linux-kernel - Re: [syzbot] [nfs?] INFO: task hung in nfsd_nl_listener_get

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Date: Mon, 17 Jun 2024 07:51:29 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Jeff Layton <jlayton@...nel.org>
Cc: syzbot <syzbot+4207adf14e7c0981d28d@...kaller.appspotmail.com>,
 Dai.Ngo@...cle.com, chuck.lever@...cle.com, kolga@...app.com,
 linux-kernel@...r.kernel.org, linux-nfs@...r.kernel.org, neilb@...e.de,
 syzkaller-bugs@...glegroups.com, tom@...pey.com, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Paolo Abeni
 <pabeni@...hat.com>, Lorenzo Bianconi <lorenzo@...nel.org>
Subject: Re: [syzbot] [nfs?] INFO: task hung in nfsd_nl_listener_get_doit

On Mon, 17 Jun 2024 06:15:25 -0400 Jeff Layton wrote:
> We've had number of these reports recently. I think I understand what's
> happening but I'm not sure how to fix it. The problem manifests as a
> stuck nfsd_mutex:
> 
> nfsd_nl_rpc_status_get_start takes the nfsd_mutex, and it's released in
> nfsd_nl_rpc_status_get_done. These are the ->start and ->done
> operations for the rpc_status_get dumpit routine.
> 
> I think syzbot is triggering one of the two "goto errout_skb"
> conditions in netlink_dump (not sure which). In those cases we end up
> returning from that function without calling ->done, which would lead
> to the hung mutex like we see here.
> 
> Is this a bug in the netlink code, or is the rpc_status_get dumpit
> routine not using ->start and ->done correctly?

Dumps are spread over multiple recvmsg() calls, even if we error out
the next recvmsg() will dump again, until ->done() is called. And we'll
call ->done() if socket is closed without reaching the end.

But the multi-syscall nature puts us at the mercy of the user meaning
that holding locks ->start() to ->done() is a bit of a no-no.
Many of the dumps dump contents of an Xarray, so its easy to remember
an index and continue dumping from where we left off.