linux-kernel - [PATCH] drm/edid: In connector_bad_edid() cap num_of_ext by num

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Message-Id: <20211005081022.1.Ib059f9c23c2611cb5a9d760e7d0a700c1295928d@changeid>
Date:   Tue,  5 Oct 2021 08:10:28 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     dri-devel@...ts.freedesktop.org
Cc:     Rodrigo.Siqueira@....com, ville.syrjala@...ux.intel.com,
        Harry.Wentland@....com, khsieh@...eaurora.org, Jerry.Zuo@....com,
        alexander.deucher@....com,
        Douglas Anderson <dianders@...omium.org>,
        Daniel Vetter <daniel@...ll.ch>,
        David Airlie <airlied@...ux.ie>,
        Harry Wentland <harry.wentland@....com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Maxime Ripard <mripard@...nel.org>,
        Thomas Zimmermann <tzimmermann@...e.de>,
        linux-kernel@...r.kernel.org
Subject: [PATCH] drm/edid: In connector_bad_edid() cap num_of_ext by num_blocks read

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@...ux.intel.com>
Signed-off-by: Douglas Anderson <dianders@...omium.org>
---
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.

 drivers/gpu/drm/drm_edid.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 9b19eee0e1b4..ccfa08631c57 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -1843,8 +1843,9 @@ static void connector_bad_edid(struct drm_connector *connector,
 	u8 num_of_ext = 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 (num_of_ext <= num_blocks - 1)
+		connector->real_edid_checksum =
+			drm_edid_block_checksum(edid + num_of_ext * EDID_LENGTH);
 
 	if (connector->bad_edid_counter++ && !drm_debug_enabled(DRM_UT_KMS))
 		return;
-- 
2.33.0.800.g4c38ced690-goog