Re: [PATCH] x86/sgx: Fix deadlock and race conditions between fork() and EPC reclaim

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Wed, Mar 18, 2020 at 05:50:43PM +0200, Jarkko Sakkinen wrote:
> On Wed, Mar 18, 2020 at 05:39:07PM +0200, Jarkko Sakkinen wrote:
> > On Mon, Mar 16, 2020 at 10:15:39PM -0700, Sean Christopherson wrote:
> > > @@ -223,10 +229,22 @@ int sgx_encl_mm_add(struct sgx_encl *encl, struct mm_struct *mm)
> > >  
> > >  	spin_lock(&encl->mm_lock);
> > >  	list_add_rcu(&encl_mm->list, &encl->mm_list);
> > > +	/*
> > > +	 * Ensure the mm is added to the list before updating the version.
> > > +	 * Pairs with the smp_rmb() in sgx_reclaimer_block().
> > > +	 */
> > > +	smp_wmb();
> > > +	encl->mm_list_version++;
> > >  	spin_unlock(&encl->mm_lock);
> > >  
> > > -	synchronize_srcu(&encl->srcu);
> > > -
> > > +	/*
> > > +	 * DO NOT call synchronize_srcu()!  When this is called via dup_mmap(),
> > > +	 * mmap_sem is held for write in both the old mm and new mm, and the
> > > +	 * reclaimer may be holding srcu for read while waiting on down_read()
> > > +	 * for the old mm's mmap_sem, i.e. synchronizing will deadlock.
> > > +	 * Incrementing the list version ensures readers that must not race
> > > +	 * with a mm being added will see the updated list.
> > > +	 */
> 
> For this comment, please completely remove it. We either call something
> or do not call it. We do !call anything.

How on earth is someone doing to dredge up the above information without a
comment?  Anyone looking at this code without a priori knowledge of the
development history will assume the missing synchronize_srcu() is a bug.

> > >  	return 0;
> > >  }
> > >  
> > > diff --git a/arch/x86/kernel/cpu/sgx/encl.h b/arch/x86/kernel/cpu/sgx/encl.h
> > > index 44b353aa8866..f0f72e591244 100644
> > > --- a/arch/x86/kernel/cpu/sgx/encl.h
> > > +++ b/arch/x86/kernel/cpu/sgx/encl.h
> > > @@ -74,6 +74,7 @@ struct sgx_encl {
> > >  	struct mutex lock;
> > >  	struct list_head mm_list;
> > >  	spinlock_t mm_lock;
> > > +	unsigned long mm_list_version;
> > >  	struct file *backing;
> > >  	struct kref refcount;
> > >  	struct srcu_struct srcu;