Re: [PATCH v3] x86/sgx: Fix sgx_encl_may_map locking

[Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>]

On Mon, Oct 05, 2020 at 08:42:07PM +0300, Jarkko Sakkinen wrote:
> On Mon, Oct 05, 2020 at 08:55:19AM -0700, Sean Christopherson wrote:
> > On Mon, Oct 05, 2020 at 05:11:19PM +0300, Jarkko Sakkinen wrote:
> > > @@ -317,11 +319,30 @@ int sgx_encl_may_map(struct sgx_encl *encl, unsigned long start,
> > >  	if (current->personality & READ_IMPLIES_EXEC)
> > >  		return -EACCES;
> > >  
> > > -	xas_for_each(&xas, page, idx_end)
> > > -		if (!page || (~page->vm_max_prot_bits & vm_prot_bits))
> > > -			return -EACCES;
> > > +	/*
> > > +	 * No need to hold encl->lock:
> > > +	 * 1. None of the page->* get written.
> > > +	 * 2. page->vm_max_prot_bits is set in sgx_encl_page_alloc(). This
> > > +	 *    is before calling xa_insert(). After that it is never modified.
> > > +	 */
> > 
> > You forgot to cover racing with insertion, e.g. below is the snippet from my
> > original patch[*], which did the lookup without protection from encl->lock.`
> > 
> > +		/*
> > +		 * No need to take encl->lock, vm_prot_bits is set prior to
> > +		 * insertion and never changes, and racing with adding pages is
> > +		 * a userspace bug.
> > +		 */
> > +		rcu_read_lock();
> > +		page = radix_tree_lookup(&encl->page_tree, idx);
> > +		rcu_read_unlock();
> > 
> > 
> > [*]https://patchwork.kernel.org/patch/11005431/
> 
> I'm not sure why that was merged as it was but it probably was not
> because of that snippet. It had encl->lock before, so it was by all
> practical means covered then. I would have replaced encl->lock with that
> if I ever had received a patch with just that, i.e. that particular
> snippet gone to noise. That's why it was not broken before v36.

Or even if that patch was squashed (can't recall that far), it was
not intentionally not replaced. I have no idea why encl->lock was
not replaced but the code was not broken.

/Jarkko