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

From: Matthew Wilcox <willy@infradead.org>
To: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Cc: linux-sgx@vger.kernel.org,
	Haitao Huang <haitao.huang@linux.intel.com>,
	Sean Christopherson <sean.j.christopherson@intel.com>,
	Jethro Beekman <jethro@fortanix.com>,
	Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [PATCH v2] Fix the issue further discussed in:
Date: Mon, 5 Oct 2020 12:11:39 +0100	[thread overview]
Message-ID: <20201005111139.GK20115@casper.infradead.org> (raw)
In-Reply-To: <20201005031759.143544-1-jarkko.sakkinen@linux.intel.com>

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;