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

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Message-ID: <58b836cd-108d-2248-5206-1aade48153dd@gmail.com>
Date:   Sat, 30 Jan 2021 09:05:27 -0700
From:   David Ahern <dsahern@...il.com>
To:     Chinmay Agarwal <chinagar@...eaurora.org>,
        xiyou.wangcong@...il.com, Jakub Kicinski <kuba@...nel.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net,
        sharathv@...eaurora.org
Subject: Re: [PATCH] neighbour: Prevent a dead entry from updating gc_list

On 1/27/21 9:54 AM, 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")

always Cc the author(s) of commits in Fixes tag.

> Signed-off-by: Chinmay Agarwal <chinagar@...eaurora.org>
> ---
>  net/core/neighbour.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 

Reviewed-by: David Ahern <dsahern@...nel.org>