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

[Jarkko Sakkinen]

On Mon, Jan 04, 2021 at 02:22:05PM -0600, Haitao Huang wrote:
> 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.

/Jarkko