Re: [PATCH] uprobes: convert uprobe.ref to refcount_t

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Elena Reshetova <elena.reshetova@intel.com> [2019-01-16 13:20:27]:

> 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 uprobe.ref 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 uprobe.ref it might make a difference
> in following places:
>  - put_uprobe(): 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>
> ---
>  kernel/events/uprobes.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

-- 
Thanks and Regards
Srikar Dronamraju