drm/hyperv: validate VMBus packet size in receive callback

In the Linux kernel, the following vulnerability has been resolved:

drm/hyperv: validate VMBus packet size in receive callback

hyperv_receive_sub() reads msg->vid_hdr.type and dispatches into one

of four message-type branches without knowing how many bytes the host

wrote into hv->recv_buf. The completion path then runs

memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE), so the consumer that

wakes on wait_for_completion_timeout() can read up to 16 KiB of

residue from a prior message as if it were the response payload.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet that

does not cover the pipe + synthvid header. A single switch on

msg->vid_hdr.type then computes the type-specific payload size: the

three completion-driving types (SYNTHVID_VERSION_RESPONSE,

SYNTHVID_RESOLUTION_RESPONSE, SYNTHVID_VRAM_LOCATION_ACK) fall through

to a shared exit that requires that size before memcpy/complete, while

SYNTHVID_FEATURE_CHANGE validates its own payload and returns before

reading is_dirt_needed. Unknown types are dropped.

SYNTHVID_RESOLUTION_RESPONSE is variable length: the host fills

resolution_count entries, not the full SYNTHVID_MAX_RESOLUTION_COUNT

array. Validate the fixed prefix first so resolution_count can be

read, bound it against the array, then require only the count-sized

array, so the shorter responses the host actually sends are accepted.

Only run the sub-handler when vmbus_recvpacket() returned success. The

memcpy length is bytes_recvd, which is bounded by VMBUS_MAX_PACKET_SIZE

only on a successful receive; on -ENOBUFS vmbus_recvpacket() instead

reports the required length, which can exceed hv->recv_buf, so copying

bytes_recvd would read and write past the 16 KiB buffers. Gating on the

success return keeps the copy bounded. The nonzero-return path is itself

a malformed-message case and is now logged rather than silently skipped;

channel recovery is not attempted.

Rejected packets are reported via drm_err_ratelimited() rather than

silently dropped, matching the CoCo-hardened pattern in

hv_kvp_onchannelcallback().