netdev - Re: [PATCH] neighbour: Prevent a dead entry from updating gc

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Date:   Mon, 25 Jan 2021 13:07:06 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Chinmay Agarwal <chinagar@...eaurora.org>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        sharathv@...eaurora.org
Subject: Re: [PATCH] neighbour: Prevent a dead entry from updating gc_list

On Mon, Jan 25, 2021 at 11:59 AM Chinmay Agarwal
<chinagar@...eaurora.org> 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.
>
> Signed-off-by: Chinmay Agarwal <chinagar@...eaurora.org>

Please add a Fixes tag for bug fixes, in this case it is probably:

Fixes: 9c29a2f55ec0 ("neighbor: Fix locking order for gc_list changes")

And, make sure you run checkpatch.pl before sending out. For your
patch, it will definitely complain about the missing spaces around the
assignment "new=old;".

Thanks.