linux-kernel - [PATCH 3/3] auth_gss: fix deadlock that blocks rpcsec_gss_exit

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Date:   Wed,  1 May 2019 14:42:25 +0800
From:   Wenbin Zeng <wenbin.zeng@...il.com>
To:     viro@...iv.linux.org.uk, davem@...emloft.net, bfields@...ldses.org,
        jlayton@...nel.org, trond.myklebust@...merspace.com,
        anna.schumaker@...app.com, wenbinzeng@...cent.com,
        dsahern@...il.com, nicolas.dichtel@...nd.com, willy@...radead.org,
        edumazet@...gle.com, jakub.kicinski@...ronome.com,
        tyhicks@...onical.com, chuck.lever@...cle.com, neilb@...e.com
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, linux-nfs@...r.kernel.org
Subject: [PATCH 3/3] auth_gss: fix deadlock that blocks rpcsec_gss_exit_net when use-gss-proxy==1

When use-gss-proxy is set to 1, write_gssp() creates a rpc client in
gssp_rpc_create(), this increases 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 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.

This fix introduced a new callback i.e. evict in struct proc_ns_operations,
which is called in nsfs_evict. Moving rpcsec_gss_exit_net to evict path
gives it a chance to get called and avoids the above deadlock situation.

Signed-off-by: Wenbin Zeng <wenbinzeng@...cent.com>
---
 net/sunrpc/auth_gss/auth_gss.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/net/sunrpc/auth_gss/auth_gss.c b/net/sunrpc/auth_gss/auth_gss.c
index 3fd56c0..3e6bd59 100644
--- a/net/sunrpc/auth_gss/auth_gss.c
+++ b/net/sunrpc/auth_gss/auth_gss.c
@@ -2136,14 +2136,17 @@ static __net_init int rpcsec_gss_init_net(struct net *net)
 	return gss_svc_init_net(net);
 }
 
-static __net_exit void rpcsec_gss_exit_net(struct net *net)
+static void rpcsec_gss_evict_net(struct net *net)
 {
-	gss_svc_shutdown_net(net);
+	struct sunrpc_net *sn = net_generic(net, sunrpc_net_id);
+
+	if (sn->gssp_clnt)
+		gss_svc_shutdown_net(net);
 }
 
 static struct pernet_operations rpcsec_gss_net_ops = {
 	.init = rpcsec_gss_init_net,
-	.exit = rpcsec_gss_exit_net,
+	.evict = rpcsec_gss_evict_net,
 };
 
 /*
-- 
1.8.3.1