On Wed, Oct 2, 2019 at 10:35 AM Stanislav Fomichev <sdf@xxxxxxxxxx> wrote:
>
> Always use init_net flow dissector BPF program if it's attached and fall
> back to the per-net namespace one. Also, deny installing new programs if
> there is already one attached to the root namespace.
> Users can still detach their BPF programs, but can't attach any
> new ones (-EPERM).
>
> Cc: Petar Penkov <ppenkov@xxxxxxxxxx>
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> ---
> Documentation/bpf/prog_flow_dissector.rst | 3 +++
> net/core/flow_dissector.c | 11 ++++++++++-
> 2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/Documentation/bpf/prog_flow_dissector.rst b/Documentation/bpf/prog_flow_dissector.rst
> index a78bf036cadd..4d86780ab0f1 100644
> --- a/Documentation/bpf/prog_flow_dissector.rst
> +++ b/Documentation/bpf/prog_flow_dissector.rst
> @@ -142,3 +142,6 @@ BPF flow dissector doesn't support exporting all the metadata that in-kernel
> C-based implementation can export. Notable example is single VLAN (802.1Q)
> and double VLAN (802.1AD) tags. Please refer to the ``struct bpf_flow_keys``
> for a set of information that's currently can be exported from the BPF context.
> +
> +When BPF flow dissector is attached to the root network namespace (machine-wide
> +policy), users can't override it in their child network namespaces.
> diff --git a/net/core/flow_dissector.c b/net/core/flow_dissector.c
> index 7c09d87d3269..494e2016fe84 100644
> --- a/net/core/flow_dissector.c
> +++ b/net/core/flow_dissector.c
> @@ -115,6 +115,11 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
> struct bpf_prog *attached;
> struct net *net;
>
> + if (rcu_access_pointer(init_net.flow_dissector_prog)) {
> + /* Can't override root flow dissector program */
> + return -EPERM;
> + }
This is racy, shouldn't this be checked after grabbing a lock below?
> +
> net = current->nsproxy->net_ns;
> mutex_lock(&flow_dissector_mutex);
> attached = rcu_dereference_protected(net->flow_dissector_prog,
> @@ -910,7 +915,11 @@ bool __skb_flow_dissect(const struct net *net,
> WARN_ON_ONCE(!net);
> if (net) {
> rcu_read_lock();
> - attached = rcu_dereference(net->flow_dissector_prog);
> + attached =
> + rcu_dereference(init_net.flow_dissector_prog);
> +
> + if (!attached)
> + attached = rcu_dereference(net->flow_dissector_prog);
>
> if (attached) {
> struct bpf_flow_keys flow_keys;
> --
> 2.23.0.444.g18eeb5a265-goog
>
