[PATCH bpf-next v3 0/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[PATCH bpf-next v3 0/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[Stanislav Fomichev]

While having a per-net-ns flow dissector programs is convenient for
testing, security-wise it's better to have only one vetted global
flow dissector implementation.

Let's have a convention that when BPF flow dissector is installed
in the root namespace, child namespaces can't override it.

The intended use-case is to attach global BPF flow dissector
early from the init scripts/systemd. Attaching global dissector
is prohibited if some non-root namespace already has flow dissector
attached. Also, attaching to non-root namespace is prohibited
when there is flow dissector attached to the root namespace.

v3:
* drop extra check and empty line (Andrii Nakryiko)

v2:
* EPERM -> EEXIST (Song Liu)
* Make sure we don't have dissector attached to non-root namespaces
  when attaching the global one (Andrii Nakryiko)

Cc: Petar Penkov <ppenkov@google.com>

Stanislav Fomichev (2):
  bpf/flow_dissector: add mode to enforce global BPF flow dissector
  selftests/bpf: add test for BPF flow dissector in the root namespace

 Documentation/bpf/prog_flow_dissector.rst     |  3 ++
 net/core/flow_dissector.c                     | 38 +++++++++++++--
 .../selftests/bpf/test_flow_dissector.sh      | 48 ++++++++++++++++---
 3 files changed, 79 insertions(+), 10 deletions(-)

-- 
2.23.0.581.g78d2f28ef7-goog

[PATCH bpf-next v3 1/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[Stanislav Fomichev]

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 (-EEXIST).

Cc: Petar Penkov <ppenkov@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 Documentation/bpf/prog_flow_dissector.rst |  3 ++
 net/core/flow_dissector.c                 | 38 ++++++++++++++++++++---
 2 files changed, 37 insertions(+), 4 deletions(-)

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..6b4b88d1599d 100644
--- a/net/core/flow_dissector.c
+++ b/net/core/flow_dissector.c
@@ -114,19 +114,46 @@ int skb_flow_dissector_bpf_prog_attach(const union bpf_attr *attr,
 {
 	struct bpf_prog *attached;
 	struct net *net;
+	int ret = 0;
 
 	net = current->nsproxy->net_ns;
 	mutex_lock(&flow_dissector_mutex);
+
+	if (net == &init_net) {
+		/* BPF flow dissector in the root namespace overrides
+		 * any per-net-namespace one. When attaching to root,
+		 * make sure we don't have any BPF program attached
+		 * to the non-root namespaces.
+		 */
+		struct net *ns;
+
+		for_each_net(ns) {
+			if (rcu_access_pointer(ns->flow_dissector_prog)) {
+				ret = -EEXIST;
+				goto out;
+			}
+		}
+	} else {
+		/* Make sure root flow dissector is not attached
+		 * when attaching to the non-root namespace.
+		 */
+		if (rcu_access_pointer(init_net.flow_dissector_prog)) {
+			ret = -EEXIST;
+			goto out;
+		}
+	}
+
 	attached = rcu_dereference_protected(net->flow_dissector_prog,
 					     lockdep_is_held(&flow_dissector_mutex));
 	if (attached) {
 		/* Only one BPF program can be attached at a time */
-		mutex_unlock(&flow_dissector_mutex);
-		return -EEXIST;
+		ret = -EEXIST;
+		goto out;
 	}
 	rcu_assign_pointer(net->flow_dissector_prog, prog);
+out:
 	mutex_unlock(&flow_dissector_mutex);
-	return 0;
+	return ret;
 }
 
 int skb_flow_dissector_bpf_prog_detach(const union bpf_attr *attr)
@@ -910,7 +937,10 @@ 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;
-- 
2.23.0.581.g78d2f28ef7-goog

[PATCH bpf-next v3 2/2] selftests/bpf: add test for BPF flow dissector in the root namespace

[Stanislav Fomichev]

Make sure non-root namespaces get an error if root flow dissector is
attached.

Cc: Petar Penkov <ppenkov@google.com>
Acked-by: Song Liu <songliubraving@fb.com>

Signed-off-by: Stanislav Fomichev <sdf@google.com>
---
 .../selftests/bpf/test_flow_dissector.sh      | 48 ++++++++++++++++---
 1 file changed, 42 insertions(+), 6 deletions(-)

diff --git a/tools/testing/selftests/bpf/test_flow_dissector.sh b/tools/testing/selftests/bpf/test_flow_dissector.sh
index d23d4da66b83..2c3a25d64faf 100755
--- a/tools/testing/selftests/bpf/test_flow_dissector.sh
+++ b/tools/testing/selftests/bpf/test_flow_dissector.sh
@@ -18,19 +18,55 @@ fi
 # this is the case and run it with in_netns.sh if it is being run in the root
 # namespace.
 if [[ -z $(ip netns identify $$) ]]; then
+	err=0
+	if bpftool="$(which bpftool)"; then
+		echo "Testing global flow dissector..."
+
+		$bpftool prog loadall ./bpf_flow.o /sys/fs/bpf/flow \
+			type flow_dissector
+
+		if ! unshare --net $bpftool prog attach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Unexpected unsuccessful attach in namespace" >&2
+			err=1
+		fi
+
+		$bpftool prog attach pinned /sys/fs/bpf/flow/flow_dissector \
+			flow_dissector
+
+		if unshare --net $bpftool prog attach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Unexpected successful attach in namespace" >&2
+			err=1
+		fi
+
+		if ! $bpftool prog detach pinned \
+			/sys/fs/bpf/flow/flow_dissector flow_dissector; then
+			echo "Failed to detach flow dissector" >&2
+			err=1
+		fi
+
+		rm -rf /sys/fs/bpf/flow
+	else
+		echo "Skipping root flow dissector test, bpftool not found" >&2
+	fi
+
+	# Run the rest of the tests in a net namespace.
 	../net/in_netns.sh "$0" "$@"
-	exit $?
-fi
+	err=$(( $err + $? ))
 
-# Determine selftest success via shell exit code
-exit_handler()
-{
-	if (( $? == 0 )); then
+	if (( $err == 0 )); then
 		echo "selftests: $TESTNAME [PASS]";
 	else
 		echo "selftests: $TESTNAME [FAILED]";
 	fi
 
+	exit $err
+fi
+
+# Determine selftest success via shell exit code
+exit_handler()
+{
 	set +e
 
 	# Cleanup
-- 
2.23.0.581.g78d2f28ef7-goog

Re: [PATCH bpf-next v3 0/2] bpf/flow_dissector: add mode to enforce global BPF flow dissector

[Alexei Starovoitov]

On Mon, Oct 7, 2019 at 9:21 AM Stanislav Fomichev <sdf@google.com> wrote:
>
> While having a per-net-ns flow dissector programs is convenient for
> testing, security-wise it's better to have only one vetted global
> flow dissector implementation.
>
> Let's have a convention that when BPF flow dissector is installed
> in the root namespace, child namespaces can't override it.
>
> The intended use-case is to attach global BPF flow dissector
> early from the init scripts/systemd. Attaching global dissector
> is prohibited if some non-root namespace already has flow dissector
> attached. Also, attaching to non-root namespace is prohibited
> when there is flow dissector attached to the root namespace.
>
> v3:
> * drop extra check and empty line (Andrii Nakryiko)
>
> v2:
> * EPERM -> EEXIST (Song Liu)
> * Make sure we don't have dissector attached to non-root namespaces
>   when attaching the global one (Andrii Nakryiko)

Applied. Thanks