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

[Sahitya Tummala <stummala@codeaurora.org>]

Al,

Can we merge this patch, if there are no further comments?

Thanks,
Sahitya.

On Thu, Jan 03, 2019 at 04:48:15PM +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)
> 
> Fixes: 76ae281f6307 ('configfs: fix race between dentry put and lookup')
> Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
> ---
> 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);
> -- 
> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
> 

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