autofs: drop dentry reference only when it is never used

BugLink: https://bugs.launchpad.net/bugs/1822271

[ Upstream commit 63ce5f55 ]

autofs_expire_run() calls dput(dentry) to drop the reference count of
dentry.  However, dentry is read via autofs_dentry_ino(dentry) after
that.  This may result in a use-free-bug.  The patch drops the reference
count of dentry only when it is never used.

Link: http://lkml.kernel.org/r/154725122396.11260.16053424107144453867.stgit@pluto-themaw-netSigned-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Juerg Haefliger <juerg.haefliger@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

fs/autofs4/expire.c

@@ -560,7 +560,6 @@ int autofs4_expire_run(struct super_block *sb,
 	pkt.len = dentry->d_name.len;
 	memcpy(pkt.name, dentry->d_name.name, pkt.len);
 	pkt.name[pkt.len] = '\0';
-	dput(dentry);
 
 	if ( copy_to_user(pkt_p, &pkt, sizeof(struct autofs_packet_expire)) )
 		ret = -EFAULT;
@@ -573,6 +572,8 @@ int autofs4_expire_run(struct super_block *sb,
 	complete_all(&ino->expire_complete);
 	spin_unlock(&sbi->fs_lock);
 
+	dput(dentry);
+
 	return ret;
 }