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_bui_log_item.bui_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_bui_log_item.bui_refcount it might make a difference in following places: - xfs_bui_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 <keesc...@chromium.org> Reviewed-by: David Windsor <dwind...@gmail.com> Reviewed-by: Hans Liljestrand <ishkam...@gmail.com> Signed-off-by: Elena Reshetova <elena.reshet...@intel.com> --- fs/xfs/xfs_bmap_item.c | 6 +++--- fs/xfs/xfs_bmap_item.h | 2 +- fs/xfs/xfs_linux.h | 1 + 3 files changed, 5 insertions(+), 4 deletions(-) diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c index dd136f7..0f2e3d8 100644 --- a/fs/xfs/xfs_bmap_item.c +++ b/fs/xfs/xfs_bmap_item.c @@ -201,7 +201,7 @@ xfs_bui_init( buip->bui_format.bui_nextents = XFS_BUI_MAX_FAST_EXTENTS; buip->bui_format.bui_id = (uintptr_t)(void *)buip; atomic_set(&buip->bui_next_extent, 0); - atomic_set(&buip->bui_refcount, 2); + refcount_set(&buip->bui_refcount, 2); return buip; } @@ -217,8 +217,8 @@ void xfs_bui_release( struct xfs_bui_log_item *buip) { - ASSERT(atomic_read(&buip->bui_refcount) > 0); - if (atomic_dec_and_test(&buip->bui_refcount)) { + ASSERT(refcount_read(&buip->bui_refcount) > 0); + if (refcount_dec_and_test(&buip->bui_refcount)) { xfs_trans_ail_remove(&buip->bui_item, SHUTDOWN_LOG_IO_ERROR); xfs_bui_item_free(buip); } diff --git a/fs/xfs/xfs_bmap_item.h b/fs/xfs/xfs_bmap_item.h index c867daa..7048b14 100644 --- a/fs/xfs/xfs_bmap_item.h +++ b/fs/xfs/xfs_bmap_item.h @@ -61,7 +61,7 @@ struct kmem_zone; */ struct xfs_bui_log_item { struct xfs_log_item bui_item; - atomic_t bui_refcount; + refcount_t bui_refcount; atomic_t bui_next_extent; unsigned long bui_flags; /* misc flags */ struct xfs_bui_log_format bui_format; diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h index 99562ec..31658ac 100644 --- a/fs/xfs/xfs_linux.h +++ b/fs/xfs/xfs_linux.h @@ -20,6 +20,7 @@ #include <linux/types.h> #include <linux/uuid.h> +#include <linux/refcount.h> /* * Kernel specific type declarations for XFS -- 2.7.4
