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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Stefan Hajnoczi , Shuah Khan , Bobby Eshleman , "Michael S. Tsirkin" , virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, linux-kselftest@vger.kernel.org, Daan De Meyer Subject: Re: [PATCH net] vsock: lock down child_ns_mode as write-once Message-ID: References: <20260217-vsock-ns-write-once-v1-1-a1fb30f289a9@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 Content-Disposition: inline In-Reply-To: On Wed, Feb 18, 2026 at 11:43:42AM +0100, Stefano Garzarella wrote: > On Wed, Feb 18, 2026 at 11:02:02AM +0100, Stefano Garzarella wrote: > > On Tue, Feb 17, 2026 at 05:45:10PM -0800, Bobby Eshleman wrote: > > > From: Bobby Eshleman > > > > > > To improve the security posture of vsock namespacing, this patch locks > > > down the vsock child_ns_mode sysctl setting with a write-once policy. > > > The user may write to child_ns_mode only once in each namespace, making > > > changes to either local or global mode be irreversible. > > > > > > This avoids security breaches where a process in a local namespace may > > > attempt to jailbreak into the global vsock ns space by setting > > > child_ns_mode to "global", creating a new namespace, and accessing the > > > global space through the new namespace. > > > > Commit 6a997f38bdf8 ("vsock: prevent child netns mode switch from local > > to global") should avoid exactly that, so I don't get this. Can you > > elaborate more how this can happen without this patch? > > > > I think here we should talk more about what we described in > > https://lore.kernel.org/netdev/aZNNBc390y6V09qO@sgarzare-redhat/ which > > is that two administrator processes could compete in setting > > `child_ns_mode` and end up creating a namespace with an `ns_mode` > > different from the one set in `child_ns_mode`. But I would also explain > > that this can still be detected by the process by looking at `ns_mode` > > and trying again. With this patch, we avoid this and allow the > > namespace manager to set it once and be sure that it cannot be changed > > again. Oh that's right, I lost track of the original intent when writing this. > > > > > > > > Additionally, fix the test functions that this change would otherwise > > > break by adding "global-parent" and "local-parent" namespaces and using > > > them as intermediaries to spawn namespaces in the given modes. This > > > avoids the need to change "child_ns_mode" in the init_ns. nsenter must > > > be used because ip netns unshares the mount namespace so nested "ip > > > netns add" breaks exec calls from the init ns. > > > > I'm not sure what the policy is in netdev, but I would prefer to have > > selftest changes in another patch (I think earlier in the series so as > > not to break the bisection), in order to simplify backporting (e.g. in > > CentOS Stream, to keep the backport small, I didn't backport the dozens > > of patches for selftest that we did previously). Sounds good! I wasn't sure if breakage so tightly coupled should be in the same patch or not, I'm happy to split it up to ease backporting. > > > > Obviously, if it's not possible and breaks the bisection, I can safely > > skip these changes during the backport. > > > > > > > > Test run: > > > > > > 1..25 > > > ok 1 vm_server_host_client > > > ok 2 vm_client_host_server > > > ok 3 vm_loopback > > > ok 4 ns_host_vsock_ns_mode_ok > > > ok 5 ns_host_vsock_child_ns_mode_ok > > > ok 6 ns_global_same_cid_fails > > > ok 7 ns_local_same_cid_ok > > > ok 8 ns_global_local_same_cid_ok > > > ok 9 ns_local_global_same_cid_ok > > > ok 10 ns_diff_global_host_connect_to_global_vm_ok > > > ok 11 ns_diff_global_host_connect_to_local_vm_fails > > > ok 12 ns_diff_global_vm_connect_to_global_host_ok > > > ok 13 ns_diff_global_vm_connect_to_local_host_fails > > > ok 14 ns_diff_local_host_connect_to_local_vm_fails > > > ok 15 ns_diff_local_vm_connect_to_local_host_fails > > > ok 16 ns_diff_global_to_local_loopback_local_fails > > > ok 17 ns_diff_local_to_global_loopback_fails > > > ok 18 ns_diff_local_to_local_loopback_fails > > > ok 19 ns_diff_global_to_global_loopback_ok > > > ok 20 ns_same_local_loopback_ok > > > ok 21 ns_same_local_host_connect_to_local_vm_ok > > > ok 22 ns_same_local_vm_connect_to_local_host_ok > > > ok 23 ns_delete_vm_ok > > > ok 24 ns_delete_host_ok > > > ok 25 ns_delete_both_ok > > > SUMMARY: PASS=25 SKIP=0 FAIL=0 > > > > IMO this can be removed from the commit message, doesn't add much value > > other than say that all test passes. sgtm! > > > > > > > > Fixes: eafb64f40ca4 ("vsock: add netns to vsock core") > > > Signed-off-by: Bobby Eshleman > > > Suggested-by: Daan De Meyer > > > Suggested-by: Stefano Garzarella > > > --- > > > include/net/af_vsock.h | 6 +++++- > > > include/net/netns/vsock.h | 1 + > > > net/vmw_vsock/af_vsock.c | 10 ++++++---- > > > tools/testing/selftests/vsock/vmtest.sh | 35 > > > +++++++++++++++------------------ > > > 4 files changed, 28 insertions(+), 24 deletions(-) > > > > > > diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h > > > index d3ff48a2fbe0..c7de33039907 100644 > > > --- a/include/net/af_vsock.h > > > +++ b/include/net/af_vsock.h > > > @@ -276,10 +276,14 @@ 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) > > > { > > > + if (xchg(&net->vsock.child_ns_mode_locked, 1)) > > > + return false; > > > + > > > WRITE_ONCE(net->vsock.child_ns_mode, mode); > > > + return true; > > > } > > > > > > static inline enum vsock_net_mode vsock_net_child_mode(struct net *net) > > > diff --git a/include/net/netns/vsock.h b/include/net/netns/vsock.h > > > index b34d69a22fa8..8c855fff8039 100644 > > > --- a/include/net/netns/vsock.h > > > +++ b/include/net/netns/vsock.h > > > @@ -17,5 +17,6 @@ struct netns_vsock { > > > > > > enum vsock_net_mode mode; > > > enum vsock_net_mode child_ns_mode; > > > + int child_ns_mode_locked; > > > }; > > > #endif /* __NET_NET_NAMESPACE_VSOCK_H */ > > > diff --git a/net/vmw_vsock/af_vsock.c b/net/vmw_vsock/af_vsock.c > > > index 9880756d9eff..35e097f4fde8 100644 > > > --- a/net/vmw_vsock/af_vsock.c > > > +++ b/net/vmw_vsock/af_vsock.c > > > @@ -90,14 +90,15 @@ > > > * > > > * - /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. At namespace > > > + * creation, ns_mode is inherited from the parent's child_ns_mode. > > > > We just merged commit a07c33c6f2fc ("vsock: document namespace mode > > sysctls") in the net tree, so we should update also > > Documentation/admin-guide/sysctl/net.rst Indeed. > > > > > * > > > * The init_net mode is "global" and cannot be modified. > > > > Maybe we should also emphasise in the documentation and in the commit > > description that `child_ns_mode` in `init_net` also is write-once, so > > writing `local` to that one by the init process (e.g. systemd), > > essentially will make all the new namespaces in `local` mode. This could > > be useful for container/namespace managers. > > Sounds good. > > > * > > > @@ -2853,7 +2854,8 @@ 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 -EPERM; > > > > So, if `child_ns_mode` is set to `local` but locked, writing `local` > > again will return -EPERM, is this really what we want? > > > > I'm not sure if we can relax it a bit, but then we may race between > > reader and writer, so maybe it's fine like it is in this patch, but we > > should document better that any writes (even same value) after the first > > one will return -EPERM. > > I think we can try in this way: > > 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; > } > > With a comment like this near child_ns_mode_locked in struct netns_vsock: > /* 0 = unlocked > * 1 = locked to GLOBAL (VSOCK_NET_MODE_GLOBAL + 1) > * 2 = locked to LOCAL (VSOCK_NET_MODE_LOCAL + 1) > */ This looks good to me. Will change. > > While writing that, I thought that we can even remove `child_ns_mode_locked` > and use a single variable where encode the value and the state, but maybe > it's an unnecessary extra complexity. > > Stefano > > > > > About that, should we return something different, like -EBUSY ? > > Not a strong opinion, just to differentiate with the other check before. > > > > Thanks, > > Stefano > Differentiating with -EBUSY sounds useful. Will add that and document. Best, Bobby