A misbehaved user can create a steering anchor that points to a kernel
flow table and then destroy the anchor without freeing the associated
STC. This creates a problem as the kernel can't destroy the flow
table since there is still a reference to it. As a result, this can
exhaust all available flow table resources, preventing other users from
using the RDMA device.

To prevent this problem, a solution is implemented where a special flow
table with two steering rules is created when a user creates a steering
anchor for the first time. The rules include one that drops all traffic
and another that points to the kernel flow table. If the steering anchor
is destroyed, only the rule pointing to the kernel's flow table is removed.
Any traffic reaching the special flow table after that is dropped.

Since the special flow table is not destroyed when the steering anchor is
destroyed, any issues are prevented from occurring. The remaining resources
are only destroyed when the RDMA device is destroyed, which happens after
all DEVX objects are freed, including the STCs, thus mitigating the issue.

Fixes: 0c6ab0ca ("RDMA/mlx5: Expose steering anchor to userspace")
Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Link: https://lore.kernel.org/r/b4a88a871d651fa4e8f98d552553c1cfe9ba2cd6.1685960567.git.leon@kernel.orgSigned-off-by: Leon Romanovsky <leon@kernel.org>