[PATCH net] vsock: lock down child_ns_mode as write-once

[Bobby Eshleman <bobbyeshleman@gmail.com>]

From: Bobby Eshleman <bobbyeshleman@meta.com>

To improve the security posture of vsock namespacing, this patch locks
down the vsock child_ns_mode sysctl setting with a write-once policy.
The user may write to child_ns_mode only once in each namespace, making
changes to either local or global mode be irreversible.

This avoids security breaches where a process in a local namespace may
attempt to jailbreak into the global vsock ns space by setting
child_ns_mode to "global", creating a new namespace, and accessing the
global space through the new namespace.

Additionally, fix the test functions that this change would otherwise
break by adding "global-parent" and "local-parent" namespaces and using
them as intermediaries to spawn namespaces in the given modes. This
avoids the need to change "child_ns_mode" in the init_ns. nsenter must
be used because ip netns unshares the mount namespace so nested "ip
netns add" breaks exec calls from the init ns.

Test run:

1..25
ok 1 vm_server_host_client
ok 2 vm_client_host_server
ok 3 vm_loopback
ok 4 ns_host_vsock_ns_mode_ok
ok 5 ns_host_vsock_child_ns_mode_ok
ok 6 ns_global_same_cid_fails
ok 7 ns_local_same_cid_ok
ok 8 ns_global_local_same_cid_ok
ok 9 ns_local_global_same_cid_ok
ok 10 ns_diff_global_host_connect_to_global_vm_ok
ok 11 ns_diff_global_host_connect_to_local_vm_fails
ok 12 ns_diff_global_vm_connect_to_global_host_ok
ok 13 ns_diff_global_vm_connect_to_local_host_fails
ok 14 ns_diff_local_host_connect_to_local_vm_fails
ok 15 ns_diff_local_vm_connect_to_local_host_fails
ok 16 ns_diff_global_to_local_loopback_local_fails
ok 17 ns_diff_local_to_global_loopback_fails
ok 18 ns_diff_local_to_local_loopback_fails
ok 19 ns_diff_global_to_global_loopback_ok
ok 20 ns_same_local_loopback_ok
ok 21 ns_same_local_host_connect_to_local_vm_ok
ok 22 ns_same_local_vm_connect_to_local_host_ok
ok 23 ns_delete_vm_ok
ok 24 ns_delete_host_ok
ok 25 ns_delete_both_ok
SUMMARY: PASS=25 SKIP=0 FAIL=0

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Suggested-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
---
 include/net/af_vsock.h                  |  6 +++++-
 include/net/netns/vsock.h               |  1 +
 net/vmw_vsock/af_vsock.c                | 10 ++++++----
 tools/testing/selftests/vsock/vmtest.sh | 35 +++++++++++++++------------------
 4 files changed, 28 insertions(+), 24 deletions(-)

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index d3ff48a2fbe0..c7de33039907 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -276,10 +276,14 @@ static inline bool vsock_net_mode_global(struct vsock_sock *vsk)
 	return vsock_net_mode(sock_net(sk_vsock(vsk))) == VSOCK_NET_MODE_GLOBAL;
 }
 
-static inline void vsock_net_set_child_mode(struct net *net,
+static inline bool vsock_net_set_child_mode(struct net *net,
 					    enum vsock_net_mode mode)
 {
+	if (xchg(&net->vsock.child_ns_mode_locked, 1))
+		return false;
+
 	WRITE_ONCE(net->vsock.child_ns_mode, mode);
+	return true;
 }
 
 static inline enum vsock_net_mode vsock_net_child_mode(struct net *net)
diff --git a/include/net/netns/vsock.h b/include/net/netns/vsock.h
index b34d69a22fa8..8c855fff8039 100644
--- a/include/net/netns/vsock.h
+++ b/include/net/netns/vsock.h
@@ -17,5 +17,6 @@ struct netns_vsock {
 
 	enum vsock_net_mode mode;
 	enum vsock_net_mode child_ns_mode;
+	int child_ns_mode_locked;
 };
 #endif /* __NET_NET_NAMESPACE_VSOCK_H */
diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 9880756d9eff..35e097f4fde8 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -90,14 +90,15 @@
  *
  *   - /proc/sys/net/vsock/ns_mode (read-only) reports the current namespace's
  *     mode, which is set at namespace creation and immutable thereafter.
- *   - /proc/sys/net/vsock/child_ns_mode (writable) controls what mode future
+ *   - /proc/sys/net/vsock/child_ns_mode (write-once) controls what mode future
  *     child namespaces will inherit when created. The initial value matches
  *     the namespace's own ns_mode.
  *
  *   Changing child_ns_mode only affects newly created namespaces, not the
  *   current namespace or existing children. A "local" namespace cannot set
- *   child_ns_mode to "global". At namespace creation, ns_mode is inherited
- *   from the parent's child_ns_mode.
+ *   child_ns_mode to "global". child_ns_mode is write-once, so that it may
+ *   be configured and locked down by a namespace manager. At namespace
+ *   creation, ns_mode is inherited from the parent's child_ns_mode.
  *
  *   The init_net mode is "global" and cannot be modified.
  *
@@ -2853,7 +2854,8 @@ static int vsock_net_child_mode_string(const struct ctl_table *table, int write,
 		    new_mode == VSOCK_NET_MODE_GLOBAL)
 			return -EPERM;
 
-		vsock_net_set_child_mode(net, new_mode);
+		if (!vsock_net_set_child_mode(net, new_mode))
+			return -EPERM;
 	}
 
 	return 0;
diff --git a/tools/testing/selftests/vsock/vmtest.sh b/tools/testing/selftests/vsock/vmtest.sh
index dc8dbe74a6d0..e1e78b295e41 100755
--- a/tools/testing/selftests/vsock/vmtest.sh
+++ b/tools/testing/selftests/vsock/vmtest.sh
@@ -210,16 +210,17 @@ check_result() {
 }
 
 add_namespaces() {
-	local orig_mode
-	orig_mode=$(cat /proc/sys/net/vsock/child_ns_mode)
+	ip netns add "global-parent" 2>/dev/null
+	echo "global" | ip netns exec "global-parent" \
+		tee /proc/sys/net/vsock/child_ns_mode &>/dev/null
+	ip netns add "local-parent" 2>/dev/null
+	echo "local" | ip netns exec "local-parent" \
+		tee /proc/sys/net/vsock/child_ns_mode &>/dev/null
 
-	for mode in "${NS_MODES[@]}"; do
-		echo "${mode}" > /proc/sys/net/vsock/child_ns_mode
-		ip netns add "${mode}0" 2>/dev/null
-		ip netns add "${mode}1" 2>/dev/null
-	done
-
-	echo "${orig_mode}" > /proc/sys/net/vsock/child_ns_mode
+	nsenter --net=/var/run/netns/global-parent ip netns add "global0" 2>/dev/null
+	nsenter --net=/var/run/netns/global-parent ip netns add "global1" 2>/dev/null
+	nsenter --net=/var/run/netns/local-parent ip netns add "local0" 2>/dev/null
+	nsenter --net=/var/run/netns/local-parent ip netns add "local1" 2>/dev/null
 }
 
 init_namespaces() {
@@ -237,6 +238,8 @@ del_namespaces() {
 		log_host "removed ns ${mode}0"
 		log_host "removed ns ${mode}1"
 	done
+	ip netns del "global-parent" &>/dev/null
+	ip netns del "local-parent" &>/dev/null
 }
 
 vm_ssh() {
@@ -287,7 +290,7 @@ check_args() {
 }
 
 check_deps() {
-	for dep in vng ${QEMU} busybox pkill ssh ss socat; do
+	for dep in vng ${QEMU} busybox pkill ssh ss socat nsenter; do
 		if [[ ! -x $(command -v "${dep}") ]]; then
 			echo -e "skip:    dependency ${dep} not found!\n"
 			exit "${KSFT_SKIP}"
@@ -1231,12 +1234,8 @@ test_ns_local_same_cid_ok() {
 }
 
 test_ns_host_vsock_child_ns_mode_ok() {
-	local orig_mode
-	local rc
-
-	orig_mode=$(cat /proc/sys/net/vsock/child_ns_mode)
+	local rc="${KSFT_PASS}"
 
-	rc="${KSFT_PASS}"
 	for mode in "${NS_MODES[@]}"; do
 		local ns="${mode}0"
 
@@ -1246,15 +1245,13 @@ test_ns_host_vsock_child_ns_mode_ok() {
 			continue
 		fi
 
-		if ! echo "${mode}" > /proc/sys/net/vsock/child_ns_mode; then
-			log_host "child_ns_mode should be writable to ${mode}"
+		if ! echo "${mode}" | ip netns exec "${ns}" \
+			tee /proc/sys/net/vsock/child_ns_mode &>/dev/null; then
 			rc="${KSFT_FAIL}"
 			continue
 		fi
 	done
 
-	echo "${orig_mode}" > /proc/sys/net/vsock/child_ns_mode
-
 	return "${rc}"
 }
 

---
base-commit: 77c5e3fdd2793f478e6fdae55c9ea85b21d06f8f
change-id: 20260217-vsock-ns-write-once-8834d684e0a2

Best regards,
-- 
Bobby Eshleman <bobbyeshleman@meta.com>