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Date:   Mon, 1 Feb 2021 17:09:20 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Chinmay Agarwal <chinagar@codeaurora.org>
Cc:     xiyou.wangcong@gmail.com, netdev@vger.kernel.org,
        davem@davemloft.net, sharathv@codeaurora.org
Subject: Re: [PATCH] neighbour: Prevent a dead entry from updating gc_list
Message-ID: <20210201170920.6fa1204e@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
In-Reply-To: <20210127165453.GA20514@chinagar-linux.qualcomm.com>
References: <20210127165453.GA20514@chinagar-linux.qualcomm.com>
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On Wed, 27 Jan 2021 22:24:54 +0530 Chinmay Agarwal wrote:
> Following race condition was detected:
> <CPU A, t0> - neigh_flush_dev() is under execution and calls
> neigh_mark_dead(n) marking the neighbour entry 'n' as dead.
> 
> <CPU B, t1> - Executing: __netif_receive_skb() ->
> __netif_receive_skb_core() -> arp_rcv() -> arp_process().arp_process()
> calls __neigh_lookup() which takes a reference on neighbour entry 'n'.
> 
> <CPU A, t2> - Moves further along neigh_flush_dev() and calls
> neigh_cleanup_and_release(n), but since reference count increased in t2,
> 'n' couldn't be destroyed.
> 
> <CPU B, t3> - Moves further along, arp_process() and calls
> neigh_update()-> __neigh_update() -> neigh_update_gc_list(), which adds
> the neighbour entry back in gc_list(neigh_mark_dead(), removed it
> earlier in t0 from gc_list)
> 
> <CPU B, t4> - arp_process() finally calls neigh_release(n), destroying
> the neighbour entry.
> 
> This leads to 'n' still being part of gc_list, but the actual
> neighbour structure has been freed.
> 
> The situation can be prevented from happening if we disallow a dead
> entry to have any possibility of updating gc_list. This is what the
> patch intends to achieve.
> 
> Fixes: 9c29a2f55ec0 ("neighbor: Fix locking order for gc_list changes")
> Signed-off-by: Chinmay Agarwal <chinagar@codeaurora.org>

Applied, thanks!