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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 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: 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 > > > > 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. 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 > > >