Re: [RESEND][PATCH] omapdrm: fix missing check on list iterator

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Hi,

On 14/04/2022 09:14, Xiaomeng Tong wrote:
The bug is here:
	bus_flags = connector->display_info.bus_flags;

The list iterator 'connector-' will point to a bogus position containing
HEAD if the list is empty or no element is found. This case must
be checked before any use of the iterator, otherwise it will lead
to a invalid memory access.

To fix this bug, add an check. Use a new value 'iter' as the list
iterator, while use the old value 'connector' as a dedicated variable
to point to the found element.

Cc: stable@xxxxxxxxxxxxxxx
Fixes: ("drm/omap: Add support for drm_panel")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@xxxxxxxxx>
---
  drivers/gpu/drm/omapdrm/omap_encoder.c | 14 +++++++++-----
  1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/omapdrm/omap_encoder.c b/drivers/gpu/drm/omapdrm/omap_encoder.c
index 4dd05bc732da..d648ab4223b1 100644
--- a/drivers/gpu/drm/omapdrm/omap_encoder.c
+++ b/drivers/gpu/drm/omapdrm/omap_encoder.c
@@ -76,14 +76,16 @@ static void omap_encoder_mode_set(struct drm_encoder *encoder,
  	struct omap_encoder *omap_encoder = to_omap_encoder(encoder);
  	struct omap_dss_device *output = omap_encoder->output;
  	struct drm_device *dev = encoder->dev;
-	struct drm_connector *connector;
+	struct drm_connector *connector = NULL, *iter;
  	struct drm_bridge *bridge;
  	struct videomode vm = { 0 };
  	u32 bus_flags;
- list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
-		if (connector->encoder == encoder)
+	list_for_each_entry(iter, &dev->mode_config.connector_list, head) {
+		if (iter->encoder == encoder) {
+			connector = iter;
  			break;
+		}
  	}

When does this bug happen? How do you get omap_encoder_mode_set() called for an encoder with a connector that is not valid?

drm_display_mode_to_videomode(adjusted_mode, &vm);
@@ -106,8 +108,10 @@ static void omap_encoder_mode_set(struct drm_encoder *encoder,
  		omap_encoder_update_videomode_flags(&vm, bus_flags);
  	}
- bus_flags = connector->display_info.bus_flags;
-	omap_encoder_update_videomode_flags(&vm, bus_flags);
+	if (connector) {
+		bus_flags = connector->display_info.bus_flags;
+		omap_encoder_update_videomode_flags(&vm, bus_flags);
+	}
/* Set timings for all devices in the display pipeline. */
  	dss_mgr_set_timings(output, &vm);

How does this fix the issue? You just skip the lines that set up the videomode, but then pass that videomode to dss_mgr_set_timings()...

 Tomi



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