Re: [PATCH net v3 2/3] vsock: lock down child_ns_mode as write-once

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Feb 23, 2026 at 02:38:33PM -0800, Bobby Eshleman wrote:
>From: Bobby Eshleman <bobbyeshleman@meta.com>
>
>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>
>---
> 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 <sgarzare@redhat.com>

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
>