From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [net-next PATCH] net: ptr_ring: otherwise safe empty checks can overrun array bounds Date: Tue, 2 Jan 2018 18:53:08 +0200 Message-ID: <20180102183322-mutt-send-email-mst@kernel.org> References: <20171228035024.14699.69453.stgit@john-Precision-Tower-5810> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: jakub.kicinski@netronome.com, davem@davemloft.net, xiyou.wangcong@gmail.com, jiri@resnulli.us, netdev@vger.kernel.org To: John Fastabend Return-path: Received: from mx1.redhat.com ([209.132.183.28]:33650 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750960AbeABQxJ (ORCPT ); Tue, 2 Jan 2018 11:53:09 -0500 Content-Disposition: inline In-Reply-To: <20171228035024.14699.69453.stgit@john-Precision-Tower-5810> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, Dec 27, 2017 at 07:50:25PM -0800, John Fastabend wrote: > 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 > --- > include/linux/ptr_ring.h | 7 ++++++- > 1 file changed, 6 insertions(+), 1 deletion(-) > > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h > index 6866df4..13fb06a 100644 > --- a/include/linux/ptr_ring.h > +++ b/include/linux/ptr_ring.h > @@ -447,7 +447,12 @@ static inline int ptr_ring_consume_batched_bh(struct ptr_ring *r, > > static inline void **__ptr_ring_init_queue_alloc(unsigned int size, gfp_t gfp) > { > - return kcalloc(size, sizeof(void *), gfp); > + /* Allocate an extra dummy element at end of ring to avoid consumer head > + * or produce head access past the end of the array. Possible when > + * producer/consumer operations and __ptr_ring_peek operations run in > + * parallel. > + */ > + return kcalloc(size + 1, sizeof(void *), gfp); > } > > static inline void __ptr_ring_set_size(struct ptr_ring *r, int size) Well the peek will return a false negative then, won't it? So I kind of prefer just fixing the consumer. The first step I think would look something like the below untested patch. Pls take a look. I suspect we'll need a memory barrier too. I wonder though: are false positives or negatives ever a problem? Would it be a big deal to just take a lock there, and avoid trying to support a lockless peek? It would definitely be more straight-forward to just remove the promise to support a lockless peek. Thoughts? --> ptr_ring: keep consumer_head valid at all times The comment near __ptr_ring_peek says: * If ring is never resized, and if the pointer is merely * tested, there's no need to take the lock - see e.g. __ptr_ring_empty. but this was in fact never possible. Fixes: c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array") Signed-off-by: Michael S. Tsirkin --- diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h index 37b4bb2..802375f 100644 --- a/include/linux/ptr_ring.h +++ b/include/linux/ptr_ring.h @@ -236,22 +236,28 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) /* Fundamentally, what we want to do is update consumer * index and zero out the entry so producer can reuse it. * Doing it naively at each consume would be as simple as: - * r->queue[r->consumer++] = NULL; - * if (unlikely(r->consumer >= r->size)) - * r->consumer = 0; + * consumer = r->consumer; + * r->queue[consumer++] = NULL; + * if (unlikely(consumer >= r->size)) + * consumer = 0; + * r->consumer = consumer; * but that is suboptimal when the ring is full as producer is writing * out new entries in the same cache line. Defer these updates until a * batch of entries has been consumed. */ - int head = r->consumer_head++; + /* Note: we must keep consumer_head valid at all times for __ptr_ring_peek + * to work correctly. + */ + int consumer_head = r->consumer_head; + int head = consumer_head++; /* Once we have processed enough entries invalidate them in * the ring all at once so producer can reuse their space in the ring. * We also do this when we reach end of the ring - not mandatory * but helps keep the implementation simple. */ - if (unlikely(r->consumer_head - r->consumer_tail >= r->batch || - r->consumer_head >= r->size)) { + if (unlikely(consumer_head - r->consumer_tail >= r->batch || + consumer_head >= r->size)) { /* Zero out entries in the reverse order: this way we touch the * cache line that producer might currently be reading the last; * producer won't make progress and touch other cache lines @@ -259,12 +265,13 @@ static inline void __ptr_ring_discard_one(struct ptr_ring *r) */ while (likely(head >= r->consumer_tail)) r->queue[head--] = NULL; - r->consumer_tail = r->consumer_head; + r->consumer_tail = consumer_head; } - if (unlikely(r->consumer_head >= r->size)) { - r->consumer_head = 0; + if (unlikely(consumer_head >= r->size)) { + consumer_head = 0; r->consumer_tail = 0; } + r->consumer_head = consumer_head; } static inline void *__ptr_ring_consume(struct ptr_ring *r)