Messages in this thread Patch in this message | | | From | Elena Reshetova <> | Subject | [PATCH 2/5] fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to refcount_t | Date | Wed, 29 Nov 2017 13:38:00 +0200 |
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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_efi_log_item.efi_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_efi_log_item.efi_refcount it might make a difference in following places: - xfs_efi_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 <keescook@chromium.org> Reviewed-by: David Windsor <dwindsor@gmail.com> Reviewed-by: Hans Liljestrand <ishkamiel@gmail.com> Signed-off-by: Elena Reshetova <elena.reshetova@intel.com> --- fs/xfs/xfs_extfree_item.c | 6 +++--- fs/xfs/xfs_extfree_item.h | 2 +- 2 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/xfs_extfree_item.c b/fs/xfs/xfs_extfree_item.c index 44f8c54..dfe2bcb 100644 --- a/fs/xfs/xfs_extfree_item.c +++ b/fs/xfs/xfs_extfree_item.c @@ -220,7 +220,7 @@ xfs_efi_init( efip->efi_format.efi_nextents = nextents; efip->efi_format.efi_id = (uintptr_t)(void *)efip; atomic_set(&efip->efi_next_extent, 0); - atomic_set(&efip->efi_refcount, 2); + refcount_set(&efip->efi_refcount, 2); return efip; } @@ -290,8 +290,8 @@ void xfs_efi_release( struct xfs_efi_log_item *efip) { - ASSERT(atomic_read(&efip->efi_refcount) > 0); - if (atomic_dec_and_test(&efip->efi_refcount)) { + ASSERT(refcount_read(&efip->efi_refcount) > 0); + if (refcount_dec_and_test(&efip->efi_refcount)) { xfs_trans_ail_remove(&efip->efi_item, SHUTDOWN_LOG_IO_ERROR); xfs_efi_item_free(efip); } diff --git a/fs/xfs/xfs_extfree_item.h b/fs/xfs/xfs_extfree_item.h index a32c794..fadf736 100644 --- a/fs/xfs/xfs_extfree_item.h +++ b/fs/xfs/xfs_extfree_item.h @@ -64,7 +64,7 @@ struct kmem_zone; */ typedef struct xfs_efi_log_item { xfs_log_item_t efi_item; - atomic_t efi_refcount; + refcount_t efi_refcount; atomic_t efi_next_extent; unsigned long efi_flags; /* misc flags */ xfs_efi_log_format_t efi_format; -- 2.7.4
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