autofs: drop dentry reference only when it is never used

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>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/autofs/expire.c b/fs/autofs/expire.c
index d441244b79df..28d9c2b1b3bb 100644
--- a/fs/autofs/expire.c
+++ b/fs/autofs/expire.c
@@ -596,7 +596,6 @@ int autofs_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;
@@ -609,6 +608,8 @@ int autofs_expire_run(struct super_block *sb,
         complete_all(&ino->expire_complete);
         spin_unlock(&sbi->fs_lock);
 
+        dput(dentry);
+
         return ret;
 }