linux-kernel - Re: [PATCH 09/12] KVM: SEV: Do COPY_ENC_CONTEXT

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Date:   Mon, 29 Nov 2021 23:08:46 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        pgonda@...gle.com
Subject: Re: [PATCH 09/12] KVM: SEV: Do COPY_ENC_CONTEXT_FROM with both VMs
 locked

On Mon, Nov 22, 2021, Paolo Bonzini wrote:
> Now that we have a facility to lock two VMs with deadlock
> protection, use it for the creation of mirror VMs as well.  One of
> COPY_ENC_CONTEXT_FROM(dst, src) and COPY_ENC_CONTEXT_FROM(src, dst)
> would always fail, so the combination is nonsensical and it is okay to
> return -EBUSY if it is attempted.
> 
> This sidesteps the question of what happens if a VM is
> MOVE_ENC_CONTEXT_FROM'd at the same time as it is
> COPY_ENC_CONTEXT_FROM'd: the locking prevents that from
> happening.
> 
> Cc: Peter Gonda <pgonda@...gle.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/kvm/svm/sev.c | 69 +++++++++++++++++-------------------------
>  1 file changed, 27 insertions(+), 42 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index c1eb1c83401d..025d9731b66c 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -1548,6 +1548,9 @@ static int sev_lock_two_vms(struct kvm *dst_kvm, struct kvm *src_kvm)
>  	struct kvm_sev_info *dst_sev = &to_kvm_svm(dst_kvm)->sev_info;
>  	struct kvm_sev_info *src_sev = &to_kvm_svm(src_kvm)->sev_info;
>  
> +	if (dst_kvm == src_kvm)
> +		return -EINVAL;

This should go into 

  KVM: SEV: cleanup locking for KVM_CAP_VM_MOVE_ENC_CONTEXT_FROM
 
because before that, KVM would never attempt mutex_lock() on the second VM, one
of the SEV || !SEV was guaranteed to fail.

With that change,

Reviewed-by: Sean Christopherson <seanjc@...gle.com>