configfs: Fix use-after-free when accessing sd->s_dentry

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
(https://github.com/linux-test-project/ltp/blob/master/testcases/kernel/fs/racer/fs_racer_file_list.sh)

Fixes: 76ae281f6307 ('configfs: fix race between dentry put and lookup')
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 5e7932d..22203a3 100644 (file)
--- 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 */
                spin_lock(&configfs_dirent_lock);
-               /* 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;
 
                spin_unlock(&configfs_dirent_lock);