Re: [PATCH net v2 2/3] vsock: lock down child_ns_mode as write-once
From: Stefano Garzarella <sgarzare@redhat.com>
Date: 2026-02-19 10:36:00
Also in:
kvm, linux-doc, linux-kselftest, lkml, virtualization
Subsystem:
networking [general], the rest, virtio and vhost vsock driver, vm sockets (af_vsock) · Maintainers:
"David S. Miller", Eric Dumazet, Jakub Kicinski, Paolo Abeni, Linus Torvalds, Stefan Hajnoczi, Stefano Garzarella
On Wed, Feb 18, 2026 at 10:10:37AM -0800, Bobby Eshleman wrote:
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.
quoted hunk ↗ jump to hunk
--- 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)
}
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 hunk ↗ jump to hunk
} /* 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 hunk ↗ jump to hunk
}; 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
quoted hunk ↗ jump to hunk
}; #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