On Thu, Feb 19, 2026 at 08:20:54AM -0800, Bobby Eshleman wrote:

On Thu, Feb 19, 2026 at 11:35:52AM +0100, Stefano Garzarella wrote:

quoted

On Wed, Feb 18, 2026 at 10:10:37AM -0800, Bobby Eshleman wrote:

quoted

From: Bobby Eshleman <redacted>

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")
Signed-off-by: Bobby Eshleman <redacted>
Suggested-by: Daan De Meyer <redacted>
Suggested-by: Stefano Garzarella <sgarzare@redhat.com>

nit: usually the S-o-b of the author is the last when sending a patch.

Ah good to know, thanks. Will change.

quoted

quoted

---
include/net/af_vsock.h    | 20 +++++++++++++++++---
include/net/netns/vsock.h |  9 ++++++++-
net/vmw_vsock/af_vsock.c  | 15 ++++++++++-----
3 files changed, 35 insertions(+), 9 deletions(-)

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index d3ff48a2fbe0..9bd42147626d 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h

@@ -276,15 +276,29 @@ 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 locked = mode + VSOCK_NET_MODE_LOCKED;
+	int cur;
+
+	cur = READ_ONCE(net->vsock.child_ns_mode);
+	if (cur == locked)
+		return true;
+	if (cur >= VSOCK_NET_MODE_LOCKED)
+		return false;
+
+	if (try_cmpxchg(&net->vsock.child_ns_mode, &cur, locked))
+		return true;
+
+	return cur == locked;

Sorry, it took me a while to get it entirely :-(
This overcomplication is exactly what I wanted to avoid when I proposed the
change in v1:
https://lore.kernel.org/netdev/aZWUmbiH11Eh3Y4v@sgarzare-redhat/ (local)

Glad you thought so too, because I actually think your original proposed
snippet in that thread is the best/simplest so far.

quoted

quoted

}

static inline enum vsock_net_mode vsock_net_child_mode(struct net *net)
{
-	return READ_ONCE(net->vsock.child_ns_mode);
+	int mode = READ_ONCE(net->vsock.child_ns_mode);
+
+	return mode & (VSOCK_NET_MODE_LOCKED - 1);

This is working just because VSOCK_NET_MODE_LOCKED == 2, so IMO this should
at least set as value in the enum and documented on top of vsock_net_mode.

quoted

}

/* Return true if two namespaces pass the mode rules. Otherwise, return false.

diff --git a/include/net/netns/vsock.h b/include/net/netns/vsock.h
index b34d69a22fa8..d20ab6269342 100644
--- a/include/net/netns/vsock.h
+++ b/include/net/netns/vsock.h

@@ -7,6 +7,7 @@

enum vsock_net_mode {
	VSOCK_NET_MODE_GLOBAL,
	VSOCK_NET_MODE_LOCAL,
+	VSOCK_NET_MODE_LOCKED,

This is not really a mode, so IMO should not be part of `enum
vsock_net_mode`. If you really want it, maybe we can add both
VSOCK_NET_MODE_GLOBAL_LOCKED and VSOCK_NET_MODE_LOCAL_LOCKED, which can be
less error prone if we will touch this enum one day.

quoted

};

struct netns_vsock {

@@ -16,6 +17,12 @@ struct netns_vsock {

	u32 port;

	enum vsock_net_mode mode;
-	enum vsock_net_mode child_ns_mode;
+
+	/* 0 (GLOBAL)
+	 * 1 (LOCAL)
+	 * 2 (GLOBAL + LOCKED)
+	 * 3 (LOCAL + LOCKED)
+	 */
+	int child_ns_mode;

Sorry, I don't like this too much, since it seems too complicated to read
and to maintain, If we really want to use just one variable, maybe we can
use -1 as UNSET for child_ns_mode. If it is UNSET, vsock_net_child_mode()
can just return `mode` since it's the default that we also documented, if
it's set, it means that is locked with the value specified.

Maybe with code is easier, I mean something like this:

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index d3ff48a2fbe0..fcd5b538df35 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h

@@ -276,15 +276,25 @@ 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 old = VSOCK_NET_CHILD_NS_UNSET;
+
+	if (try_cmpxchg(&net->vsock.child_ns_mode, &old, mode))
+		return true;
+
+	return old == mode;
 }
 static inline enum vsock_net_mode vsock_net_child_mode(struct net *net)
 {
-	return READ_ONCE(net->vsock.child_ns_mode);
+	int mode = READ_ONCE(net->vsock.child_ns_mode);
+
+	if (mode == VSOCK_NET_CHILD_NS_UNSET)
+		return net->vsock.mode;
+
+	return mode;
 }
 /* Return true if two namespaces pass the mode rules. Otherwise, return false.

diff --git a/include/net/netns/vsock.h b/include/net/netns/vsock.h
index b34d69a22fa8..bf52baf7d7a7 100644
--- a/include/net/netns/vsock.h
+++ b/include/net/netns/vsock.h

@@ -9,6 +9,8 @@ enum vsock_net_mode {
 	VSOCK_NET_MODE_LOCAL,
 };
+#define VSOCK_NET_CHILD_NS_UNSET (-1)
+
 struct netns_vsock {
 	struct ctl_table_header *sysctl_hdr;

@@ -16,6 +18,13 @@ struct netns_vsock {
 	u32 port;
 	enum vsock_net_mode mode;
-	enum vsock_net_mode child_ns_mode;
+
+	/* Write-once child namespace mode, must be initialized to
+	 * VSOCK_NET_CHILD_NS_UNSET. Transitions once from UNSET to a
+	 * vsock_net_mode value via try_cmpxchg on first sysctl write.
+	 * While UNSET, vsock_net_child_mode() returns the namespace's
+	 * own mode since it's the default.
+	 */
+	int child_ns_mode;
 };
 #endif /* __NET_NET_NAMESPACE_VSOCK_H */

diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c
index 9880756d9eff..f0cb7c6a8212 100644
--- a/net/vmw_vsock/af_vsock.c
+++ b/net/vmw_vsock/af_vsock.c

@@ -2853,7 +2853,8 @@ 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;

@@ -2922,7 +2923,7 @@ static void vsock_net_init(struct net *net)
 	else
 		net->vsock.mode = vsock_net_child_mode(current->nsproxy->net_ns);
-	net->vsock.child_ns_mode = net->vsock.mode;
+	net->vsock.child_ns_mode = VSOCK_NET_CHILD_NS_UNSET;
 }
 static __net_init int vsock_sysctl_init_net(struct net *net)

If you like it, please add my Co-developed-by and S-o-b.

Will do!

quoted

BTW, let's discuss here more about it and agree before sending a new
version, so this should also allow other to comment eventually.

Thanks,
Stefano

Tbh, I like your original proposal from v1 best (copied below). I like
that the whole locking mechanism is self-contained there in one place,
and doesn't ripple out elsewhere into the code (e.g.,
vsock_net_child_mode() carrying logic around UNSET). Wdyt?

Initially, yes, I liked that one too, especially because, being a patch 
for net, it remains very small and clear to read. But now, after 
spending some time on how to reuse `child_ns_mode` for that, I also like 
the last version I sent using UNSET so that we don't have the same 
information in two variables.

I'm truly conflicted, but not a strong preference, so if you like more 
the one with `child_ns_mode_locked`, let's go with that, we can always 
change it in the future.

Jacub, Paolo, any preference?

static inline bool vsock_net_set_child_mode(struct net *net,
				    enum vsock_net_mode mode)
{
int new_locked = mode + 1;
int old_locked = 0;

If we are going to use this one, maybe a macro for 0, or a comment here 
+ on top of child_ns_mode_locked should be better.

Thanks,
Stefano

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;
}


Best,
Bobby

quoted

quoted

};
#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