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 xlog_ticket.t_ref 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 xlog_ticket.t_ref it might make a difference in following places: - xfs_log_ticket_put(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart 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/xfs/xfs_log.c | 10 +++++----- fs/xfs/xfs_log_priv.h | 2 +- 2 files changed, 6 insertions(+), 6 deletions(-) diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c index a503af9..6e1528f 100644 --- a/fs/xfs/xfs_log.c +++ b/fs/xfs/xfs_log.c @@ -3600,8 +3600,8 @@ void xfs_log_ticket_put( xlog_ticket_t *ticket) { - ASSERT(atomic_read(&ticket->t_ref) > 0); - if (atomic_dec_and_test(&ticket->t_ref)) + ASSERT(refcount_read(&ticket->t_ref) > 0); + if (refcount_dec_and_test(&ticket->t_ref)) kmem_zone_free(xfs_log_ticket_zone, ticket); } @@ -3609,8 +3609,8 @@ xlog_ticket_t * xfs_log_ticket_get( xlog_ticket_t *ticket) { - ASSERT(atomic_read(&ticket->t_ref) > 0); - atomic_inc(&ticket->t_ref); + ASSERT(refcount_read(&ticket->t_ref) > 0); + refcount_inc(&ticket->t_ref); return ticket; } @@ -3732,7 +3732,7 @@ xlog_ticket_alloc( unit_res = xfs_log_calc_unit_res(log->l_mp, unit_bytes); - atomic_set(&tic->t_ref, 1); + refcount_set(&tic->t_ref, 1); tic->t_task = current; INIT_LIST_HEAD(&tic->t_queue); tic->t_unit_res = unit_res; diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h index 1299759..7482145 100644 --- a/fs/xfs/xfs_log_priv.h +++ b/fs/xfs/xfs_log_priv.h @@ -168,7 +168,7 @@ typedef struct xlog_ticket { struct list_head t_queue; /* reserve/write queue */ struct task_struct *t_task; /* task that owns this ticket */ xlog_tid_t t_tid; /* transaction identifier : 4 */ - atomic_t t_ref; /* ticket reference count : 4 */ + refcount_t t_ref; /* ticket reference count : 4 */ int t_curr_res; /* current reservation in bytes : 4 */ int t_unit_res; /* unit reservation in bytes : 4 */ char t_ocnt; /* original count : 1 */ -- 2.7.4 -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
