drm/dp/mst: make mst i2c transfer code more robust.

commit ae491542 upstream.

This zeroes the msg so no random stack data ends up getting
sent, it also limits the function to not accepting > 4
i2c msgs.
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -2771,12 +2771,13 @@ static int drm_dp_mst_i2c_xfer(struct i2c_adapter *adapter, struct i2c_msg *msgs
 	if (msgs[num - 1].flags & I2C_M_RD)
 		reading = true;
 
-	if (!reading) {
+	if (!reading || (num - 1 > DP_REMOTE_I2C_READ_MAX_TRANSACTIONS)) {
 		DRM_DEBUG_KMS("Unsupported I2C transaction for MST device\n");
 		ret = -EIO;
 		goto out;
 	}
 
+	memset(&msg, 0, sizeof(msg));
 	msg.req_type = DP_REMOTE_I2C_READ;
 	msg.u.i2c_read.num_transactions = num - 1;
 	msg.u.i2c_read.port_number = port->port_num;

include/drm/drm_dp_mst_helper.h

@@ -253,6 +253,7 @@ struct drm_dp_remote_dpcd_write {
 	u8 *bytes;
 };
 
+#define DP_REMOTE_I2C_READ_MAX_TRANSACTIONS 4
 struct drm_dp_remote_i2c_read {
 	u8 num_transactions;
 	u8 port_number;
@@ -262,7 +263,7 @@ struct drm_dp_remote_i2c_read {
 		u8 *bytes;
 		u8 no_stop_bit;
 		u8 i2c_transaction_delay;
-	} transactions[4];
+	} transactions[DP_REMOTE_I2C_READ_MAX_TRANSACTIONS];
 	u8 read_i2c_device_id;
 	u8 num_bytes_read;
 };