[PATCH v2 2/2] drm/hyperv: validate VMBus packet size in receive callback

[Berkant Koc <me@berkoc.com>]

hyperv_receive() reads bytes_recvd from vmbus_recvpacket() but does not
forward that value to hyperv_receive_sub(). The sub-handler reads
msg->vid_hdr.type and msg->feature_chg.is_dirt_needed without knowing
how many bytes the host actually wrote, so a short packet leaves the
parser reading bytes that the host did not write in this packet. The
unconditional memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE) on the
wait-completion path also copies the full 16 KiB recv_buf regardless
of bytes_recvd, including any residue from the prior message.

Pass bytes_recvd into hyperv_receive_sub() and reject any packet shorter
than the pipe + synthvid header. The dirt-feature branch additionally
requires the feature_change payload size before reading is_dirt_needed.
The init_buf copy now uses bytes_recvd as the length argument, which
keeps it bounded by VMBUS_MAX_PACKET_SIZE through the new upper check.

Unchanged from v1.

Fixes: 76c56a5affeb ("drm/hyperv: Add DRM driver for hyperv synthetic video device")
Cc: stable@vger.kernel.org # 5.14+
Signed-off-by: Berkant Koc <me@berkoc.com>
---
 drivers/gpu/drm/hyperv/hyperv_drm_proto.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
index 3b5065fe06e4..cdab4895dd40 100644
--- a/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
+++ b/drivers/gpu/drm/hyperv/hyperv_drm_proto.c
@@ -423,26 +423,35 @@ static int hyperv_get_supported_resolution(struct hv_device *hdev)
 	return 0;
 }
 
-static void hyperv_receive_sub(struct hv_device *hdev)
+static void hyperv_receive_sub(struct hv_device *hdev, u32 bytes_recvd)
 {
 	struct hyperv_drm_device *hv = hv_get_drvdata(hdev);
 	struct synthvid_msg *msg;
+	size_t hdr_size;
 
 	if (!hv)
 		return;
 
+	hdr_size = sizeof(struct pipe_msg_hdr) +
+		   sizeof(struct synthvid_msg_hdr);
+	if (bytes_recvd < hdr_size || bytes_recvd > VMBUS_MAX_PACKET_SIZE)
+		return;
+
 	msg = (struct synthvid_msg *)hv->recv_buf;
 
 	/* Complete the wait event */
 	if (msg->vid_hdr.type == SYNTHVID_VERSION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_RESOLUTION_RESPONSE ||
 	    msg->vid_hdr.type == SYNTHVID_VRAM_LOCATION_ACK) {
-		memcpy(hv->init_buf, msg, VMBUS_MAX_PACKET_SIZE);
+		memcpy(hv->init_buf, msg, bytes_recvd);
 		complete(&hv->wait);
 		return;
 	}
 
 	if (msg->vid_hdr.type == SYNTHVID_FEATURE_CHANGE) {
+		if (bytes_recvd < hdr_size +
+		    sizeof(struct synthvid_feature_change))
+			return;
 		hv->dirt_needed = msg->feature_chg.is_dirt_needed;
 		if (hv->dirt_needed)
 			hyperv_hide_hw_ptr(hv->hdev);
@@ -469,7 +478,7 @@ static void hyperv_receive(void *ctx)
 				       &bytes_recvd, &req_id);
 		if (bytes_recvd > 0 &&
 		    recv_buf->pipe_hdr.type == PIPE_MSG_DATA)
-			hyperv_receive_sub(hdev);
+			hyperv_receive_sub(hdev, bytes_recvd);
 	} while (bytes_recvd > 0 && ret == 0);
 }
 
-- 
2.47.3