vsock: lock down child_ns_mode as write-once

Two administrator processes may race when setting child_ns_mode as one
process sets child_ns_mode to "local" and then creates a namespace, but
another process changes child_ns_mode to "global" between the write and
the namespace creation. The first process ends up with a namespace in
"global" mode instead of "local". While this can be detected after the
fact by reading ns_mode and retrying, it is fragile and error-prone.

Make child_ns_mode write-once so that a namespace manager can set it
once and be sure it won't change. Writing a different value after the
first write returns -EBUSY. This applies to all namespaces, including
init_net, where an init process can write "local" to lock all future
namespaces into local mode.

Fixes: eafb64f40ca4 ("vsock: add netns to vsock core")
Suggested-by: Daan De Meyer <daan.j.demeyer@gmail.com>
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Co-developed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260223-vsock-ns-write-once-v3-2-c0cde6959923@meta.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

1 files changed, 10 insertions, 5 deletions

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index f4062c6a1944..2f7d94d682cb 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c
@@ -90,16 +90,20 @@
  *
  *   - /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. Writing a different
+ *   value after the first write returns -EBUSY. At namespace creation, ns_mode
+ *   is inherited from the parent's child_ns_mode.
  *
- *   The init_net mode is "global" and cannot be modified.
+ *   The init_net mode is "global" and cannot be modified. The init_net
+ *   child_ns_mode is also write-once, so an init process (e.g. systemd) can
+ *   set it to "local" to ensure all new namespaces inherit local mode.
  *
  *   The modes affect the allocation and accessibility of CIDs as follows:
  *
@@ -2853,7 +2857,8 @@ 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 -EBUSY;
 	}
 
 	return 0;