From: Kumar Kartikeya Dwivedi memxor@gmail.com
commit 6855e8213e06efcaf7c02a15e12b1ae64b9a7149 upstream.
Currently, action creation using ACT API in replace mode is buggy. When invoking for non-existent action index 42,
tc action replace action bpf obj foo.o sec <xyz> index 42
kernel creates the action, fills up the netlink response, and then just deletes the action after notifying userspace.
tc action show action bpf
doesn't list the action.
This happens due to the following sequence when ovr = 1 (replace mode) is enabled:
tcf_idr_check_alloc is used to atomically check and either obtain reference for existing action at index, or reserve the index slot using a dummy entry (ERR_PTR(-EBUSY)).
This is necessary as pointers to these actions will be held after dropping the idrinfo lock, so bumping the reference count is necessary as we need to insert the actions, and notify userspace by dumping their attributes. Finally, we drop the reference we took using the tcf_action_put_many call in tcf_action_add. However, for the case where a new action is created due to free index, its refcount remains one. This when paired with the put_many call leads to the kernel setting up the action, notifying userspace of its creation, and then tearing it down. For existing actions, the refcount is still held so they remain unaffected.
Fortunately due to rtnl_lock serialization requirement, such an action with refcount == 1 will not be concurrently deleted by anything else, at best CLS API can move its refcount up and down by binding to it after it has been published from tcf_idr_insert_many. Since refcount is atleast one until put_many call, CLS API cannot delete it. Also __tcf_action_put release path already ensures deterministic outcome (either new action will be created or existing action will be reused in case CLS API tries to bind to action concurrently) due to idr lock serialization.
We fix this by making refcount of newly created actions as 2 in ACT API replace mode. A relaxed store will suffice as visibility is ensured only after the tcf_idr_insert_many call.
Note that in case of creation or overwriting using CLS API only (i.e. bind = 1), overwriting existing action object is not allowed, and any such request is silently ignored (without error).
The refcount bump that occurs in tcf_idr_check_alloc call there for existing action will pair with tcf_exts_destroy call made from the owner module for the same action. In case of action creation, there is no existing action, so no tcf_exts_destroy callback happens.
This means no code changes for CLS API.
Fixes: cae422f379f3 ("net: sched: use reference counting action init") Signed-off-by: Kumar Kartikeya Dwivedi memxor@gmail.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/sched/act_api.c | 3 +++ 1 file changed, 3 insertions(+)
--- a/net/sched/act_api.c +++ b/net/sched/act_api.c @@ -1029,6 +1029,9 @@ struct tc_action *tcf_action_init_1(stru if (!name) a->hw_stats = hw_stats;
+ if (!bind && ovr && err == ACT_P_CREATED) + refcount_set(&a->tcfa_refcnt, 2); + return a;
err_out: