netdev - Re: [PATCH net v2] ipv6: fix suspecious RCU usage warning

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Date:   Wed, 10 Mar 2021 11:54:47 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Wei Wang <weiwan@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        syzbot <syzkaller@...glegroups.com>,
        David Ahern <dsahern@...nel.org>,
        Petr Machata <petrm@...dia.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net v2] ipv6: fix suspecious RCU usage warning

On Tue, Mar 09, 2021 at 06:20:35PM -0800, Wei Wang wrote:
> Syzbot reported the suspecious RCU usage in nexthop_fib6_nh() when
> called from ipv6_route_seq_show(). The reason is ipv6_route_seq_start()
> calls rcu_read_lock_bh(), while nexthop_fib6_nh() calls
> rcu_dereference_rtnl().
> The fix proposed is to add a variant of nexthop_fib6_nh() to use
> rcu_dereference_bh_rtnl() for ipv6_route_seq_show().
> 
> The reported trace is as follows:
> ./include/net/nexthop.h:416 suspicious rcu_dereference_check() usage!
> 
> other info that might help us debug this:
> 
> rcu_scheduler_active = 2, debug_locks = 1
> 2 locks held by syz-executor.0/17895:
>      at: seq_read+0x71/0x12a0 fs/seq_file.c:169
>      at: seq_file_net include/linux/seq_file_net.h:19 [inline]
>      at: ipv6_route_seq_start+0xaf/0x300 net/ipv6/ip6_fib.c:2616

[...]

> 
> Fixes: f88d8ea67fbdb ("ipv6: Plumb support for nexthop object in a fib6_info")
> Reported-by: syzbot <syzkaller@...glegroups.com>
> Signed-off-by: Wei Wang <weiwan@...gle.com>
> Cc: David Ahern <dsahern@...nel.org>
> Cc: Ido Schimmel <idosch@...sch.org>
> Cc: Petr Machata <petrm@...dia.com>
> Cc: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Ido Schimmel <idosch@...dia.com>