On Mon, Feb 23, 2026 at 02:38:33PM -0800, Bobby Eshleman wrote:
From: Bobby Eshleman <[email protected]>
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 <[email protected]>
Suggested-by: Stefano Garzarella <[email protected]>
Co-developed-by: Stefano Garzarella <[email protected]>
Signed-off-by: Stefano Garzarella <[email protected]>
Signed-off-by: Bobby Eshleman <[email protected]>
---
include/net/af_vsock.h | 13 +++++++++++--
include/net/netns/vsock.h | 3 +++
net/vmw_vsock/af_vsock.c | 15 ++++++++++-----
3 files changed, 24 insertions(+), 7 deletions(-)
I still slightly prefer the version I proposed here:
https://lore.kernel.org/netdev/aZbR2H2oDyIAxDef@sgarzare-redhat/
But this definitely affects the code less, so for `net` I think this
patch is better:
Reviewed-by: Stefano Garzarella <[email protected]>
Eventually, we can discuss in net-next patch whether to change the
implementation to the other one, but for the user nothing changes at
all.
Thanks,
Stefano
diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index d3ff48a2fbe0..533d8e75f7bb 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -276,10 +276,19 @@ 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)
{
- WRITE_ONCE(net->vsock.child_ns_mode, mode);
+ int new_locked = mode + 1;
+ int old_locked = 0; /* unlocked */
+
+ if (try_cmpxchg(&net->vsock.child_ns_mode_locked,
+ &old_locked, new_locked)) {
+ WRITE_ONCE(net->vsock.child_ns_mode, mode);
+ return true;
+ }
+
+ return old_locked == new_locked;
}
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..dc8cbe45f406 100644
--- a/include/net/netns/vsock.h
+++ b/include/net/netns/vsock.h
@@ -17,5 +17,8 @@ struct netns_vsock {
enum vsock_net_mode mode;
enum vsock_net_mode child_ns_mode;
+
+ /* 0 = unlocked, 1 = locked to global, 2 = locked to local */
+ 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..50044a838c89 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;
--
2.47.3
