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 xfs_rui_log_item.rui_refcount 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 xfs_rui_log_item.rui_refcount it might make a difference in following places: - xfs_rui_release(): decrement in refcount_dec_and_test() only provides RELEASE ordering and control dependency on success vs. fully ordered atomic counterpart 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/xfs/xfs_rmap_item.c | 6 +++--- fs/xfs/xfs_rmap_item.h | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-) diff --git a/fs/xfs/xfs_rmap_item.c b/fs/xfs/xfs_rmap_item.c index f3b139c..f914829f 100644 --- a/fs/xfs/xfs_rmap_item.c +++ b/fs/xfs/xfs_rmap_item.c @@ -204,7 +204,7 @@ xfs_rui_init( ruip->rui_format.rui_nextents = nextents; ruip->rui_format.rui_id = (uintptr_t)(void *)ruip; atomic_set(&ruip->rui_next_extent, 0); - atomic_set(&ruip->rui_refcount, 2); + refcount_set(&ruip->rui_refcount, 2); return ruip; } @@ -243,8 +243,8 @@ void xfs_rui_release( struct xfs_rui_log_item *ruip) { - ASSERT(atomic_read(&ruip->rui_refcount) > 0); - if (atomic_dec_and_test(&ruip->rui_refcount)) { + ASSERT(refcount_read(&ruip->rui_refcount) > 0); + if (refcount_dec_and_test(&ruip->rui_refcount)) { xfs_trans_ail_remove(&ruip->rui_item, SHUTDOWN_LOG_IO_ERROR); xfs_rui_item_free(ruip); } diff --git a/fs/xfs/xfs_rmap_item.h b/fs/xfs/xfs_rmap_item.h index 340c968..1e425a9 100644 --- a/fs/xfs/xfs_rmap_item.h +++ b/fs/xfs/xfs_rmap_item.h @@ -64,7 +64,7 @@ struct kmem_zone; */ struct xfs_rui_log_item { struct xfs_log_item rui_item; - atomic_t rui_refcount; + refcount_t rui_refcount; atomic_t rui_next_extent; unsigned long rui_flags; /* misc flags */ struct xfs_rui_log_format rui_format; -- 2.7.4
