linux-kernel - Re: [PATCH] fs: Handle register

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Date:   Thu, 6 Apr 2017 14:21:29 -0500
From:   Goldwyn Rodrigues <rgoldwyn@...e.de>
To:     Nikolay Borisov <nborisov@...e.com>, dvyukov@...gle.com
Cc:     viro@...iv.linux.org.uk, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, syzkaller@...glegroups.com
Subject: Re: [PATCH] fs: Handle register_shrinker failure



On 03/24/2017 02:55 AM, Nikolay Borisov wrote:
> register_shrinker allocates dynamic memory and thus is susceptible to failures
> under low-memory situation. Currently,get_userns ignores the return value of
> register_shrinker, potentially exposing not fully initialised object. This
> can lead to a NULL-ptr deref everytime shrinker->nr_deferred is referenced.
> 
> Fix this by failing to register the filesystem in case there is not enough
> memory to fully construct the shrinker object.
> 
> Signed-off-by: Nikolay Borisov <nborisov@...e.com>

Looks good, though the situation seems rare.

Reviewed-by: Goldwyn Rodrigues <rgoldwyn@...e.com>

> ---
>  fs/super.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/super.c b/fs/super.c
> index b8b6a086c03b..964b18447c92 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -518,7 +518,19 @@ struct super_block *sget_userns(struct file_system_type *type,
>  	hlist_add_head(&s->s_instances, &type->fs_supers);
>  	spin_unlock(&sb_lock);
>  	get_filesystem(type);
> -	register_shrinker(&s->s_shrink);
> +	err = register_shrinker(&s->s_shrink);
> +	if (err) {
> +		spin_lock(&sb_lock);
> +		list_del(&s->s_list);
> +		hlist_del(&s->s_instances);
> +		spin_unlock(&sb_lock);
> +
> +		up_write(&s->s_umount);
> +		destroy_super(s);
> +		put_filesystem(type);
> +		return ERR_PTR(err);
> +	}
> +
>  	return s;
>  }
>  
> 

-- 
Goldwyn