[PATCH AUTOSEL 3.18 17/18] autofs: drop dentry reference only when it is never used

[Sasha Levin <sashal@kernel.org>]

From: Pan Bian <bianpan2016@163.com>

[ Upstream commit 63ce5f552beb9bdb41546b3a26c4374758b21815 ]

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-net
Signed-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>
---
 fs/autofs4/expire.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/autofs4/expire.c b/fs/autofs4/expire.c
index 513b8e5d151cb..4cd4b20c69dbf 100644
--- a/fs/autofs4/expire.c
+++ b/fs/autofs4/expire.c
@@ -570,7 +570,6 @@ 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;
@@ -583,6 +582,8 @@ int autofs4_expire_run(struct super_block *sb,
 	complete_all(&ino->expire_complete);
 	spin_unlock(&sbi->fs_lock);
 
+	dput(dentry);
+
 	return ret;
 }
 
-- 
2.19.1