This is my first patch which I tested to work on OpenBSD and then confirmed compiling in Linux.

From c4f815bf0fa55a3a68caa7c721ea3dd8ac5c3d29 Mon Sep 17 00:00:00 2001

From: Greg Steuck linux-drm@nest.cx Date: Thu, 15 Apr 2021 21:44:57 -0700 Subject: [PATCH] drm/drm_edid: Read within initialized memory bounds (fix off by one)

The problem manifests as a memory out of bounds kernel panic in OpenBSD which uses this code. The buggy error reporting code path likely never runs with nominal hardware.

drm_do_get_edid at carp used to invoke connector_bad_edid was num_exts of 1 even though edid at that point is only allocated a memory block of size EDID_LENGTH. This in turn led to drm_edid_block_checksum trying to read memory in the range [edid + EDID_LENGTH, edid + 2*EDID_LENGTH) i.e. outside the allocated boundaries. A similar if a bit more complicated analysis applies to the other call to connector_bad_edid. Switching to using valid_extensions limits connector_bad_edid memory reading to the memory previously written by drm_do_get_edid.

OpenBSD bug report https://marc.info/?l=openbsd-bugs&m=161794843427437

Signed-off-by: Greg Steuck linux-drm@nest.cx --- drivers/gpu/drm/drm_edid.c | 10 +++++----- 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c index 81d5f2524246..f731bf7ac715 100644 --- a/drivers/gpu/drm/drm_edid.c +++ b/drivers/gpu/drm/drm_edid.c @@ -1831,20 +1831,20 @@ drm_do_probe_ddc_edid(void *data, u8 *buf, unsigned int block, size_t len) }

static void connector_bad_edid(struct drm_connector *connector, - u8 *edid, int num_blocks) + u8 *edid, int valid_extensions) { int i; 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); + drm_edid_block_checksum(edid + valid_extensions * EDID_LENGTH);

if (connector->bad_edid_counter++ && !drm_debug_enabled(DRM_UT_KMS)) return;

drm_dbg_kms(connector->dev, "%s: EDID is invalid:\n", connector->name); - for (i = 0; i < num_blocks; i++) { + for (i = 0; i <= valid_extensions; i++) { u8 *block = edid + i * EDID_LENGTH; char prefix[20];

@@ -1983,7 +1983,7 @@ struct edid *drm_do_get_edid(struct drm_connector *connector, if (valid_extensions != edid[0x7e]) { u8 *base;

- connector_bad_edid(connector, edid, edid[0x7e] + 1); + connector_bad_edid(connector, edid, valid_extensions);

edid[EDID_LENGTH-1] += edid[0x7e] - valid_extensions; edid[0x7e] = valid_extensions; @@ -2011,7 +2011,7 @@ struct edid *drm_do_get_edid(struct drm_connector *connector, return (struct edid *)edid;

carp: - connector_bad_edid(connector, edid, 1); + connector_bad_edid(connector, edid, 0); out: kfree(edid); return NULL;