Subject: Re: possible deadlock in rtnl_lock (5)

From: Florian Westphal <[email-masked]>
Date: Tue, 27 Mar 2018 13:50:01 +0200

syzbot <[email-masked]> wrote:
[ cc Julian and trimming cc list ]

> syzkaller688027/4497 is trying to acquire lock:
>  (rtnl_mutex){+.+.}, at: [<00000000bb14d7fb>] rtnl_lock+0x17/0x20
> net/core/rtnetlink.c:74

> but task is already holding lock:
> IPVS: stopping backup sync thread 4495 ...
>  (rtnl_mutex){+.+.}, at: [<00000000bb14d7fb>] rtnl_lock+0x17/0x20
> net/core/rtnetlink.c:74
> 
> other info that might help us debug this:
>  Possible unsafe locking scenario:
> 
>        CPU0
>        ----
>   lock(rtnl_mutex);
>   lock(rtnl_mutex);
> 
>  *** DEADLOCK ***
> 
>  May be due to missing lock nesting notation

Looks like this is real, commit e0b26cc997d57305b4097711e12e13992580ae34
("ipvs: call rtnl_lock early") added rtnl_lock when starting sync thread
but socket close invokes rtnl_lock too:

> stack backtrace:
>  rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
>  ip_mc_drop_socket+0x88/0x230 net/ipv4/igmp.c:2643
>  inet_release+0x4e/0x1c0 net/ipv4/af_inet.c:413
>  sock_release+0x8d/0x1e0 net/socket.c:595
>  start_sync_thread+0x2213/0x2b70 net/netfilter/ipvs/ip_vs_sync.c:1924
>  do_ip_vs_set_ctl+0x1139/0x1cc0 net/netfilter/ipvs/ip_vs_ctl.c:2389