• Elena Reshetova's avatar
    posix_acl: convert posix_acl.a_refcount from atomic_t to refcount_t · 66717260
    Elena Reshetova authored
    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 posix_acl.a_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 posix_acl.a_refcount it might make a difference
    in following places:
     - get_cached_acl(): increment in refcount_inc_not_zero() only
       guarantees control dependency on success vs. fully ordered
       atomic counterpart. However this operation is performed under
       rcu_read_lock(), so this should be fine.
     - posix_acl_release(): decrement in refcount_dec_and_test() only
       provides RELEASE ordering and control dependency on success
       vs. fully ordered atomic counterpart
    Suggested-by: default avatarKees Cook <keescook@chromium.org>
    Reviewed-by: default avatarDavid Windsor <dwindsor@gmail.com>
    Reviewed-by: default avatarHans Liljestrand <ishkamiel@gmail.com>
    Signed-off-by: default avatarElena Reshetova <elena.reshetova@intel.com>
    Signed-off-by: default avatarJaegeuk Kim <jaegeuk@kernel.org>
    66717260