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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Al Viro <viro@zeniv.linux.org.uk>
Subject: [PATCH 4.4 122/134] lock_parent() needs to recheck if dentry got __dentry_killed under it
Date:   Mon, 19 Mar 2018 19:06:45 +0100
Message-Id: <20180319171906.890740695@linuxfoundation.org>
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4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Al Viro <viro@zeniv.linux.org.uk>

commit 3b821409632ab778d46e807516b457dfa72736ed upstream.

In case when dentry passed to lock_parent() is protected from freeing only
by the fact that it's on a shrink list and trylock of parent fails, we
could get hit by __dentry_kill() (and subsequent dentry_kill(parent))
between unlocking dentry and locking presumed parent.  We need to recheck
that dentry is alive once we lock both it and parent *and* postpone
rcu_read_unlock() until after that point.  Otherwise we could return
a pointer to struct dentry that already is rcu-scheduled for freeing, with
->d_lock held on it; caller's subsequent attempt to unlock it can end
up with memory corruption.

Cc: stable@vger.kernel.org # 3.12+, counting backports
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/dcache.c |   11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -634,11 +634,16 @@ again:
 		spin_unlock(&parent->d_lock);
 		goto again;
 	}
-	rcu_read_unlock();
-	if (parent != dentry)
+	if (parent != dentry) {
 		spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
-	else
+		if (unlikely(dentry->d_lockref.count < 0)) {
+			spin_unlock(&parent->d_lock);
+			parent = NULL;
+		}
+	} else {
 		parent = NULL;
+	}
+	rcu_read_unlock();
 	return parent;
 }