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 bdi_writeback_congested.refcnt is used as pure reference counter. Convert it to refcount_t and fix up the operations. Suggested-by: Kees Cook <[email protected]> Reviewed-by: David Windsor <[email protected]> Reviewed-by: Hans Liljestrand <[email protected]> Signed-off-by: Elena Reshetova <[email protected]> --- include/linux/backing-dev-defs.h | 3 ++- include/linux/backing-dev.h | 4 ++-- mm/backing-dev.c | 14 ++++++++------ 3 files changed, 12 insertions(+), 9 deletions(-) diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h index b7c7be6..429fe3b 100644 --- a/include/linux/backing-dev-defs.h +++ b/include/linux/backing-dev-defs.h @@ -4,6 +4,7 @@ #include <linux/list.h> #include <linux/radix-tree.h> #include <linux/rbtree.h> +#include <linux/refcount.h> #include <linux/spinlock.h> #include <linux/percpu_counter.h> #include <linux/percpu-refcount.h> @@ -75,7 +76,7 @@ enum wb_reason { */ struct bdi_writeback_congested { unsigned long state; /* WB_[a]sync_congested flags */ - atomic_t refcnt; /* nr of attached wb's and blkg */ + refcount_t refcnt; /* nr of attached wb's and blkg */ #ifdef CONFIG_CGROUP_WRITEBACK struct backing_dev_info *__bdi; /* the associated bdi, set to NULL diff --git a/include/linux/backing-dev.h b/include/linux/backing-dev.h index e6f5037..f3b38c4 100644 --- a/include/linux/backing-dev.h +++ b/include/linux/backing-dev.h @@ -401,13 +401,13 @@ static inline bool inode_cgwb_enabled(struct inode *inode) static inline struct bdi_writeback_congested * wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp) { - atomic_inc(&bdi->wb_congested->refcnt); + refcount_inc(&bdi->wb_congested->refcnt); return bdi->wb_congested; } static inline void wb_congested_put(struct bdi_writeback_congested *congested) { - if (atomic_dec_and_test(&congested->refcnt)) + if (refcount_dec_and_test(&congested->refcnt)) kfree(congested); } diff --git a/mm/backing-dev.c b/mm/backing-dev.c index 74b52df..e92a20f 100644 --- a/mm/backing-dev.c +++ b/mm/backing-dev.c @@ -440,14 +440,17 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp) node = &parent->rb_left; else if (congested->blkcg_id > blkcg_id) node = &parent->rb_right; - else - goto found; + else { + refcount_inc(&congested->refcnt); + goto found; + } } if (new_congested) { /* !found and storage for new one already allocated, insert */ congested = new_congested; new_congested = NULL; + refcount_set(&congested->refcnt, 1); rb_link_node(&congested->rb_node, parent, node); rb_insert_color(&congested->rb_node, &bdi->cgwb_congested_tree); goto found; @@ -460,13 +463,12 @@ wb_congested_get_create(struct backing_dev_info *bdi, int blkcg_id, gfp_t gfp) if (!new_congested) return NULL; - atomic_set(&new_congested->refcnt, 0); + refcount_set(&new_congested->refcnt, 0); new_congested->__bdi = bdi; new_congested->blkcg_id = blkcg_id; goto retry; found: - atomic_inc(&congested->refcnt); spin_unlock_irqrestore(&cgwb_lock, flags); kfree(new_congested); return congested; @@ -483,7 +485,7 @@ void wb_congested_put(struct bdi_writeback_congested *congested) unsigned long flags; local_irq_save(flags); - if (!atomic_dec_and_lock(&congested->refcnt, &cgwb_lock)) { + if (!refcount_dec_and_lock(&congested->refcnt, &cgwb_lock)) { local_irq_restore(flags); return; } @@ -793,7 +795,7 @@ static int cgwb_bdi_init(struct backing_dev_info *bdi) if (!bdi->wb_congested) return -ENOMEM; - atomic_set(&bdi->wb_congested->refcnt, 1); + refcount_set(&bdi->wb_congested->refcnt, 1); err = wb_init(&bdi->wb, bdi, 1, GFP_KERNEL); if (err) { -- 2.7.4
