On Thu, Feb 19, 2026 at 11:35:52AM +0100, Stefano Garzarella wrote:
> On Wed, Feb 18, 2026 at 10:10:37AM -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")
> > Signed-off-by: Bobby Eshleman <[email protected]>
> > Suggested-by: Daan De Meyer <[email protected]>
> > Suggested-by: Stefano Garzarella <[email protected]>
>
> nit: usually the S-o-b of the author is the last when sending a patch.
Ah good to know, thanks. Will change.
>
> > ---
> > 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/
Glad you thought so too, because I actually think your original proposed
snippet in that thread is the best/simplest so far.
>
>
> > }
> >
> > 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.
Will do!
>
> 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?
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 (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
>
> > };
> > #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
> >
>
