of one-off "try lock" to detect a mean userspace.
> 
> Got it now. Thanks to you both, for the explanation. By the way, just
> to make sure I completely follow, I assume that if a "double
> migration" occurs, then user space is mis-behaving -- correct?

Yep.

> But presumably, we need to reason about how to respond to such mis-behavior
> so that buggy or malicious user-space code cannot stumble over/exploit this
> scenario?

That's what the anti-deadlock flag is for. :-)  With that in place, there's no
meaningful difference between say a bad userspace doing double migrate and a bad
userspace migrating from garbage, e.g. passing in a bogus fd.

  reply	other threads:[~2021-08-20 14:50 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-19 15:49 [PATCH 0/2 V4] Add AMD SEV and SEV-ES intra host migration support Peter Gonda
2021-08-19 15:49 ` [PATCH 1/2 V4] KVM, SEV: Add support for SEV intra host migration Peter Gonda
2021-08-19 16:23   ` Marc Orr
2021-08-19 21:00     ` Peter Gonda
2021-08-19 22:58       ` Sean Christopherson
2021-08-20  6:35         ` Marc Orr
2021-08-20 14:50           ` Sean Christopherson [this message]
2021-08-20 20:53         ` Marc Orr
2021-08-23 16:39           ` Peter Gonda
2021-08-19 15:49 ` [PATCH 2/2 V4] KVM, SEV: Add support for SEV-ES " Peter Gonda
2021-08-20 21:00   ` Marc Orr
2021-08-23 16:38     ` Peter Gonda

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