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 13:33:16 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     mst@...hat.com
Cc:     john.fastabend@...il.com, jakub.kicinski@...ronome.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: "Michael S. Tsirkin" <mst@...hat.com>
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 <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.
> 
> 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.