linux-kernel - Re: [PATCH net 2/2] auth_gss: Fix deadlock that blocks rpcsec_gss_exit

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Message-ID: <77051a059fa19a7ae2390fbda7f8ab6f09514dfc.camel@hammerspace.com>
Date:   Tue, 28 Sep 2021 14:04:49 +0000
From:   Trond Myklebust <trondmy@...merspace.com>
To:     "bfields@...ldses.org" <bfields@...ldses.org>
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Subject: Re: [PATCH net 2/2] auth_gss: Fix deadlock that blocks
 rpcsec_gss_exit_net when use-gss-proxy==1

On Tue, 2021-09-28 at 09:49 -0400, bfields@...ldses.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.

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