[PATCH v2 1/2] vsock/vmci: make the cached_peer dgram decision race-safe

["Bartłomiej Dmitruk" <bartlomiej.dmitruk@isec.pl>]

vmci_transport_allow_dgram() cached its result in vsock->cached_peer and
vsock->cached_peer_allow_dgram with an unsynchronized check-then-set. The
function runs both in the lockless receive tasklet
(vmci_transport_recv_dgram_cb(), no socket lock) and in the lock_sock() send
path; lock_sock() does not exclude bottom halves, so the two contexts race on
those fields and can return a stale 'allow' for a VMCI_PRIVILEGE_FLAG_RESTRICTED
peer. It is also a plain data race. The in-code comment claiming the fields
are never modified outside create/destruct is contradicted by the send path.

Keep the O(1) cache -- it avoids an O(N) vmci_ctx_get() lookup on every
datagram in the bottom-half receive path -- but pack the peer CID and the
decision into a single word accessed with READ_ONCE()/WRITE_ONCE(). A race
then only forces a recompute and can never return a stale allow.

This was found by code inspection; I do not have VMCI hardware to test on
(compile-tested only).

Fixes: d021c344051a ("VSOCK: Introduce VM Sockets")
Signed-off-by: Bartłomiej Dmitruk <bartlomiej.dmitruk@isec.pl>
Assisted-by: Claude (Anthropic)
---
v2: keep an O(1) cache made race-safe rather than dropping it entirely; an
    earlier revision removed the cache, which the Sashiko AI review flagged as
    an O(N)-per-datagram fast-path regression. Split out per Stefano
    Garzarella; independent of namespace support.
v1: https://lore.kernel.org/netdev/20260917220225.56200-1-bartlomiej.dmitruk@isec.pl/

diff --git a/include/net/af_vsock.h b/include/net/af_vsock.h
index 5549298c1..97968ac53 100644
--- a/include/net/af_vsock.h
+++ b/include/net/af_vsock.h
@@ -39,10 +39,13 @@ struct vsock_sock {
 	 * modified outsided of socket create or destruct.
 	 */
 	bool trusted;
-	bool cached_peer_allow_dgram;	/* Dgram communication allowed to
-					 * cached peer?
-					 */
-	u32 cached_peer;  /* Context ID of last dgram destination check. */
+	/* Cached dgram access decision for the last peer, packed as
+	 * (cid << 32) | VALID | ALLOW and accessed via READ_ONCE()/
+	 * WRITE_ONCE() so the lockless receive tasklet and the
+	 * lock_sock() send path cannot race to a stale decision.
+	 * See vmci_transport_allow_dgram().
+	 */
+	u64 cached_peer_access;
 	const struct cred *owner;
 	/* Rest are SOCK_STREAM only. */
 	long connect_timeout;
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
@@ -524,23 +524,38 @@
  * only if it is trusted as described in vmci_transport_is_trusted.
  */
 
+/* Packing for vsk->cached_peer_access. */
+#define VMCI_DGRAM_ACCESS_VALID		BIT_ULL(0)
+#define VMCI_DGRAM_ACCESS_ALLOW		BIT_ULL(1)
+#define VMCI_DGRAM_ACCESS_CID_SHIFT	32
+
 static bool vmci_transport_allow_dgram(struct vsock_sock *vsock, u32 peer_cid)
 {
+	u64 access;
+
 	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;
-		}
-	}
+	/* Cache the trusted/restricted decision for the last peer to avoid the
+	 * O(N) vmci_ctx_get() lookup on every datagram.  Read/update it through
+	 * a single word so a race between the lockless receive tasklet and the
+	 * lock_sock() send path only forces a recompute -- it can never return a
+	 * stale allow for a restricted peer.
+	 */
+	access = READ_ONCE(vsock->cached_peer_access);
+	if ((access & VMCI_DGRAM_ACCESS_VALID) &&
+	    (u32)(access >> VMCI_DGRAM_ACCESS_CID_SHIFT) == peer_cid)
+		return !!(access & VMCI_DGRAM_ACCESS_ALLOW);
 
-	return vsock->cached_peer_allow_dgram;
+	access = VMCI_DGRAM_ACCESS_VALID |
+		 ((u64)peer_cid << VMCI_DGRAM_ACCESS_CID_SHIFT);
+	if (vmci_transport_is_trusted(vsock, peer_cid) ||
+	    !(vmci_context_get_priv_flags(peer_cid) &
+	      VMCI_PRIVILEGE_FLAG_RESTRICTED))
+		access |= VMCI_DGRAM_ACCESS_ALLOW;
+
+	WRITE_ONCE(vsock->cached_peer_access, access);
+	return !!(access & VMCI_DGRAM_ACCESS_ALLOW);
 }
 
 static int