linux-kernel - Re: [PATCH 1/2] fsnotify: convert fsnotify_group.refcnt from atomic_t to refcount

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Date:   Tue, 31 Oct 2017 11:58:43 +0100
From:   Jan Kara <jack@...e.cz>
To:     Elena Reshetova <elena.reshetova@...el.com>
Cc:     jack@...e.cz, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, amir73il@...il.com,
        peterz@...radead.org, keescook@...omium.org
Subject: Re: [PATCH 1/2] fsnotify: convert fsnotify_group.refcnt from
 atomic_t to refcount_t

On Fri 20-10-17 13:26:01, Elena Reshetova wrote:
> atomic_t variables are currently used to implement reference
> counters with the following properties:
>  - counter is initialized to 1 using atomic_set()
>  - a resource is freed upon counter reaching zero
>  - once counter reaches zero, its further
>    increments aren't allowed
>  - counter schema uses basic atomic operations
>    (set, inc, inc_not_zero, dec_and_test, etc.)
> 
> Such atomic variables should be converted to a newly provided
> refcount_t type and API that prevents accidental counter overflows
> and underflows. This is important since overflows and underflows
> can lead to use-after-free situation and be exploitable.
> 
> The variable fsnotify_group.refcnt is used as pure reference counter.
> Convert it to refcount_t and fix up the operations.
> 
> Suggested-by: Kees Cook <keescook@...omium.org>
> Reviewed-by: David Windsor <dwindsor@...il.com>
> Reviewed-by: Hans Liljestrand <ishkamiel@...il.com>
> Signed-off-by: Elena Reshetova <elena.reshetova@...el.com>

Thanks for the patch. I've added it to my tree.

								Honza

> ---
>  fs/notify/group.c                | 6 +++---
>  include/linux/fsnotify_backend.h | 3 ++-
>  2 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/notify/group.c b/fs/notify/group.c
> index 3235753..b7a4b6a 100644
> --- a/fs/notify/group.c
> +++ b/fs/notify/group.c
> @@ -107,7 +107,7 @@ void fsnotify_destroy_group(struct fsnotify_group *group)
>   */
>  void fsnotify_get_group(struct fsnotify_group *group)
>  {
> -	atomic_inc(&group->refcnt);
> +	refcount_inc(&group->refcnt);
>  }
>  
>  /*
> @@ -115,7 +115,7 @@ void fsnotify_get_group(struct fsnotify_group *group)
>   */
>  void fsnotify_put_group(struct fsnotify_group *group)
>  {
> -	if (atomic_dec_and_test(&group->refcnt))
> +	if (refcount_dec_and_test(&group->refcnt))
>  		fsnotify_final_destroy_group(group);
>  }
>  
> @@ -131,7 +131,7 @@ struct fsnotify_group *fsnotify_alloc_group(const struct fsnotify_ops *ops)
>  		return ERR_PTR(-ENOMEM);
>  
>  	/* set to 0 when there a no external references to this group */
> -	atomic_set(&group->refcnt, 1);
> +	refcount_set(&group->refcnt, 1);
>  	atomic_set(&group->num_marks, 0);
>  	atomic_set(&group->user_waits, 0);
>  
> diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h
> index c6c6931..20a57ba 100644
> --- a/include/linux/fsnotify_backend.h
> +++ b/include/linux/fsnotify_backend.h
> @@ -17,6 +17,7 @@
>  #include <linux/types.h>
>  #include <linux/atomic.h>
>  #include <linux/user_namespace.h>
> +#include <linux/refcount.h>
>  
>  /*
>   * IN_* from inotfy.h lines up EXACTLY with FS_*, this is so we can easily
> @@ -135,7 +136,7 @@ struct fsnotify_group {
>  	 * inotify_init() and the refcnt will hit 0 only when that fd has been
>  	 * closed.
>  	 */
> -	atomic_t refcnt;		/* things with interest in this group */
> +	refcount_t refcnt;		/* things with interest in this group */
>  
>  	const struct fsnotify_ops *ops;	/* how this group handles things */
>  
> -- 
> 2.7.4
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR