linux-kernel - [PATCH 0/4] MOVE/COPY_ENC_CONTEXT

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Message-Id: <20211117163809.1441845-1-pbonzini@redhat.com>
Date:   Wed, 17 Nov 2021 11:38:05 -0500
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     pgonda@...gle.com, seanjc@...gle.com
Subject: [PATCH 0/4] MOVE/COPY_ENC_CONTEXT_FROM locking cleanup and tests

Patches 1 and 2 are the long-awaited tests for COPY_ENC_CONTEXT_FROM,
based on the ones for intra-host migration.  The aim of patches 3
and 4 is to simplify the locking for COPY_ENC_CONTEXT_FROM, and solving
(by sidestepping the question) the problem of a VM's encryption
context being moved from and copied from at the same time.

These patches are an alternative to Sean's patch with subject "KVM:
SEV: Explicitly document that there are no TOCTOU races in copy ASID"
(https://lore.kernel.org/kvm/76c7c752-f1b0-f100-03dd-364366eff02f@redhat.com/T/).

There is another bug: a VM that is the owner of a copied context must not
be migrated, otherwise you could have a dangling ASID:

1. copy context from A to B (gets ref to A)
2. move context from A to L (moves ASID from A to L)
3. close L (releases ASID from L, B still references it)

The right way to do the handoff instead is to create a fresh mirror VM
on the destination first:

1. copy context from A to B (gets ref to A)
[later] 2. close B (releases ref to A)
3. move context from A to L (moves ASID from A to L)
4. copy context from L to M

I'll take a look at this later, probably next week after this series has
been reviewed.

Paolo

Paolo Bonzini (4):
  selftests: sev_migrate_tests: free all VMs
  selftests: sev_migrate_tests: add tests for
    KVM_CAP_VM_COPY_ENC_CONTEXT_FROM
  KVM: SEV: cleanup locking for KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM
  KVM: SEV: Do COPY_ENC_CONTEXT_FROM with both VMs locked

 arch/x86/kvm/svm/sev.c                        | 118 ++++++++----------
 .../selftests/kvm/x86_64/sev_migrate_tests.c  | 113 +++++++++++++++--
 2 files changed, 155 insertions(+), 76 deletions(-)

-- 
2.27.0