[PATCH AUTOSEL 4.4 08/13] configfs: Fix use-after-free when accessing sd->s_dentry

From: Sasha Levin
Date: Sat Jun 08 2019 - 07:53:26 EST

From: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>

[ Upstream commit f6122ed2a4f9c9c1c073ddf6308d1b2ac10e0781 ]

In the vfs_statx() context, during path lookup, the dentry gets
added to sd->s_dentry via configfs_attach_attr(). In the end,
vfs_statx() kills the dentry by calling path_put(), which invokes
configfs_d_iput(). Ideally, this dentry must be removed from
sd->s_dentry but it doesn't if the sd->s_count >= 3. As a result,
sd->s_dentry is holding reference to a stale dentry pointer whose
memory is already freed up. This results in use-after-free issue,
when this stale sd->s_dentry is accessed later in
configfs_readdir() path.

This issue can be easily reproduced, by running the LTP test case -
sh fs_racer_file_list.sh /config

Fixes: 76ae281f6307 ('configfs: fix race between dentry put and lookup')
Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
Signed-off-by: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
fs/configfs/dir.c | 14 ++++++--------
1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index a7a1b218f308..8e709b641b55 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -58,15 +58,13 @@ static void configfs_d_iput(struct dentry * dentry,
if (sd) {
/* Coordinate with configfs_readdir */
- /* 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.
+ /*
+ * Set sd->s_dentry to null only when this dentry is the one
+ * that is going to be killed. Otherwise 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)
+ if (sd->s_dentry == dentry)
sd->s_dentry = NULL;