netdev - Re: [syzbot] [net?] WARNING: refcount bug in inet_twsk

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Message-ID: <20240811133200.GC13736@breakpoint.cc>
Date: Sun, 11 Aug 2024 15:32:00 +0200
From: Florian Westphal <fw@...len.de>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com,
	davem@...emloft.net, dsahern@...nel.org, edumazet@...gle.com,
	kuba@...nel.org, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, pabeni@...hat.com,
	syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [net?] WARNING: refcount bug in inet_twsk_kill

Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> From: syzbot <syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com>
> Date: Sat, 10 Aug 2024 18:29:20 -0700
> > Hello,
> > 
> > syzbot found the following issue on:
> > 
> > HEAD commit:    33e02dc69afb Merge tag 'sound-6.10-rc1' of git://git.kerne..
> > git tree:       upstream
> > console output: https://syzkaller.appspot.com/x/log.txt?x=117f3182980000
> > kernel config:  https://syzkaller.appspot.com/x/.config?x=25544a2faf4bae65
> > dashboard link: https://syzkaller.appspot.com/bug?extid=8ea26396ff85d23a8929
> > compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
> > 
> > Unfortunately, I don't have any reproducer for this issue yet.
> > 
> > Downloadable assets:
> > disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/7bc7510fe41f/non_bootable_disk-33e02dc6.raw.xz
> > vmlinux: https://storage.googleapis.com/syzbot-assets/573c88ac3233/vmlinux-33e02dc6.xz
> > kernel image: https://storage.googleapis.com/syzbot-assets/760a52b9a00a/bzImage-33e02dc6.xz
> > 
> > IMPORTANT: if you fix the issue, please add the following tag to the commit:
> > Reported-by: syzbot+8ea26396ff85d23a8929@...kaller.appspotmail.com
> > 
> > ------------[ cut here ]------------
> > refcount_t: decrement hit 0; leaking memory.
> > WARNING: CPU: 3 PID: 1396 at lib/refcount.c:31 refcount_warn_saturate+0x1ed/0x210 lib/refcount.c:31
> 
> Eric, this is the weird report I was talking about at netdevconf :)
> 
> It seems refcount_dec(&tw->tw_dr->tw_refcount) is somehow done earlier
> than refcount_inc().
> 
> I started to see the same splat at a very low rate after consuming
> commit b334b924c9b7 ("net: tcp/dccp: prepare for tw_timer un-pinning").

Could you share a splat you saw?

Was it also for the fallback tcp_hashinfo and/or from error
unwindinding after a ops->init() failure on netns creation?

I don't think it makes sense following this specific report given
b334b924c9b7 made changes in this area, but a post-b334b924c9b7
would be more relevant I think.