From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Miller Subject: Re: [net-next PATCH] net: ptr_ring: otherwise safe empty checks can overrun array bounds Date: Tue, 02 Jan 2018 13:33:16 -0500 (EST) Message-ID: <20180102.133316.589391273753601377.davem@davemloft.net> References: <20171228035024.14699.69453.stgit@john-Precision-Tower-5810> <20180102.115219.1101472320429215260.davem@davemloft.net> <20180102190107-mutt-send-email-mst@kernel.org> Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: john.fastabend@gmail.com, jakub.kicinski@netronome.com, xiyou.wangcong@gmail.com, jiri@resnulli.us, netdev@vger.kernel.org To: mst@redhat.com Return-path: Received: from shards.monkeyblade.net ([184.105.139.130]:50728 "EHLO shards.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751029AbeABSdS (ORCPT ); Tue, 2 Jan 2018 13:33:18 -0500 In-Reply-To: <20180102190107-mutt-send-email-mst@kernel.org> Sender: netdev-owner@vger.kernel.org List-ID: From: "Michael S. Tsirkin" Date: Tue, 2 Jan 2018 19:01:33 +0200 > On Tue, Jan 02, 2018 at 11:52:19AM -0500, David Miller wrote: >> From: John Fastabend >> Date: Wed, 27 Dec 2017 19:50:25 -0800 >> >> > When running consumer and/or producer operations and empty checks in >> > parallel its possible to have the empty check run past the end of the >> > array. The scenario occurs when an empty check is run while >> > __ptr_ring_discard_one() is in progress. Specifically after the >> > consumer_head is incremented but before (consumer_head >= ring_size) >> > check is made and the consumer head is zeroe'd. >> > >> > To resolve this, without having to rework how consumer/producer ops >> > work on the array, simply add an extra dummy slot to the end of the >> > array. Even if we did a rework to avoid the extra slot it looks >> > like the normal case checks would suffer some so best to just >> > allocate an extra pointer. >> > >> > Reported-by: Jakub Kicinski >> > Fixes: c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array") >> > Signed-off-by: John Fastabend >> >> Applied, thanks John. > > I think that patch is wrong. I'd rather it got reverted. I agree with John's logic, the asynchronous test is always safe in this parallel access case and John's change solves the out-of-bounds test. If you propose a cleaner way to handle this as a follow-on patch I'll be happy to consider it. Thanks.