[PATCH 3.2 084/185] configfs: fix race between dentry put and lookup

From: Ben Hutchings
Date: Sat Dec 28 2013 - 21:34:22 EST

3.2.54-rc1 review patch. If anyone has any objections, please let me know.


From: Junxiao Bi <junxiao.bi@xxxxxxxxxx>

commit 76ae281f6307331aa063288edb6422ae99f435f0 upstream.

A race window in configfs, it starts from one dentry is UNHASHED and end
before configfs_d_iput is called. In this window, if a lookup happen,
since the original dentry was UNHASHED, so a new dentry will be
allocated, and then in configfs_attach_attr(), sd->s_dentry will be
updated to the new dentry. Then in configfs_d_iput(),
BUG_ON(sd->s_dentry != dentry) will be triggered and system panic.

sys_open: sys_close:
... fput
__d_drop <--- dentry unhashed here,
but sd->dentry still point
to this dentry.

configfs_attach_attr---> update sd->s_dentry
to new allocated dentry here.

configfs_d_iput <--- BUG_ON(sd->s_dentry != dentry)
triggered here.

To fix it, change configfs_d_iput to not update sd->s_dentry if
sd->s_count > 2, that means there are another dentry is using the sd
beside the one that is going to be put. Use configfs_dirent_lock in
configfs_attach_attr to sync with configfs_d_iput.

With the following steps, you can reproduce the bug.

1. enable ocfs2, this will mount configfs at /sys/kernel/config and
fill configure in it.

2. run the following script.
while [ 1 ]; do cat /sys/kernel/config/cluster/$your_cluster_name/idle_timeout_ms > /dev/null; done &
while [ 1 ]; do cat /sys/kernel/config/cluster/$your_cluster_name/idle_timeout_ms > /dev/null; done &

Signed-off-by: Junxiao Bi <junxiao.bi@xxxxxxxxxx>
Cc: Joel Becker <jlbec@xxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
fs/configfs/dir.c | 16 ++++++++++++++--
1 file changed, 14 insertions(+), 2 deletions(-)

--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -56,10 +56,19 @@ static void configfs_d_iput(struct dentr
struct configfs_dirent *sd = dentry->d_fsdata;

if (sd) {
- BUG_ON(sd->s_dentry != dentry);
/* Coordinate with configfs_readdir */
- sd->s_dentry = NULL;
+ /* Coordinate with configfs_attach_attr where will increase
+ * sd->s_count and update sd->s_dentry to new allocated one.
+ * Only set sd->dentry to null when this dentry is the only
+ * sd owner.
+ * If not do so, configfs_d_iput may run just after
+ * configfs_attach_attr and set sd->s_dentry to null
+ * even it's still in use.
+ */
+ if (atomic_read(&sd->s_count) <= 2)
+ sd->s_dentry = NULL;
@@ -436,8 +445,11 @@ static int configfs_attach_attr(struct c
struct configfs_attribute * attr = sd->s_element;
int error;

+ spin_lock(&configfs_dirent_lock);
dentry->d_fsdata = configfs_get(sd);
sd->s_dentry = dentry;
+ spin_unlock(&configfs_dirent_lock);
error = configfs_create(dentry, (attr->ca_mode & S_IALLUGO) | S_IFREG,
if (error) {

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