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    Subject[PATCH 5.10 015/545] KVM: nVMX: Snapshot pre-VM-Enter DEBUGCTL for !nested_run_pending case
    Date
    From: Sean Christopherson <seanjc@google.com>

    commit 764643a6be07445308e492a528197044c801b3ba upstream.

    If a nested run isn't pending, snapshot vmcs01.GUEST_IA32_DEBUGCTL
    irrespective of whether or not VM_ENTRY_LOAD_DEBUG_CONTROLS is set in
    vmcs12. When restoring nested state, e.g. after migration, without a
    nested run pending, prepare_vmcs02() will propagate
    nested.vmcs01_debugctl to vmcs02, i.e. will load garbage/zeros into
    vmcs02.GUEST_IA32_DEBUGCTL.

    If userspace restores nested state before MSRs, then loading garbage is a
    non-issue as loading DEBUGCTL will also update vmcs02. But if usersepace
    restores MSRs first, then KVM is responsible for propagating L2's value,
    which is actually thrown into vmcs01, into vmcs02.

    Restoring L2 MSRs into vmcs01, i.e. loading all MSRs before nested state
    is all kinds of bizarre and ideally would not be supported. Sadly, some
    VMMs do exactly that and rely on KVM to make things work.

    Note, there's still a lurking SMM bug, as propagating vmcs01's DEBUGCTL
    to vmcs02 across RSM may corrupt L2's DEBUGCTL. But KVM's entire VMX+SMM
    emulation is flawed as SMI+RSM should not toouch _any_ VMCS when use the
    "default treatment of SMIs", i.e. when not using an SMI Transfer Monitor.

    Link: https://lore.kernel.org/all/Yobt1XwOfb5M6Dfa@google.com
    Fixes: 8fcc4b5923af ("kvm: nVMX: Introduce KVM_CAP_NESTED_STATE")
    Cc: stable@vger.kernel.org
    Signed-off-by: Sean Christopherson <seanjc@google.com>
    Message-Id: <20220614215831.3762138-3-seanjc@google.com>
    Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kvm/vmx/nested.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- a/arch/x86/kvm/vmx/nested.c
    +++ b/arch/x86/kvm/vmx/nested.c
    @@ -3337,7 +3337,8 @@ enum nvmx_vmentry_status nested_vmx_ente
    if (likely(!evaluate_pending_interrupts) && kvm_vcpu_apicv_active(vcpu))
    evaluate_pending_interrupts |= vmx_has_apicv_interrupt(vcpu);

    - if (!(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
    + if (!vmx->nested.nested_run_pending ||
    + !(vmcs12->vm_entry_controls & VM_ENTRY_LOAD_DEBUG_CONTROLS))
    vmx->nested.vmcs01_debugctl = vmcs_read64(GUEST_IA32_DEBUGCTL);
    if (kvm_mpx_supported() &&
    (!vmx->nested.nested_run_pending ||

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