Re: [PATCH v3] x86/sgx: Synchronize encl->srcu in sgx_encl_release().

[Haitao Huang]

On Wed, 16 Dec 2020 07:49:20 -0600, Jarkko Sakkinen <jarkko@xxxxxxxxxx>  
wrote:

> Add synchronize_srcu_expedited() to sgx_encl_release() to catch a grace
> period initiated by sgx_mmu_notifier_release().
>
> A trivial example of a failing sequence with tasks A and B:
>
> 1. A: -> sgx_release()
> 2. B: -> sgx_mmu_notifier_release()
> 3. B: -> list_del_rcu()
> 3. A: -> sgx_encl_release()
> 4. A: -> cleanup_srcu_struct()
>
> The loop in sgx_release() observes an empty list because B has removed  
> its
> entry in the middle, and calls cleanup_srcu_struct() before B has a  
> chance
> to calls synchronize_srcu().
>
> Fixes: 1728ab54b4be ("x86/sgx: Add a page reclaimer")
> Suggested-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> Suggested-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
> Signed-off-by: Jarkko Sakkinen <jarkko@xxxxxxxxxx>
> ---
> v3: Fine-tuned tags, and added missing change log for v2.
> v2: Switch to synchronize_srcu_expedited().
>  arch/x86/kernel/cpu/sgx/encl.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/arch/x86/kernel/cpu/sgx/encl.c  
> b/arch/x86/kernel/cpu/sgx/encl.c
> index ee50a5010277..fe7256db6e73 100644
> --- a/arch/x86/kernel/cpu/sgx/encl.c
> +++ b/arch/x86/kernel/cpu/sgx/encl.c
> @@ -438,6 +438,12 @@ void sgx_encl_release(struct kref *ref)
>  	if (encl->backing)
>  		fput(encl->backing);
> +	/*
> +	 * Each sgx_mmu_notifier_release() starts a grace period. Therefore, an
> +	 * additional sync is required here.
> +	 */
> +	synchronize_srcu_expedited(&encl->srcu);
> +
>  	cleanup_srcu_struct(&encl->srcu);
> 	WARN_ON_ONCE(!list_empty(&encl->mm_list));

Tested-by: Haitao Huang <haitao.huang@xxxxxxxxxxxxxxx>
Thanks
Haitao