[PATCH v2] drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

[Douglas Anderson]

In commit e11f5bd8228f ("drm: Add support for DP 1.4 Compliance edid
corruption test") the function connector_bad_edid() started assuming
that the memory for the EDID passed to it was big enough to hold
`edid[0x7e] + 1` blocks of data (1 extra for the base block). It
completely ignored the fact that the function was passed `num_blocks`
which indicated how much memory had been allocated for the EDID.

Let's fix this by adding a bounds check.

This is important for handling the case where there's an error in the
first block of the EDID. In that case we will call
connector_bad_edid() without having re-allocated memory based on
`edid[0x7e]`.

Fixes: e11f5bd8228f ("drm: Add support for DP 1.4 Compliance edid corruption test")
Reported-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
Signed-off-by: Douglas Anderson <dianders at chromium.org>
Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>
---
This problem report came up in the context of a patch I sent out [1]
and this is my attempt at a fix. The problem predates my patch,
though. I don't personally know anything about DP compliance testing
and what should be happening here, nor do I apparently have any
hardware that actually reports a bad EDID. Thus this is just compile
tested. I'm hoping that someone here can test this and make sure it
seems OK to them.

Changes in v2:
- Added a comment/changed math to help make it easier to grok.

 drivers/gpu/drm/drm_edid.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 9c9463ec5465..0383d97c306f 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1840,11 +1840,20 @@ 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;
+
+	/*
+	 * 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;
-- 
2.33.0.800.g4c38ced690-goog