Re: [PATCH 1/3] KVM: x86: Don't re-acquire SRCU lock in complete_emulated_io()

Maxim Levitsky
2022-04-19T08:55:31Z

On Fri, 2022-04-15 at 00:43 +0000, Sean Christopherson wrote:
> Don't re-acquire SRCU in complete_emulated_io() now that KVM acquires the
> lock in kvm_arch_vcpu_ioctl_run(). More importantly, don't overwrite
> vcpu->srcu_idx. If the index acquired by complete_emulated_io() differs
> from the one acquired by kvm_arch_vcpu_ioctl_run(), KVM will effectively
> leak a lock and hang if/when synchronize_srcu() is invoked for the
> relevant grace period.
>
> Fixes: 8d25b7beca7e ("KVM: x86: pull kvm->srcu read-side to kvm_arch_vcpu_ioctl_run")
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/x86.c | 7 +------
> 1 file changed, 1 insertion(+), 6 deletions(-)
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index ab336f7c82e4..f35fe09de59d 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10450,12 +10450,7 @@ static int vcpu_run(struct kvm_vcpu *vcpu)
>
> static inline int complete_emulated_io(struct kvm_vcpu *vcpu)
> {
> - int r;
> -
> - vcpu->srcu_idx = srcu_read_lock(&vcpu->kvm->srcu);
> - r = kvm_emulate_instruction(vcpu, EMULTYPE_NO_DECODE);
> - srcu_read_unlock(&vcpu->kvm->srcu, vcpu->srcu_idx);
> - return r;
> + return kvm_emulate_instruction(vcpu, EMULTYPE_NO_DECODE);
> }
>
> static int complete_emulated_pio(struct kvm_vcpu *vcpu)

I wonder how this did work....

Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>

Best regards,
Maxim Levitsky