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

[Dave Airlie]

This zeroes the msg so no random stack data ends up getting
sent, it also limits the function to not accepting > 4
i2c msgs.

Cc: stable at vger.kernel.org
Signed-off-by: Dave Airlie <airlied at redhat.com>
---
 drivers/gpu/drm/drm_dp_mst_topology.c | 3 ++-
 include/drm/drm_dp_mst_helper.h       | 3 ++-
 2 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index bf27a07..5bca390 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -2801,12 +2801,13 @@ 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;
diff --git a/include/drm/drm_dp_mst_helper.h b/include/drm/drm_dp_mst_helper.h
index 0f408b0..5340099 100644
--- a/include/drm/drm_dp_mst_helper.h
+++ b/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;
 };
-- 
2.4.3