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@xxxxxxxxxxxx>
> Reviewed-by: David Windsor <dwindsor@xxxxxxxxx>
> Reviewed-by: Hans Liljestrand <ishkamiel@xxxxxxxxx>
> Signed-off-by: Elena Reshetova <elena.reshetova@xxxxxxxxx>
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@xxxxxxxx>
SUSE Labs, CR
