Subject: Re: [net-next PATCH] net: ptr_ring: otherwise safe empty checks can overrun array bounds

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 2 Jan 2018 18:53:08 +0200

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 <[email-masked]>
> Fixes: c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array")
> Signed-off-by: John Fastabend <[email-masked]>

> ---
>  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 <[email-masked]>

---

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)