Re: [PATCH] RDMA/uverbs: Remove needs_kfree_rcu from uverbs_obj_type_class

[Jason Gunthorpe <jgg@ziepe.ca>]

On Mon, Jan 13, 2020 at 02:33:10PM +0000, Jason Gunthorpe wrote:
> After device disassociation the uapi_objects are destroyed and freed,
> however it is still possible that core code can be holding a kref on the
> uobject. When it finally goes to uverbs_uobject_free() via the kref_put()
> it can trigger a use-after-free on the uapi_object.
> 
> Since needs_kfree_rcu is a micro optimization that only benefits file
> uobjects, just get rid of it. There is no harm in using kfree_rcu even if
> it isn't required, and the number of involved objects is small.
> 
> Signed-off-by: Michael Guralnik <michaelgur@mellanox.com>
> Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> ---
>  drivers/infiniband/core/rdma_core.c | 23 +----------------------
>  include/rdma/uverbs_types.h         |  1 -
>  2 files changed, 1 insertion(+), 23 deletions(-)
> 
> This should go before the 'refactoring fd usage' series as more
> testing has shown the reworked code can trivially trigger this
> existing bug.

Applied to for-next

Though it seems this might not be an existing problem as none of the
existing kref users can outlive disassociation. Nevertheless it is
very surprising that the kref becomes a segfault after disassociation.

Jason