um: virtio_uml: fix heap overflow in vhost_user_get_config()

vhost_user_get_config() sizes its reply buffer from header + payload:

	size_t payload_size = sizeof(msg->payload.config) + cfg_size;
	size_t msg_size = sizeof(msg->header) + payload_size;
	...
	msg = kzalloc(msg_size, GFP_KERNEL);

but then receives the response with msg_size as the payload cap:

	rc = vhost_user_recv_resp(vu_dev, msg, msg_size);

vhost_user_recv() reads the header, then full_read()s header.size bytes
into &msg->payload after checking only "size > max_payload_size". The
payload area is msg_size - sizeof(msg->header) == payload_size bytes, so
passing msg_size as the cap lets a peer set header.size up to msg_size
and write sizeof(struct vhost_user_msg_header) (12) bytes past the end
of the kzalloc(msg_size) buffer -- a slab out-of-bounds write of
peer-controlled content.

Pass payload_size, which is the size the payload area was allocated for.

A malicious or compromised vhost-user backend triggers this during
device probe (vu_get -> vhost_user_get_config) with no guest action,
which matters in the confidential-computing model where the guest does
not trust the device backend.

Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Reviewed-by: Richard Weinberger <richard@nod.at>
Link: https://patch.msgid.link/20260622124750.1694957-1-michael.bommarito@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

0 files changed, 0 insertions, 0 deletions