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[82.53.134.58]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4834d8334a8sm680068285e9.12.2026.02.19.02.35.53 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Feb 2026 02:35:54 -0800 (PST) Date: Thu, 19 Feb 2026 11:35:52 +0100 From: Stefano Garzarella To: Bobby Eshleman Cc: "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Stefan Hajnoczi , Shuah Khan , Bobby Eshleman , "Michael S. Tsirkin" , Jonathan Corbet , Shuah Khan , virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-doc@vger.kernel.org, Daan De Meyer Subject: Re: [PATCH net v2 2/3] vsock: lock down child_ns_mode as write-once Message-ID: References: <20260218-vsock-ns-write-once-v2-0-19e4c50d509a@meta.com> <20260218-vsock-ns-write-once-v2-2-19e4c50d509a@meta.com> Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260218-vsock-ns-write-once-v2-2-19e4c50d509a@meta.com> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: FfNZqAcAyCn9GnsYj0LQo26UJjCzeBsugVni4wJF6zc_1771497356 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline On Wed, Feb 18, 2026 at 10:10:37AM -0800, Bobby Eshleman wrote: >From: Bobby Eshleman > >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 >Suggested-by: Daan De Meyer >Suggested-by: Stefano Garzarella 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 >