drm: Hold mode_config.lock to prevent hotplug whilst setting up crtcs

The fb_helper->connector_count is modified when a new connector is
constructed following a hotplug event (e.g. DP-MST). This causes trouble
for drm_setup_crtcs() and friends that assume that fb_helper is
constant:

[ 1250.872997] BUG: KASAN: slab-out-of-bounds in drm_setup_crtcs+0x320/0xf80 at addr ffff88074cdd2608
[ 1250.873020] Write of size 40 by task kworker/u8:3/480
[ 1250.873039] CPU: 2 PID: 480 Comm: kworker/u8:3 Tainted: G     U          4.9.0-rc6+ #285
[ 1250.873043] Hardware name:                  /NUC6i3SYB, BIOS SYSKLi35.86A.0024.2015.1027.2142 10/27/2015
[ 1250.873050] Workqueue: events_unbound async_run_entry_fn
[ 1250.873056]  ffff88070f9d78f0 ffffffff814b72aa ffff88074e40c5c0 ffff88074cdd2608
[ 1250.873067]  ffff88070f9d7918 ffffffff8124ff3c ffff88070f9d79b0 ffff88074cdd2600
[ 1250.873079]  ffff88074e40c5c0 ffff88070f9d79a0 ffffffff812501e4 0000000000000005
[ 1250.873090] Call Trace:
[ 1250.873099]  [<ffffffff814b72aa>] dump_stack+0x67/0x9d
[ 1250.873106]  [<ffffffff8124ff3c>] kasan_object_err+0x1c/0x70
[ 1250.873113]  [<ffffffff812501e4>] kasan_report_error+0x204/0x4f0
[ 1250.873120]  [<ffffffff81698df0>] ? drm_dev_printk+0x140/0x140
[ 1250.873127]  [<ffffffff81250ac3>] kasan_report+0x53/0x60
[ 1250.873134]  [<ffffffff81688b40>] ? drm_setup_crtcs+0x320/0xf80
[ 1250.873142]  [<ffffffff8124f18e>] check_memory_region+0x13e/0x1a0
[ 1250.873147]  [<ffffffff8124f5f3>] memset+0x23/0x40
[ 1250.873154]  [<ffffffff81688b40>] drm_setup_crtcs+0x320/0xf80
[ 1250.873161]  [<ffffffff810be7c5>] ? wake_up_q+0x45/0x80
[ 1250.873169]  [<ffffffff81b0c180>] ? mutex_lock_nested+0x5a0/0x5a0
[ 1250.873176]  [<ffffffff8168a0e6>] drm_fb_helper_initial_config+0x206/0x7a0
[ 1250.873183]  [<ffffffff81689ee0>] ? drm_fb_helper_set_par+0x90/0x90
[ 1250.873303]  [<ffffffffa0b68690>] ? intel_fbdev_fini+0x140/0x140 [i915]
[ 1250.873387]  [<ffffffffa0b686b2>] intel_fbdev_initial_config+0x22/0x40 [i915]
[ 1250.873391]  [<ffffffff810b50ff>] async_run_entry_fn+0x7f/0x270
[ 1250.873394]  [<ffffffff810a64b0>] process_one_work+0x3d0/0x960
[ 1250.873398]  [<ffffffff810a641d>] ? process_one_work+0x33d/0x960
[ 1250.873401]  [<ffffffff810a60e0>] ? max_active_store+0xf0/0xf0
[ 1250.873406]  [<ffffffff810f6f9d>] ? do_raw_spin_lock+0x10d/0x1a0
[ 1250.873413]  [<ffffffff810a767d>] worker_thread+0x8d/0x840
[ 1250.873419]  [<ffffffff810a75f0>] ? create_worker+0x2e0/0x2e0
[ 1250.873426]  [<ffffffff810b0454>] kthread+0x194/0x1c0
[ 1250.873432]  [<ffffffff810b02c0>] ? kthread_park+0x60/0x60
[ 1250.873438]  [<ffffffff810f095d>] ? trace_hardirqs_on+0xd/0x10
[ 1250.873446]  [<ffffffff810b02c0>] ? kthread_park+0x60/0x60
[ 1250.873453]  [<ffffffff810b02c0>] ? kthread_park+0x60/0x60
[ 1250.873457]  [<ffffffff81b12277>] ret_from_fork+0x27/0x40
[ 1250.873460] Object at ffff88074cdd2608, in cache kmalloc-32 size: 32

However, when holding the mode_config.lock around the fb_helper, we have
to be careful of any callbacks that may reenter the fb_helper and so try
to reacquire the mode_config.lock (e.g. register_framebuffer). To avoid
the mutex recursion, we have to rearrange the sequence to move the
registration into the caller outside of the mode_config.lock.

v2: drop the 1; following the lockdep assertion inside the for(;;), I
anticipated an error that doesn't happen!

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98826
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20161129120217.7344-1-chris@chris-wilson.co.uk

drivers/gpu/drm/drm_fb_helper.c

@@ -97,6 +97,10 @@ static LIST_HEAD(kernel_fb_helper_list);
  * mmap page writes.
  */
 
+#define drm_fb_helper_for_each_connector(fbh, i__) \
+	for (({ lockdep_assert_held(&(fbh)->dev->mode_config.mutex); }), \
+	     i__ = 0; i__ < (fbh)->connector_count; i__++)
+
 /**
  * drm_fb_helper_single_add_all_connectors() - add all connectors to fbdev
  * 					       emulation helper
@@ -130,7 +134,7 @@ int drm_fb_helper_single_add_all_connectors(struct drm_fb_helper *fb_helper)
 	mutex_unlock(&dev->mode_config.mutex);
 	return 0;
 fail:
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		struct drm_fb_helper_connector *fb_helper_connector =
 			fb_helper->connector_info[i];
 
@@ -565,7 +569,7 @@ static void drm_fb_helper_dpms(struct fb_info *info, int dpms_mode)
 			continue;
 
 		/* Walk the connectors & encoders on this fb turning them on/off */
-		for (j = 0; j < fb_helper->connector_count; j++) {
+		drm_fb_helper_for_each_connector(fb_helper, j) {
 			connector = fb_helper->connector_info[j]->connector;
 			connector->funcs->dpms(connector, dpms_mode);
 			drm_object_property_set_value(&connector->base,
@@ -1469,7 +1473,6 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 	int ret = 0;
 	int crtc_count = 0;
 	int i;
-	struct fb_info *info;
 	struct drm_fb_helper_surface_size sizes;
 	int gamma_size = 0;
 
@@ -1485,7 +1488,7 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 		sizes.surface_depth = sizes.surface_bpp = preferred_bpp;
 
 	/* first up get a count of crtcs now in use and new min/maxes width/heights */
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		struct drm_fb_helper_connector *fb_helper_conn = fb_helper->connector_info[i];
 		struct drm_cmdline_mode *cmdline_mode;
 
@@ -1572,8 +1575,6 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 	if (ret < 0)
 		return ret;
 
-	info = fb_helper->fbdev;
-
 	/*
 	 * Set the fb pointer - usually drm_setup_crtcs does this for hotplug
 	 * events, but at init time drm_setup_crtcs needs to be called before
@@ -1585,20 +1586,6 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper,
 		if (fb_helper->crtc_info[i].mode_set.num_connectors)
 			fb_helper->crtc_info[i].mode_set.fb = fb_helper->fb;
 
-
-	info->var.pixclock = 0;
-	if (register_framebuffer(info) < 0)
-		return -EINVAL;
-
-	dev_info(fb_helper->dev->dev, "fb%d: %s frame buffer device\n",
-			info->node, info->fix.id);
-
-	if (list_empty(&kernel_fb_helper_list)) {
-		register_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
-	}
-
-	list_add(&fb_helper->kernel_fb_list, &kernel_fb_helper_list);
-
 	return 0;
 }
 
@@ -1730,7 +1717,7 @@ static int drm_fb_helper_probe_connector_modes(struct drm_fb_helper *fb_helper,
 	int count = 0;
 	int i;
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		connector = fb_helper->connector_info[i]->connector;
 		count += connector->funcs->fill_modes(connector, maxX, maxY);
 	}
@@ -1830,7 +1817,7 @@ static void drm_enable_connectors(struct drm_fb_helper *fb_helper,
 	struct drm_connector *connector;
 	int i = 0;
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		connector = fb_helper->connector_info[i]->connector;
 		enabled[i] = drm_connector_enabled(connector, true);
 		DRM_DEBUG_KMS("connector %d enabled? %s\n", connector->base.id,
@@ -1841,7 +1828,7 @@ static void drm_enable_connectors(struct drm_fb_helper *fb_helper,
 	if (any_enabled)
 		return;
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		connector = fb_helper->connector_info[i]->connector;
 		enabled[i] = drm_connector_enabled(connector, false);
 	}
@@ -1862,7 +1849,7 @@ static bool drm_target_cloned(struct drm_fb_helper *fb_helper,
 		return false;
 
 	count = 0;
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		if (enabled[i])
 			count++;
 	}
@@ -1873,7 +1860,7 @@ static bool drm_target_cloned(struct drm_fb_helper *fb_helper,
 
 	/* check the command line or if nothing common pick 1024x768 */
 	can_clone = true;
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		if (!enabled[i])
 			continue;
 		fb_helper_conn = fb_helper->connector_info[i];
@@ -1899,8 +1886,7 @@ static bool drm_target_cloned(struct drm_fb_helper *fb_helper,
 	can_clone = true;
 	dmt_mode = drm_mode_find_dmt(fb_helper->dev, 1024, 768, 60, false);
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
-
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		if (!enabled[i])
 			continue;
 
@@ -1931,7 +1917,7 @@ static int drm_get_tile_offsets(struct drm_fb_helper *fb_helper,
 	int i;
 	int hoffset = 0, voffset = 0;
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		fb_helper_conn = fb_helper->connector_info[i];
 		if (!fb_helper_conn->connector->has_tile)
 			continue;
@@ -1965,7 +1951,7 @@ static bool drm_target_preferred(struct drm_fb_helper *fb_helper,
 	int i;
 
 retry:
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		fb_helper_conn = fb_helper->connector_info[i];
 
 		if (conn_configured & BIT_ULL(i))
@@ -2128,6 +2114,9 @@ static void drm_setup_crtcs(struct drm_fb_helper *fb_helper)
 	width = dev->mode_config.max_width;
 	height = dev->mode_config.max_height;
 
+	/* prevent concurrent modification of connector_count by hotplug */
+	lockdep_assert_held(&fb_helper->dev->mode_config.mutex);
+
 	crtcs = kcalloc(fb_helper->connector_count,
 			sizeof(struct drm_fb_helper_crtc *), GFP_KERNEL);
 	modes = kcalloc(fb_helper->connector_count,
@@ -2141,7 +2130,6 @@ static void drm_setup_crtcs(struct drm_fb_helper *fb_helper)
 		goto out;
 	}
 
-
 	drm_enable_connectors(fb_helper, enabled);
 
 	if (!(fb_helper->funcs->initial_config &&
@@ -2170,7 +2158,7 @@ static void drm_setup_crtcs(struct drm_fb_helper *fb_helper)
 		drm_fb_helper_modeset_release(fb_helper,
 					      &fb_helper->crtc_info[i].mode_set);
 
-	for (i = 0; i < fb_helper->connector_count; i++) {
+	drm_fb_helper_for_each_connector(fb_helper, i) {
 		struct drm_display_mode *mode = modes[i];
 		struct drm_fb_helper_crtc *fb_crtc = crtcs[i];
 		struct drm_fb_offset *offset = &offsets[i];
@@ -2247,7 +2235,9 @@ out:
 int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
 {
 	struct drm_device *dev = fb_helper->dev;
+	struct fb_info *info;
 	int count = 0;
+	int ret;
 
 	if (!drm_fbdev_emulation)
 		return 0;
@@ -2256,7 +2246,6 @@ int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
 	count = drm_fb_helper_probe_connector_modes(fb_helper,
 						    dev->mode_config.max_width,
 						    dev->mode_config.max_height);
-	mutex_unlock(&dev->mode_config.mutex);
 	/*
 	 * we shouldn't end up with no modes here.
 	 */
@@ -2264,8 +2253,26 @@ int drm_fb_helper_initial_config(struct drm_fb_helper *fb_helper, int bpp_sel)
 		dev_info(fb_helper->dev->dev, "No connectors reported connected with modes\n");
 
 	drm_setup_crtcs(fb_helper);
+	ret = drm_fb_helper_single_fb_probe(fb_helper, bpp_sel);
+	mutex_unlock(&dev->mode_config.mutex);
+	if (ret)
+		return ret;
 
-	return drm_fb_helper_single_fb_probe(fb_helper, bpp_sel);
+	info = fb_helper->fbdev;
+	info->var.pixclock = 0;
+	ret = register_framebuffer(info);
+	if (ret < 0)
+		return ret;
+
+	dev_info(dev->dev, "fb%d: %s frame buffer device\n",
+		 info->node, info->fix.id);
+
+	if (list_empty(&kernel_fb_helper_list))
+		register_sysrq_key('v', &sysrq_drm_fb_helper_restore_op);
+
+	list_add(&fb_helper->kernel_fb_list, &kernel_fb_helper_list);
+
+	return 0;
 }
 EXPORT_SYMBOL(drm_fb_helper_initial_config);