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Date:   Fri, 8 Jan 2021 08:34:16 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Your Real Name <zhutong@...zon.com>
Cc:     David Miller <davem@...emloft.net>, sashal@...nel.org,
        edumazet@...gle.com, vvs@...tuozzo.com, netdev@...r.kernel.org,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] neighbour: Disregard DEAD dst in neigh_update

On Thu, Jan 07, 2021 at 09:36:37PM -0500, Your Real Name wrote:
> On Tue, Jan 05, 2021 at 04:05:21PM -0800, David Miller wrote: 
> > 
> > 
> > From: Tong Zhu <zhutong@...zon.com>
> > Date: Wed, 30 Dec 2020 17:54:23 -0500
> > 
> > > In 4.x kernel a dst in DST_OBSOLETE_DEAD state is associated
> > > with loopback net_device and leads to loopback neighbour. It
> > > leads to an ethernet header with all zero addresses.
> > >
> > > A very troubling case is working with mac80211 and ath9k.
> > > A packet with all zero source MAC address to mac80211 will
> > > eventually fail ieee80211_find_sta_by_ifaddr in ath9k (xmit.c).
> > > As result, ath9k flushes tx queue (ath_tx_complete_aggr) without
> > > updating baw (block ack window), damages baw logic and disables
> > > transmission.
> > >
> > > Signed-off-by: Tong Zhu <zhutong@...zon.com>
> > 
> > Please repost with an appropriate Fixes: tag.
> > 
> > Thanks.
> 
> I had a second thought on this. This fix should go mainline too. This is a 
> case we are sending out queued packets when arp reply from the neighbour 
> comes in. With 5.x kernel, a dst in DST_OBSOLETE_DEAD state leads to dropping
> of this packet. It is not as bad as with 4.x kernel that may end up with an
> all-zero mac address packet out to ethernet or choking up ath9k when using 
> block ack. Dropping the packet is still wrong. I’ll repost as a fix to
> mainline and target backport to 4.x LTS releases.

That's how kernel development works, please read
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how stable kernels are allowed to accept patches.

good luck!

greg k-h

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