• Elena Reshetova's avatar
    lockd: convert nlm_host.h_count from atomic_t to refcount_t · fee21fb5
    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 nlm_host.h_count  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 nlm_host.h_count it might make a difference
    in following places:
     - nlmsvc_release_host(): decrement in refcount_dec()
       provides RELEASE ordering, while original atomic_dec()
       was fully unordered. Since the change is for better, it
       should not matter.
     - nlmclnt_release_host(): decrement in refcount_dec_and_test() only
       provides RELEASE ordering and control dependency on success
       vs. fully ordered atomic counterpart. It doesn't seem to
       matter in this case since object freeing happens under mutex
       lock anyway.
    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 avatarTrond Myklebust <trond.myklebust@primarydata.com>
    fee21fb5
host.c 17.2 KB