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

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Date:   Tue, 02 Jan 2018 11:52:19 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     john.fastabend@...il.com
Cc:     jakub.kicinski@...ronome.com, mst@...hat.com,
        xiyou.wangcong@...il.com, jiri@...nulli.us, netdev@...r.kernel.org
Subject: Re: [net-next PATCH] net: ptr_ring: otherwise safe empty checks
 can overrun array bounds

From: John Fastabend <john.fastabend@...il.com>
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 <jakub.kicinski@...ronome.com>
> Fixes: c5ad119fb6c09 ("net: sched: pfifo_fast use skb_array")
> Signed-off-by: John Fastabend <john.fastabend@...il.com>

Applied, thanks John.