linux-kernel - [PATCH AUTOSEL 4.4 25/26] autofs: drop dentry reference only when it is never used

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Date:   Sat, 23 Feb 2019 16:10:46 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Pan Bian <bianpan2016@....com>, Ian Kent <raven@...maw.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 4.4 25/26] autofs: drop dentry reference only when it is never used

From: Pan Bian <bianpan2016@....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@....com>
Signed-off-by: Ian Kent <raven@...maw.net>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
Signed-off-by: Sasha Levin <sashal@...nel.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 7a5a598a2d945..0d8b9c4f27f21 100644
--- a/fs/autofs4/expire.c
+++ b/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;
 }
 
-- 
2.19.1