Re: [PATCH net v2 2/3] vsock: lock down child_ns_mode as write-once

[Stefano Garzarella <sgarzare@redhat.com>]

On Wed, Feb 18, 2026 at 10:10:37AM -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")
>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>

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

>---
> 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/


> }
>
> 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.

> }
>
> /* 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.

> };
>
> 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.

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

> };
> #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
>