Re: [PATCH net 2/2] auth_gss: Fix deadlock that blocks rpcsec_gss_exit_net when use-gss-proxy==1

[Trond Myklebust <trondmy@hammerspace.com>]

On Tue, 2021-09-28 at 10:57 -0400, bfields@fieldses.org wrote:
> On Tue, Sep 28, 2021 at 02:27:33PM +0000, Trond Myklebust wrote:
> > On Tue, 2021-09-28 at 10:17 -0400, bfields@fieldses.org wrote:
> > > On Tue, Sep 28, 2021 at 02:04:49PM +0000, Trond Myklebust wrote:
> > > > On Tue, 2021-09-28 at 09:49 -0400, bfields@fieldses.org wrote:
> > > > > On Tue, Sep 28, 2021 at 01:30:17PM +0000, Trond Myklebust
> > > > > wrote:
> > > > > > On Tue, 2021-09-28 at 11:14 +0800, Wang Hai wrote:
> > > > > > > When use-gss-proxy is set to 1, write_gssp() creates a
> > > > > > > rpc
> > > > > > > client
> > > > > > > in
> > > > > > > gssp_rpc_create(), this increases the netns refcount by
> > > > > > > 2,
> > > > > > > these
> > > > > > > refcounts are supposed to be released in
> > > > > > > rpcsec_gss_exit_net(),
> > > > > > > but
> > > > > > > it
> > > > > > > will never happen because rpcsec_gss_exit_net() is
> > > > > > > triggered
> > > > > > > only
> > > > > > > when
> > > > > > > the netns refcount gets to 0, specifically:
> > > > > > >     refcount=0 -> cleanup_net() -> ops_exit_list ->
> > > > > > > rpcsec_gss_exit_net
> > > > > > > It is a deadlock situation here, refcount will never get
> > > > > > > to 0
> > > > > > > unless
> > > > > > > rpcsec_gss_exit_net() is called. So, in this case, the
> > > > > > > netns
> > > > > > > refcount
> > > > > > > should not be increased.
> > > > > > > 
> > > > > > > In this case, xprt will take a netns refcount which is
> > > > > > > not
> > > > > > > supposed
> > > > > > > to be taken. Add a new flag to rpc_create_args called
> > > > > > > RPC_CLNT_CREATE_NO_NET_REF for not increasing the netns
> > > > > > > refcount.
> > > > > > > 
> > > > > > > It is safe not to hold the netns refcount, because when
> > > > > > > cleanup_net(), it
> > > > > > > will hold the gssp_lock and then shut down the rpc client
> > > > > > > synchronously.
> > > > > > > 
> > > > > > > 
> > > > > > I don't like this solution at all. Adding this kind of flag
> > > > > > is
> > > > > > going to
> > > > > > lead to problems down the road.
> > > > > > 
> > > > > > Is there any reason whatsoever why we need this RPC client
> > > > > > to
> > > > > > exist
> > > > > > when there is no active knfsd server? IOW: Is there any
> > > > > > reason
> > > > > > why
> > > > > > we
> > > > > > shouldn't defer creating this RPC client for when knfsd
> > > > > > starts
> > > > > > up
> > > > > > in
> > > > > > this net namespace, and why we can't shut it down when
> > > > > > knfsd
> > > > > > shuts
> > > > > > down?
> > > > > 
> > > > > The rpc create is done in the context of the process that
> > > > > writes
> > > > > to
> > > > > /proc/net/rpc/use-gss-proxy to get the right namespaces.  I
> > > > > don't
> > > > > know
> > > > > how hard it would be capture that information for a later
> > > > > create.
> > > > > 
> > > > 
> > > > svcauth_gss_proxy_init() uses the net namespace SVC_NET(rqstp)
> > > > (i.e.
> > > > the knfsd namespace) in the call to
> > > > gssp_accept_sec_context_upcall().
> > > > 
> > > > IOW: the net namespace used in the call to find the RPC client
> > > > is
> > > > the
> > > > one set up by knfsd, and so if use-gss-proxy was set in a
> > > > different
> > > > namespace than the one used by knfsd, then it won't be found.
> > > 
> > > Right.  If you've got multiple containers, you don't want to find
> > > a
> > > gss-proxy from a different container.
> > > 
> > 
> > Exactly. So there is no namespace context to capture in the RPC
> > client
> > other than what's already in knfsd.
> > 
> > The RPC client doesn't capture any other process context. It can
> > cache
> > a user cred in order to capture the user namespace, but that
> > information appears to be unused by this gssd RPC client.
> 
> OK, that's good to know, thanks.
> 
> It's doing a path lookup (it uses an AF_LOCAL socket), and I'm not
> assuming that will get the same result across containers.  Is there
> an
> easy way to do just that path lookup here and delay the res till
> knfsd
> startup?
> 

What is the use case here? Starting the gssd daemon or knfsd in
separate chrooted environments? We already know that they have to be
started in the same net namespace, which pretty much ensures it has to
be the same container.

-- 
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com