On Fri, Sep 18, 2026 at 12:02:24AM +0200, Bartłomiej Dmitruk wrote:
vmci_transport_allow_dgram() gates delivery of incoming VMCI datagrams. It
has two problems, both in this one function, fixed together here.
1. Namespace bypass: the send hook vmci_transport_dgram_allow() refuses
datagrams when the socket's netns is not in global mode
(vsock_net_mode_global()), but the receive path never checks it. A socket
bound in a non-global (local) netns therefore receives datagrams from
peers it could never send to, defeating namespace isolation.
Is the VMCI transport supporting namespaces? (CCing Bobby)
2. Data race / stale ACL: the decision was cached in vsock->cached_peer and
vsock->cached_peer_allow_dgram with an unsynchronized check-then-set.
The function is called both from the lockless receive tasklet
(vmci_transport_recv_dgram_cb(), no socket lock) and from the lock_sock()
send path; lock_sock() does not exclude bottom halves, so the two contexts
race on those fields. An interleave can return a stale 'allow' for a
VMCI_PRIVILEGE_FLAG_RESTRICTED peer (and it is a plain data race
regardless). The in-code comment claiming the fields are never modified
outside create/destruct is contradicted by the send path.
2 issues -> 2 fixes, no?
In this case mainly to backport them properly, since for example we
added namespace support only recently.
Please also add the right Fixes tag and all maintainers in CC.
Stefano
Add the symmetric net-mode check and drop the cache, evaluating the decision
on every datagram; there is no shared mutable state left to race. The now
unused cached_peer{,_allow_dgram} members can be removed in a follow-up.
Signed-off-by: Bartłomiej Dmitruk <[email protected]>
---
diff --git a/net/vmw_vsock/vmci_transport.c b/net/vmw_vsock/vmci_transport.c
--- a/net/vmw_vsock/vmci_transport.c
+++ b/net/vmw_vsock/vmci_transport.c
@@ -529,18 +529,25 @@
if (VMADDR_CID_HYPERVISOR == peer_cid)
return true;
- if (vsock->cached_peer != peer_cid) {
- vsock->cached_peer = peer_cid;
- if (!vmci_transport_is_trusted(vsock, peer_cid) &&
- (vmci_context_get_priv_flags(peer_cid) &
- VMCI_PRIVILEGE_FLAG_RESTRICTED)) {
- vsock->cached_peer_allow_dgram = false;
- } else {
- vsock->cached_peer_allow_dgram = true;
- }
- }
+ /* Enforce the per-netns mode on the receive path, symmetrically with
+ * the send hook vmci_transport_dgram_allow(): a socket in a non-global
+ * (local) netns must not receive datagrams it could never send.
+ */
+ if (!vsock_net_mode_global(vsock))
+ return false;
- return vsock->cached_peer_allow_dgram;
+ /* Evaluate on every datagram instead of caching the decision in
+ * vsock->cached_peer{,_allow_dgram}: those fields were an
+ * unsynchronized check-then-set shared between the lockless receive
+ * tasklet and the lock_sock() send path, which could return a stale
+ * 'allow' for a restricted peer.
+ */
+ if (!vmci_transport_is_trusted(vsock, peer_cid) &&
+ (vmci_context_get_priv_flags(peer_cid) &
+ VMCI_PRIVILEGE_FLAG_RESTRICTED))
+ return false;
+
+ return true;
}
static int
