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") Suggested-by: Daan De Meyer Suggested-by: Stefano Garzarella Co-developed-by: Stefano Garzarella Signed-off-by: Stefano Garzarella Signed-off-by: Bobby Eshleman --- include/net/af_vsock.h | 13 +++++++++++-- include/net/netns/vsock.h | 3 +++ net/vmw_vsock/af_vsock.c | 15 ++++++++++----- 3 files changed, 24 insertions(+), 7 deletions(-) diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h index d3ff48a2fbe0..533d8e75f7bb 100644 --- a/include/net/af_vsock.h +++ b/include/net/af_vsock.h @@ -276,10 +276,19 @@ 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 new_locked = mode + 1; + int old_locked = 0; /* unlocked */ + + 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; } 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..dc8cbe45f406 100644 --- a/include/net/netns/vsock.h +++ b/include/net/netns/vsock.h @@ -17,5 +17,8 @@ struct netns_vsock { enum vsock_net_mode mode; enum vsock_net_mode child_ns_mode; + + /* 0 = unlocked, 1 = locked to global, 2 = locked to local */ + 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..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