drm/mst: Avoid processing partially received up/down message transactions

Currently we may process up/down message transactions containing
uninitialized data. This can happen if there was an error during the
reception of any message in the transaction, but we happened to receive
the last message correctly with the end-of-message flag set.

To avoid this abort the reception of the transaction when the first
error is detected, rejecting any messages until a message with the
start-of-message flag is received (which will start a new transaction).
This is also what the DP 1.4 spec 2.11.8.2 calls for in this case.

In addtion this also prevents receiving bogus transactions without the
first message with the the start-of-message flag set.

v2:
- unchanged
v3:
- git add the part that actually skips messages after an error in
  drm_dp_sideband_msg_build()

Cc: Dave Airlie <airlied@redhat.com>
Cc: Lyude <lyude@redhat.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Lyude <lyude@redhat.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719134632.13366-1-imre.deak@intel.com

drivers/gpu/drm/drm_dp_mst_topology.c

@@ -330,6 +330,13 @@ static bool drm_dp_sideband_msg_build(struct drm_dp_sideband_msg_rx *msg,
 			return false;
 		}
 
+		/*
+		 * ignore out-of-order messages or messages that are part of a
+		 * failed transaction
+		 */
+		if (!recv_hdr.somt && !msg->have_somt)
+			return false;
+
 		/* get length contained in this portion */
 		msg->curchunk_len = recv_hdr.msg_len;
 		msg->curchunk_hdrlen = hdrlen;
@@ -2164,7 +2171,7 @@ out_unlock:
 }
 EXPORT_SYMBOL(drm_dp_mst_topology_mgr_resume);
 
-static void drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up)
+static bool drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up)
 {
 	int len;
 	u8 replyblock[32];
@@ -2179,12 +2186,12 @@ static void drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up)
 			       replyblock, len);
 	if (ret != len) {
 		DRM_DEBUG_KMS("failed to read DPCD down rep %d %d\n", len, ret);
-		return;
+		return false;
 	}
 	ret = drm_dp_sideband_msg_build(msg, replyblock, len, true);
 	if (!ret) {
 		DRM_DEBUG_KMS("sideband msg build failed %d\n", replyblock[0]);
-		return;
+		return false;
 	}
 	replylen = msg->curchunk_len + msg->curchunk_hdrlen;
 
@@ -2198,25 +2205,30 @@ static void drm_dp_get_one_sb_msg(struct drm_dp_mst_topology_mgr *mgr, bool up)
 		if (ret != len) {
 			DRM_DEBUG_KMS("failed to read a chunk (len %d, ret %d)\n",
 				      len, ret);
-			return;
+			return false;
 		}
 
 		ret = drm_dp_sideband_msg_build(msg, replyblock, len, false);
 		if (!ret) {
 			DRM_DEBUG_KMS("failed to build sideband msg\n");
-			return;
+			return false;
 		}
 
 		curreply += len;
 		replylen -= len;
 	}
+	return true;
 }
 
 static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 {
 	int ret = 0;
 
-	drm_dp_get_one_sb_msg(mgr, false);
+	if (!drm_dp_get_one_sb_msg(mgr, false)) {
+		memset(&mgr->down_rep_recv, 0,
+		       sizeof(struct drm_dp_sideband_msg_rx));
+		return 0;
+	}
 
 	if (mgr->down_rep_recv.have_eomt) {
 		struct drm_dp_sideband_msg_tx *txmsg;
@@ -2272,7 +2284,12 @@ static int drm_dp_mst_handle_down_rep(struct drm_dp_mst_topology_mgr *mgr)
 static int drm_dp_mst_handle_up_req(struct drm_dp_mst_topology_mgr *mgr)
 {
 	int ret = 0;
-	drm_dp_get_one_sb_msg(mgr, true);
+
+	if (!drm_dp_get_one_sb_msg(mgr, true)) {
+		memset(&mgr->up_req_recv, 0,
+		       sizeof(struct drm_dp_sideband_msg_rx));
+		return 0;
+	}
 
 	if (mgr->up_req_recv.have_eomt) {
 		struct drm_dp_sideband_msg_req_body msg;