Re: [PATCH v2] Fix the issue further discussed in:

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

On Mon, Oct 05, 2020 at 12:11:39PM +0100, Matthew Wilcox wrote:
> On Mon, Oct 05, 2020 at 06:17:59AM +0300, Jarkko Sakkinen wrote:
> > @@ -317,10 +318,31 @@ 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)
> > +	/*
> > +	 * 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.
> > +	 */
> > +	xas_lock(&xas);
> > +	xas_for_each(&xas, page, idx_end) {
> > +		if (++count % XA_CHECK_SCHED)
> > +			continue;
> 
> This really doesn't do what you think it does.
> 
> 	int ret = 0;
> 	int count = 0;
> 
> 	xas_lock(&xas);
> 	while (xas.index < idx_end) {
> 		struct sgx_page *page = xas_next(&xas);
> 
> 		if (!page || (~page->vm_max_prot_bits & vm_prot_bits)) {
> 			ret = -EACCESS;
> 			break;
> 		}
> 
> 		if (++count % XA_CHECK_SCHED)
> 			continue;
> 		xas_pause(&xas);
> 		xas_unlock(&xas);
> 		cond_resched();
> 		xas_lock(&xas);
> 	}
> 	xas_unlock(&xas);
> 
> 	return ret;

No mine certainly does not, it locks up the system if the loop succeeds
(i.e. does not return -EACCESS) :-) Unfortunately had by mistake the v1
patch (xa_load()) in the kernel that I used to test.

/Jarkko