[PATCH] drm/qxl: Fix out-of-bounds read in qxl_update_offset_props()

[Julio Faracco <jcfaracco@gmail.com>]

Function qxl_update_offset_props() dereferences
qdev->client_monitors_config->heads[output->index] for every connector.
The connector list holds qxl_num_crtc entries (default 4), but heads[] is
only allocated for client_monitors_config->count entries. When the client
reports fewer monitors than qxl_num_crtc (e.g. a guest started with
heads=1), output->index exceeds count and the read runs off the end:

    BUG: KASAN: slab-out-of-bounds in
         qxl_display_read_client_monitors_config+0x61e/0x6f0 [qxl]
    Read of size 4 ... located 16 bytes to the right of allocated
    32-byte region

Factor the index-vs-count check already open-coded in
qxl_add_monitors_config_modes() and qxl_conn_detect() into a
qxl_output_get_client_head() helper that returns the head or NULL when
the config is absent or does not cover the output's index, and use it
in all three places.

Signed-off-by: Julio Faracco <jcfaracco@gmail.com>
---
 drivers/gpu/drm/qxl/qxl_display.c | 34 +++++++++++++++++++++++--------
 1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/drivers/gpu/drm/qxl/qxl_display.c b/drivers/gpu/drm/qxl/qxl_display.c
index 75d47e6b9142..92cc240dc9c2 100644
--- a/drivers/gpu/drm/qxl/qxl_display.c
+++ b/drivers/gpu/drm/qxl/qxl_display.c
@@ -147,6 +147,21 @@ static int qxl_display_copy_rom_client_monitors_config(struct qxl_device *qdev)
 	return status;
 }
 
+/*
+ * Return the client monitors config head for @output, or NULL when the
+ * client monitors config is absent or does not cover this output's index.
+ */
+static struct qxl_head *qxl_output_get_client_head(struct qxl_output *output)
+{
+	struct qxl_device *qdev = to_qxl(output->base.dev);
+	struct qxl_monitors_config *cfg = qdev->client_monitors_config;
+
+	if (!cfg || output->index >= cfg->count)
+		return NULL;
+
+	return &cfg->heads[output->index];
+}
+
 static void qxl_update_offset_props(struct qxl_device *qdev)
 {
 	struct drm_device *dev = &qdev->ddev;
@@ -157,7 +172,9 @@ static void qxl_update_offset_props(struct qxl_device *qdev)
 	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
 		output = drm_connector_to_qxl_output(connector);
 
-		head = &qdev->client_monitors_config->heads[output->index];
+		head = qxl_output_get_client_head(output);
+		if (!head)
+			continue;
 
 		drm_object_property_set_value(&connector->base,
 			dev->mode_config.suggested_x_property, head->x);
@@ -262,12 +279,11 @@ static int qxl_add_monitors_config_modes(struct drm_connector *connector)
 		return 0;
 	if (h >= qxl_num_crtc)
 		return 0;
-	if (!qdev->client_monitors_config)
-		return 0;
-	if (h >= qdev->client_monitors_config->count)
+
+	head = qxl_output_get_client_head(output);
+	if (!head)
 		return 0;
 
-	head = &qdev->client_monitors_config->heads[h];
 	DRM_DEBUG_KMS("head %d is %dx%d\n", h, head->width, head->height);
 
 	return qxl_add_mode(connector, head->width, head->height, true);
@@ -1103,15 +1119,17 @@ static enum drm_connector_status qxl_conn_detect(
 		drm_connector_to_qxl_output(connector);
 	struct drm_device *ddev = connector->dev;
 	struct qxl_device *qdev = to_qxl(ddev);
+	struct qxl_head *head;
 	bool connected = false;
 
 	/* The first monitor is always connected */
 	if (!qdev->client_monitors_config) {
 		if (output->index == 0)
 			connected = true;
-	} else
-		connected = qdev->client_monitors_config->count > output->index &&
-		     qxl_head_enabled(&qdev->client_monitors_config->heads[output->index]);
+	} else {
+		head = qxl_output_get_client_head(output);
+		connected = head && qxl_head_enabled(head);
+	}
 
 	DRM_DEBUG("#%d connected: %d\n", output->index, connected);
 
-- 
2.55.0