Commit d3fc4a23 authored Jan 02, 2019 by Sahitya Tummala Committed by Gerrit - the friendly Code Review server Jan 10, 2019

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

Change-Id: I5ac8556837bbdf2e5d0540516efb106ec53d7536
Fixes: 76ae281f ('configfs: fix race between dentry put and lookup')
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>

parent d25fbc3d

fs/configfs/dir.c

+4 −5

Original line number	Diff line number	Diff line
		@@ -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);