Commit 102eab95 authored by Bobby Eshleman's avatar Bobby Eshleman Committed by Paolo Abeni
Browse files

vsock: lock down child_ns_mode as write-once 



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: eafb64f4 ("vsock: add netns to vsock core")
Suggested-by: default avatarDaan De Meyer <daan.j.demeyer@gmail.com>
Suggested-by: default avatarStefano Garzarella <sgarzare@redhat.com>
Co-developed-by: default avatarStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: default avatarStefano Garzarella <sgarzare@redhat.com>
Signed-off-by: default avatarBobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: default avatarStefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20260223-vsock-ns-write-once-v3-2-c0cde6959923@meta.com


Signed-off-by: default avatarPaolo Abeni <pabeni@redhat.com>
parent a382a342

include/net/af_vsock.h

+11 −2
Original line number Diff line number Diff line
@@ -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)

{

	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)

include/net/netns/vsock.h

+3 −0
Original line number Diff line number Diff line
@@ -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 */

net/vmw_vsock/af_vsock.c

+10 −5
Original line number Diff line number Diff line
@@ -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;