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

In commit 8d1604a7d1ec ("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: 8d1604a7d1ec ("drm: Add support for DP 1.4 Compliance edid corruption test")
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211005192905.v2.1.Ib059f9c23c2611cb5a9d760e7d0a700c1295928d@changeid
Signed-off-by: Dave Airlie <airlied@redhat.com>

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 6325877c5fd6e642169f350fa17d08593aaa95a0..ea9a79bc958391f6714249b438c96d65b94f080c 100644 (file)
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1834,11 +1834,20 @@ 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;