Re: [syzbot] [nfs?] INFO: task hung in nfsd_nl_listener_get_doit

[Jakub Kicinski <kuba@kernel.org>]

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.