drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

From Douglas Anderson
a7b45024f66f9ec769e8dbb1a51ae83cd05929c7 in linux 5.10.y/5.10.75
97794170b696856483f74b47bfb6049780d2d3a0 in mainline linux

1 files changed, 11 insertions, 5 deletions

diff --git a/sys/dev/pci/drm/drm_edid.c b/sys/dev/pci/drm/drm_edid.c
index 65a9f5a61d5..9a232baf9b1 100644
--- a/sys/dev/pci/drm/drm_edid.c
+++ b/sys/dev/pci/drm/drm_edid.c
@@ -1835,14 +1835,20 @@ static void connector_bad_edid(struct drm_connector *connector,
 			       u8 *edid, int num_blocks)
 {
 	int i;
-	u8 num_of_ext = edid[0x7e];
+	u8 last_block;
 
-	if (num_of_ext > num_blocks)
-		num_of_ext = num_blocks;
+	/*
+	 * 0x7e in the EDID is the number of extension blocks. The EDID
+	 * is 1 (base block) + num_ext_blocks big. That means we can think
+	 * of 0x7e in the EDID of the _index_ of the last block in the
+	 * combined chunk of memory.
+	 */
+	last_block = edid[0x7e];
 
 	/* Calculate real checksum for the last edid extension block data */
-	connector->real_edid_checksum =
-		drm_edid_block_checksum(edid + num_of_ext * EDID_LENGTH);
+	if (last_block < num_blocks)
+		connector->real_edid_checksum =
+			drm_edid_block_checksum(edid + last_block * EDID_LENGTH);
 
 	if (connector->bad_edid_counter++ && !drm_debug_enabled(DRM_UT_KMS))
 		return;