lockd: convert nlm_lockowner.count from atomic_t to refcount_t
authorElena Reshetova <elena.reshetova@intel.com>
Wed, 29 Nov 2017 11:15:45 +0000 (13:15 +0200)
committerJ. Bruce Fields <bfields@redhat.com>
Thu, 21 Dec 2017 20:24:18 +0000 (15:24 -0500)
commit8bb3ea77933e9796f8f15a5492481a96af8302d6
tree396618db808f663f82cc4bed8243546644455b4b
parentbe819f7b66031c4a21fdc8edc47a3ecd4cac635d
lockd: convert nlm_lockowner.count from atomic_t to refcount_t

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_lockowner.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_lockowner.count it might make a difference
in following places:
 - nlm_put_lockowner(): decrement in refcount_dec_and_lock() only
   provides RELEASE ordering, control dependency on success and
   holds a spin lock on success vs. fully ordered atomic counterpart.
   No changes in spin lock guarantees.

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>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
fs/lockd/clntproc.c
include/linux/lockd/lockd.h