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[82.53.134.58]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-483a31b3e0dsm22057385e9.1.2026.02.19.08.36.54 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Feb 2026 08:36:55 -0800 (PST) Date: Thu, 19 Feb 2026 17:36:50 +0100 From: Stefano Garzarella To: Bobby Eshleman , Jakub Kicinski , Paolo Abeni 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: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: On Thu, Feb 19, 2026 at 08:20:54AM -0800, Bobby Eshleman wrote: >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? Initially, yes, I liked that one too, especially because, being a patch for net, it remains very small and clear to read. But now, after spending some time on how to reuse `child_ns_mode` for that, I also like the last version I sent using UNSET so that we don't have the same information in two variables. I'm truly conflicted, but not a strong preference, so if you like more the one with `child_ns_mode_locked`, let's go with that, we can always change it in the future. Jacub, Paolo, any preference? > >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 we are going to use this one, maybe a macro for 0, or a comment here + on top of child_ns_mode_locked should be better. Thanks, Stefano > > 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 >> > >> >