[PATCH] drm/qxl: validate client monitor index before access

[Jiale Yao <yaojiale02@163.com>]

client_monitors_config is allocated with space for count heads, while QXL
creates qxl_num_crtc outputs whose indexes range from zero to
qxl_num_crtc - 1.  The monitor count may be smaller than qxl_num_crtc, but
qxl_update_offset_props() and qxl_conn_get_modes() use the output index to
access heads[] without checking it against count.

Starting QEMU with "-device qxl-vga,xres=1024,yres=768" provides one
client monitor while the driver creates four outputs by default.  KASAN
reported the first out-of-bounds access during probe, with
qxl_update_offset_props() inlined into its caller:

  BUG: KASAN: slab-out-of-bounds in
  qxl_display_read_client_monitors_config+0x2379/0x2e30
  Read of size 4 at addr ffff88810403e170 by task swapper/0/1
  Call Trace:
   qxl_display_read_client_monitors_config+0x2379/0x2e30
   qxl_modeset_init+0x6a6/0x7b0
   qxl_pci_probe+0x150/0x2d0

The same configuration reaches the unchecked access in
qxl_conn_get_modes() when userspace enumerates connectors.

Skip offset updates for outputs that have no corresponding client head,
and retain the default preferred mode dimensions in qxl_conn_get_modes()
when its output index is outside the client head array.

Fixes: 7dea0941f880 ("drm/qxl: use suggested x/y offset properties to pass guest prefs")
Fixes: 1b043677d4be ("drm/qxl: add qxl_add_mode helper function")
Cc: stable@vger.kernel.org
Signed-off-by: Jiale Yao <yaojiale02@163.com>
---
 drivers/gpu/drm/qxl/qxl_display.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/qxl/qxl_display.c b/drivers/gpu/drm/qxl/qxl_display.c
index 0719fc6a52d5..25f786ab7f1b 100644
--- a/drivers/gpu/drm/qxl/qxl_display.c
+++ b/drivers/gpu/drm/qxl/qxl_display.c
@@ -158,6 +158,9 @@ 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);
 
+		if (output->index >= qdev->client_monitors_config->count)
+			continue;
+
 		head = &qdev->client_monitors_config->heads[output->index];
 
 		drm_object_property_set_value(&connector->base,
@@ -1056,7 +1059,8 @@ static int qxl_conn_get_modes(struct drm_connector *connector)
 	unsigned int pheight = 768;
 	int ret = 0;
 
-	if (qdev->client_monitors_config) {
+	if (qdev->client_monitors_config &&
+	    output->index < qdev->client_monitors_config->count) {
 		struct qxl_head *head;
 		head = &qdev->client_monitors_config->heads[output->index];
 		if (head->width)
-- 
2.34.1