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SubjectRe: [PATCH] configfs: Fix use-after-free when accessing sd->s_dentry
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@codeaurora.org>
> ---
> 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).

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