From: "J. Bruce Fields" <bfields@redhat.com>
Date: Wed, 15 Jul 2020 13:31:36 -0400
Subject: [PATCH] nfsd4: fix NULL dereference in nfsd/clients display code
Git-commit: 9affa435817711861d774f5626c393c80f16d044
Patch-mainline: v5.8-rc7
References: git-fixes

We hold the cl_lock here, and that's enough to keep stateid's from going
away, but it's not enough to prevent the files they point to from going
away.  Take fi_lock and a reference and check for NULL, as we do in
other code.

Reported-by: NeilBrown <neilb@suse.de>
Fixes: 78599c42ae3c ("nfsd4: add file to display list of client's opens")
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Acked-by: NeilBrown <neilb@suse.com>

---
 fs/nfsd/nfs4state.c |   16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -506,6 +506,17 @@ find_any_file(struct nfs4_file *f)
 	return ret;
 }
 
+static struct file *find_deleg_file(struct nfs4_file *f)
+{
+	struct file *ret = NULL;
+
+	spin_lock(&f->fi_lock);
+	if (f->fi_deleg_file)
+		ret = get_file(f->fi_deleg_file);
+	spin_unlock(&f->fi_lock);
+	return ret;
+}
+
 static atomic_long_t num_delegations;
 unsigned long max_delegations;
 
@@ -2429,7 +2440,9 @@ static int nfs4_show_deleg(struct seq_fi
 
 	ds = delegstateid(st);
 	nf = st->sc_file;
-	file = nf->fi_deleg_file;
+	file = find_deleg_file(nf);
+	if (!file)
+		return 0;
 
 	seq_printf(s, "- 0x%16phN: { type: deleg, ", &st->sc_stateid);
 
@@ -2441,6 +2454,7 @@ static int nfs4_show_deleg(struct seq_fi
 
 	nfs4_show_superblock(s, file);
 	seq_printf(s, " }\n");
+	fput(file);
 
 	return 0;
 }