Re: [PATCH v8 09/18] nfsd: use percpu_ref to interlock nfsd_destroy_serv and nfsd_open_local_fh

[Mike Snitzer <snitzer@kernel.org>]

On Thu, Jun 27, 2024 at 07:24:54AM -0400, Jeff Layton wrote:
> On Wed, 2024-06-26 at 14:24 -0400, Mike Snitzer wrote:
> > Introduce nfsd_serv_get and nfsd_serv_put and update the nfsd code to
> > prevent nfsd_destroy_serv from destroying nn->nfsd_serv until any
> > client initiated localio calls to nfsd (that are _not_ in the context
> > of nfsd) are complete.
> > 
> > nfsd_open_local_fh is updated to nfsd_serv_get before opening its file
> > handle and then drop the reference using nfsd_serv_put at the end of
> > nfsd_open_local_fh.
> > 
> > This "interlock" working relies heavily on nfsd_open_local_fh()'s
> > maybe_get_net() safely dealing with the possibility that the struct
> > net (and nfsd_net by association) may have been destroyed by
> > nfsd_destroy_serv() via nfsd_shutdown_net().
> > 
> > Verified to fix an easy to hit crash that would occur if an nfsd
> > instance running in a container, with a localio client mounted, is
> > shutdown. Upon restart of the container and associated nfsd the client
> > would go on to crash due to NULL pointer dereference that occuured due
> > to the nfs client's localio attempting to nfsd_open_local_fh(), using
> > nn->nfsd_serv, without having a proper reference on nn->nfsd_serv.
> > 
> > Signed-off-by: Mike Snitzer <snitzer@kernel.org>
> > ---
> >  fs/nfsd/localio.c |  2 ++
> >  fs/nfsd/netns.h   |  8 +++++++-
> >  fs/nfsd/nfssvc.c  | 39 +++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 48 insertions(+), 1 deletion(-)
> > 
> > diff --git a/fs/nfsd/localio.c b/fs/nfsd/localio.c
> > index 48118db801b5..819589ae2008 100644
> > --- a/fs/nfsd/localio.c
> > +++ b/fs/nfsd/localio.c
> > @@ -204,6 +204,7 @@ int nfsd_open_local_fh(struct net *net,
> >  	}
> >  	nn = net_generic(net, nfsd_net_id);
> >  
> > +	nfsd_serv_get(nn);
> >  	serv = READ_ONCE(nn->nfsd_serv);
> >  	if (unlikely(!serv)) {
> >  		dprintk("%s: localio denied. Server not running\n", __func__);
> > @@ -244,6 +245,7 @@ int nfsd_open_local_fh(struct net *net,
> >  out_revertcred:
> >  	revert_creds(save_cred);
> >  out_net:
> > +	nfsd_serv_put(nn);
> >  	put_net(net);
> >  	return status;
> >  }
> > diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> > index 0c5a1d97e4ac..17a73c780ca1 100644
> > --- a/fs/nfsd/netns.h
> > +++ b/fs/nfsd/netns.h
> > @@ -13,6 +13,7 @@
> >  #include <linux/filelock.h>
> >  #include <linux/nfs4.h>
> >  #include <linux/percpu_counter.h>
> > +#include <linux/percpu-refcount.h>
> >  #include <linux/siphash.h>
> >  #include <linux/sunrpc/stats.h>
> >  #include <linux/nfslocalio.h>
> > @@ -140,7 +141,9 @@ struct nfsd_net {
> >  
> >  	struct svc_info nfsd_info;
> >  #define nfsd_serv nfsd_info.serv
> > -
> > +	struct percpu_ref nfsd_serv_ref;
> > +	struct completion nfsd_serv_confirm_done;
> > +	struct completion nfsd_serv_free_done;
> >  
> >  	/*
> >  	 * clientid and stateid data for construction of net unique COPY
> > @@ -225,6 +228,9 @@ struct nfsd_net {
> >  extern bool nfsd_support_version(int vers);
> >  extern void nfsd_netns_free_versions(struct nfsd_net *nn);
> >  
> > +void nfsd_serv_get(struct nfsd_net *nn);
> > +void nfsd_serv_put(struct nfsd_net *nn);
> > +
> >  extern unsigned int nfsd_net_id;
> >  
> >  void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn);
> > diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> > index a477d2c5088a..be5acb7a4057 100644
> > --- a/fs/nfsd/nfssvc.c
> > +++ b/fs/nfsd/nfssvc.c
> > @@ -258,6 +258,30 @@ int nfsd_minorversion(struct nfsd_net *nn, u32 minorversion, enum vers_op change
> >  	return 0;
> >  }
> >  
> > +void nfsd_serv_get(struct nfsd_net *nn)
> > +{
> > +	percpu_ref_get(&nn->nfsd_serv_ref);
> > +}
> > +
> > +void nfsd_serv_put(struct nfsd_net *nn)
> > +{
> > +	percpu_ref_put(&nn->nfsd_serv_ref);
> > +}
> > +
> > +static void nfsd_serv_done(struct percpu_ref *ref)
> > +{
> > +	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
> > +
> > +	complete(&nn->nfsd_serv_confirm_done);
> > +}
> > +
> > +static void nfsd_serv_free(struct percpu_ref *ref)
> > +{
> > +	struct nfsd_net *nn = container_of(ref, struct nfsd_net, nfsd_serv_ref);
> > +
> > +	complete(&nn->nfsd_serv_free_done);
> > +}
> > +
> >  /*
> >   * Maximum number of nfsd processes
> >   */
> > @@ -462,6 +486,7 @@ static void nfsd_shutdown_net(struct net *net)
> >  		lockd_down(net);
> >  		nn->lockd_up = false;
> >  	}
> > +	percpu_ref_exit(&nn->nfsd_serv_ref);
> >  #if IS_ENABLED(CONFIG_NFSD_LOCALIO)
> >  	list_del_rcu(&nn->nfsd_uuid.list);
> >  #endif
> > @@ -544,6 +569,13 @@ void nfsd_destroy_serv(struct net *net)
> >  	struct nfsd_net *nn = net_generic(net, nfsd_net_id);
> >  	struct svc_serv *serv = nn->nfsd_serv;
> >  
> > +	lockdep_assert_held(&nfsd_mutex);
> > +
> > +	percpu_ref_kill_and_confirm(&nn->nfsd_serv_ref, nfsd_serv_done);
> > +	wait_for_completion(&nn->nfsd_serv_confirm_done);
> > +	wait_for_completion(&nn->nfsd_serv_free_done);
> > +	/* percpu_ref_exit is called in nfsd_shutdown_net */
> > +
> 
> At this point where you're waiting on these completion vars, what stops
> the client from issuing new localio requests?

The server going down.  The interlock is with nfsd_open_local_fh, so
once any active file opens complete the server will get these
completions and the server will go down.  The client will reconnect
if/when the server comes back up.

Maybe I'm not imaginative enough, but is there risk of never-ending
overlapping file opens that prevent nfsd_destroy_serv() from
progressing?

Mike