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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: 76ae281f63 ('configfs: fix race between dentry put and lookup')
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
This commit is contained in:
Sahitya Tummala 2019-01-03 16:48:15 +05:30 committed by Christoph Hellwig
parent cd6c84d8f0
commit f6122ed2a4
1 changed files with 6 additions and 8 deletions
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14
fs/configfs/dir.c
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@ -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);