[PATCH bpf-next 1/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[Stanislav Fomichev <sdf@google.com>]

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@google.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 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;
+	}
+
 	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