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

[Al Viro]

On Thu, Jan 03, 2019 at 01:41:04PM +0530, Sahitya Tummala wrote:
> 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)
> 
> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
> ---
>  fs/configfs/dir.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
> index 39843fa..a5906c5 100644
> --- a/fs/configfs/dir.c
> +++ b/fs/configfs/dir.c
> @@ -66,7 +66,7 @@ static void configfs_d_iput(struct dentry * dentry,
>  		 * 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);

Please, update the comment above as well - it refers the increment of
sd->s_count, which becomes stale.  I agree that the problem is real
and the fix is sane (and that's what should've been done in the first
place, back in "configfs: fix race between dentry put and lookup",
which has introduced that logics).