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 nlm_lockowner.count is used as pure reference counter. Convert it to refcount_t and fix up the operations. **Important note for maintainers: Some functions from refcount_t API defined in lib/refcount.c have different memory ordering guarantees than their atomic counterparts. The full comparison can be seen in https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon in state to be merged to the documentation tree. Normally the differences should not matter since refcount_t provides enough guarantees to satisfy the refcounting use cases, but in some rare cases it might matter. Please double check that you don't have some undocumented memory guarantees for this variable usage. For the nlm_lockowner.count it might make a difference in following places: - nlm_put_lockowner(): decrement in refcount_dec_and_lock() only provides RELEASE ordering, control dependency on success and holds a spin lock on success vs. fully ordered atomic counterpart. No changes in spin lock guarantees. 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> --- fs/lockd/clntproc.c | 6 +++--- include/linux/lockd/lockd.h | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/fs/lockd/clntproc.c b/fs/lockd/clntproc.c index 066ac31..112173d 100644 --- a/fs/lockd/clntproc.c +++ b/fs/lockd/clntproc.c @@ -48,13 +48,13 @@ void nlmclnt_next_cookie(struct nlm_cookie *c) static struct nlm_lockowner *nlm_get_lockowner(struct nlm_lockowner *lockowner) { - atomic_inc(&lockowner->count); + refcount_inc(&lockowner->count); return lockowner; } static void nlm_put_lockowner(struct nlm_lockowner *lockowner) { - if (!atomic_dec_and_lock(&lockowner->count, &lockowner->host->h_lock)) + if (!refcount_dec_and_lock(&lockowner->count, &lockowner->host->h_lock)) return; list_del(&lockowner->list); spin_unlock(&lockowner->host->h_lock); @@ -105,7 +105,7 @@ static struct nlm_lockowner *nlm_find_lockowner(struct nlm_host *host, fl_owner_ res = __nlm_find_lockowner(host, owner); if (res == NULL && new != NULL) { res = new; - atomic_set(&new->count, 1); + refcount_set(&new->count, 1); new->owner = owner; new->pid = __nlm_alloc_pid(host); new->host = nlm_get_host(host); diff --git a/include/linux/lockd/lockd.h b/include/linux/lockd/lockd.h index cded0ad..86d012a 100644 --- a/include/linux/lockd/lockd.h +++ b/include/linux/lockd/lockd.h @@ -123,7 +123,7 @@ static inline struct sockaddr *nlm_srcaddr(const struct nlm_host *host) */ struct nlm_lockowner { struct list_head list; - atomic_t count; + refcount_t count; struct nlm_host *host; fl_owner_t owner; -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
