DEBIAN-CVE-2026-99123: Debian/linux vulnerability

In the Linux kernel, the following vulnerability has been resolved: vsock/virtio: validate packet source for connected sockets virtiotransportrecvpkt() looks up sockets first by the full source and destination tuple, then by destination only in the bound table. The fallback is needed for listening and connecting sockets, but sockets remain in the bound table after connect(), so it can also return a non-listening socket. The fallback does not validate the source address. In TCPSYNSENT, a RESPONSE from an unrelated source can transition the victim socket to TCPESTABLISHED while its stored remote address remains unchanged. Subsequent RW packets from that source are delivered through the same destination-only fallback. This was reproduced with capability-empty processes under different UIDs. The attacker discovered the target tuple through unprivileged AFVSOCK sockdiag and caused the victim socket to read 16 attacker-chosen bytes; the intended peer-side socket read 0 of those 16 bytes. Add vsockchecksource() to validate the transport, source port and source CID against the peer stored in a non-listening socket. The local transport is the CID exception because its packets are generated internally with VMADDRCIDLOCAL as their source, including connections using CID aliases. Use the helper after locksock() in the virtio receive path.