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_mark.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]> --- fs/notify/inotify/inotify_user.c | 4 ++-- fs/notify/mark.c | 14 +++++++------- include/linux/fsnotify_backend.h | 2 +- kernel/audit_tree.c | 2 +- 4 files changed, 11 insertions(+), 11 deletions(-) diff --git a/fs/notify/inotify/inotify_user.c b/fs/notify/inotify/inotify_user.c index 7cc7d3f..d3c20e0 100644 --- a/fs/notify/inotify/inotify_user.c +++ b/fs/notify/inotify/inotify_user.c @@ -376,7 +376,7 @@ static struct inotify_inode_mark *inotify_idr_find_locked(struct fsnotify_group fsnotify_get_mark(fsn_mark); /* One ref for being in the idr, one ref we just took */ - BUG_ON(atomic_read(&fsn_mark->refcnt) < 2); + BUG_ON(refcount_read(&fsn_mark->refcnt) < 2); } return i_mark; @@ -446,7 +446,7 @@ static void inotify_remove_from_idr(struct fsnotify_group *group, * One ref for being in the idr * one ref grabbed by inotify_idr_find */ - if (unlikely(atomic_read(&i_mark->fsn_mark.refcnt) < 2)) { + if (unlikely(refcount_read(&i_mark->fsn_mark.refcnt) < 2)) { printk(KERN_ERR "%s: i_mark=%p i_mark->wd=%d i_mark->group=%p\n", __func__, i_mark, i_mark->wd, i_mark->fsn_mark.group); /* we can't really recover with bad ref cnting.. */ diff --git a/fs/notify/mark.c b/fs/notify/mark.c index 9991f88..45f1141 100644 --- a/fs/notify/mark.c +++ b/fs/notify/mark.c @@ -105,8 +105,8 @@ static DECLARE_WORK(connector_reaper_work, fsnotify_connector_destroy_workfn); void fsnotify_get_mark(struct fsnotify_mark *mark) { - WARN_ON_ONCE(!atomic_read(&mark->refcnt)); - atomic_inc(&mark->refcnt); + WARN_ON_ONCE(!refcount_read(&mark->refcnt)); + refcount_inc(&mark->refcnt); } /* @@ -116,7 +116,7 @@ void fsnotify_get_mark(struct fsnotify_mark *mark) */ static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark) { - return atomic_inc_not_zero(&mark->refcnt); + return refcount_inc_not_zero(&mark->refcnt); } static void __fsnotify_recalc_mask(struct fsnotify_mark_connector *conn) @@ -211,7 +211,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark) /* Catch marks that were actually never attached to object */ if (!mark->connector) { - if (atomic_dec_and_test(&mark->refcnt)) + if (refcount_dec_and_test(&mark->refcnt)) fsnotify_final_mark_destroy(mark); return; } @@ -220,7 +220,7 @@ void fsnotify_put_mark(struct fsnotify_mark *mark) * We have to be careful so that traversals of obj_list under lock can * safely grab mark reference. */ - if (!atomic_dec_and_lock(&mark->refcnt, &mark->connector->lock)) + if (!refcount_dec_and_lock(&mark->refcnt, &mark->connector->lock)) return; conn = mark->connector; @@ -338,7 +338,7 @@ void fsnotify_detach_mark(struct fsnotify_mark *mark) WARN_ON_ONCE(!mutex_is_locked(&group->mark_mutex)); WARN_ON_ONCE(!srcu_read_lock_held(&fsnotify_mark_srcu) && - atomic_read(&mark->refcnt) < 1 + + refcount_read(&mark->refcnt) < 1 + !!(mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED)); spin_lock(&mark->lock); @@ -738,7 +738,7 @@ void fsnotify_init_mark(struct fsnotify_mark *mark, { memset(mark, 0, sizeof(*mark)); spin_lock_init(&mark->lock); - atomic_set(&mark->refcnt, 1); + refcount_set(&mark->refcnt, 1); fsnotify_get_group(group); mark->group = group; } diff --git a/include/linux/fsnotify_backend.h b/include/linux/fsnotify_backend.h index 20a57ba..26485b1 100644 --- a/include/linux/fsnotify_backend.h +++ b/include/linux/fsnotify_backend.h @@ -244,7 +244,7 @@ struct fsnotify_mark { __u32 mask; /* We hold one for presence in g_list. Also one ref for each 'thing' * in kernel that found and may be using this mark. */ - atomic_t refcnt; + refcount_t refcnt; /* Group this mark is for. Set on mark creation, stable until last ref * is dropped */ struct fsnotify_group *group; diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c index 011d46e..45ec960 100644 --- a/kernel/audit_tree.c +++ b/kernel/audit_tree.c @@ -1007,7 +1007,7 @@ static void audit_tree_freeing_mark(struct fsnotify_mark *entry, struct fsnotify * We are guaranteed to have at least one reference to the mark from * either the inode or the caller of fsnotify_destroy_mark(). */ - BUG_ON(atomic_read(&entry->refcnt) < 1); + BUG_ON(refcount_read(&entry->refcnt) < 1); } static const struct fsnotify_ops audit_tree_ops = { -- 2.7.4
