[PATCH v2] configfs: Fix use-after-free when accessing sd->s_dentry

[Sahitya Tummala]

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@xxxxxxxxxxxxxx>
---
v2:
- update comments relevant to the code.

 fs/configfs/dir.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index 39843fa..f113101 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -58,15 +58,14 @@ 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.
+		/*
+		 * Set sd->s_dentry to null only when this dentry is the
+		 * one that is going to be killed.
 		 * 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)
+		if (sd->s_dentry == dentry)
 			sd->s_dentry = NULL;
 
 		spin_unlock(&configfs_dirent_lock);
-- 
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