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

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

[Julio Faracco]

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

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

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [High] drm/qxl: Incomplete fix leaves out-of-bounds read in qxl_conn_get_modes()
- [High] drm/qxl: Lockless reallocation of client_monitors_config causes Use-After-Free
--

commit 3364903c8ea46fb8e3ab3b182d79c5b5a9cc250d
Author: Julio Faracco <jcfaracco@gmail.com>

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

This patch introduces a helper to safely validate bounds when the client
reports fewer monitors than configured, preventing an out-of-bounds read.
The helper is applied to several functions across the qxl display driver.

> diff --git a/drivers/gpu/drm/qxl/qxl_display.c b/drivers/gpu/drm/qxl/qxl_display.c
> --- a/drivers/gpu/drm/qxl/qxl_display.c
> +++ b/drivers/gpu/drm/qxl/qxl_display.c
> @@ -148,6 +148,21 @@ 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];

[Severity: High]
Does this code create a use-after-free race condition?

The qxl_client_monitors_config_work_func() workqueue asynchronously executes
qxl_display_copy_rom_client_monitors_config() and 
qxl_alloc_client_monitors_config(), which frees and reallocates 
qdev->client_monitors_config before acquiring the DRM modeset locks.

Since this helper reads qdev->client_monitors_config into cfg without any
synchronization locks, if the background workqueue frees the configuration
array immediately after this fetch:

    kfree(qdev->client_monitors_config);

could the subsequent bounds check (cfg->count) and array access
(&cfg->heads[output->index]) dereference freed memory?

> +}
> +
>  static void qxl_update_offset_props(struct qxl_device *qdev)

[ ... ]

> @@ -1111,15 +1127,17 @@ 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);
> +	}

[Severity: High]
Does qxl_conn_get_modes() remain vulnerable to the same out-of-bounds read?

This patch fixes the issue here and in a few other places, but 
qxl_conn_get_modes() has an identical pattern that was not updated:

    if (qdev->client_monitors_config) {
        struct qxl_head *head;
        head = &qdev->client_monitors_config->heads[output->index];
        if (head->width)
            ...

If output->index exceeds qdev->client_monitors_config->count during a mode
probe, won't this still trigger the KASAN slab-out-of-bounds memory read?

>  
>  	DRM_DEBUG("#%d connected: %d\n", output->index, connected);
>

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261008124317.32199-1-jcfaracco@gmail.com?part=1