drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup

[ Upstream commit eb76d0f ]

Protect vga_switcheroo_client_fb_set() with console lock. Avoids OOB
access in fbcon_remap_all(). Without holding the console lock the call
races with switching outputs.

VGA switcheroo calls fbcon_remap_all() when switching clients. The fbcon
function uses struct fb_info.node, which is set by register_framebuffer().
As the fb-helper code currently sets up VGA switcheroo before registering
the framebuffer, the value of node is -1 and therefore not a legal value.
For example, fbcon uses the value within set_con2fb_map() [1] as an index
into an array.

Moving vga_switcheroo_client_fb_set() after register_framebuffer() can
result in VGA switching that does not switch fbcon correctly.

Therefore move vga_switcheroo_client_fb_set() under fbcon_fb_registered(),
which already holds the console lock. Fbdev calls fbcon_fb_registered()
from within register_framebuffer(). Serializes the helper with VGA
switcheroo's call to fbcon_remap_all().

Although vga_switcheroo_client_fb_set() takes an instance of struct fb_info
as parameter, it really only needs the contained fbcon state. Moving the
call to fbcon initialization is therefore cleaner than before. Only amdgpu,
i915, nouveau and radeon support vga_switcheroo. For all other drivers,
this change does nothing.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://elixir.bootlin.com/linux/v6.17/source/drivers/video/fbdev/core/fbcon.c#L2942 # [1]
Fixes: 6a9ee8a ("vga_switcheroo: initial implementation (v15)")
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Cc: dri-devel@lists.freedesktop.org
Cc: nouveau@lists.freedesktop.org
Cc: amd-gfx@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Cc: <stable@vger.kernel.org> # v2.6.34+
Link: https://patch.msgid.link/20251105161549.98836-1-tzimmermann@suse.de
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: tdz

drivers/gpu/drm/drm_fb_helper.c

@@ -30,9 +30,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/console.h>
-#include <linux/pci.h>
 #include <linux/sysrq.h>
-#include <linux/vga_switcheroo.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_drv.h>
@@ -1637,10 +1635,6 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper)
 
 	strcpy(fb_helper->fb->comm, "[fbcon]");
 
-	/* Set the fb info for vgaswitcheroo clients. Does nothing otherwise. */
-	if (dev_is_pci(dev->dev))
-		vga_switcheroo_client_fb_set(to_pci_dev(dev->dev), fb_helper->info);
-
 	return 0;
 }
 

drivers/gpu/drm/i915/display/intel_fbdev.c

@@ -589,11 +589,8 @@ static int intel_fbdev_restore_mode(struct drm_i915_private *dev_priv)
 static void intel_fbdev_client_unregister(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
-	struct drm_device *dev = fb_helper->dev;
-	struct pci_dev *pdev = to_pci_dev(dev->dev);
 
 	if (fb_helper->info) {
-		vga_switcheroo_client_fb_set(pdev, NULL);
 		drm_fb_helper_unregister_info(fb_helper);
 	} else {
 		drm_fb_helper_unprepare(fb_helper);
@@ -620,7 +617,6 @@ static int intel_fbdev_client_hotplug(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
 	struct drm_device *dev = client->dev;
-	struct pci_dev *pdev = to_pci_dev(dev->dev);
 	int ret;
 
 	if (dev->fb_helper)
@@ -634,8 +630,6 @@ static int intel_fbdev_client_hotplug(struct drm_client_dev *client)
 	if (ret)
 		goto err_drm_fb_helper_fini;
 
-	vga_switcheroo_client_fb_set(pdev, fb_helper->info);
-
 	return 0;
 
 err_drm_fb_helper_fini:

drivers/gpu/drm/radeon/radeon_fbdev.c

@@ -300,10 +300,8 @@ static void radeon_fbdev_client_unregister(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
 	struct drm_device *dev = fb_helper->dev;
-	struct radeon_device *rdev = dev->dev_private;
 
 	if (fb_helper->info) {
-		vga_switcheroo_client_fb_set(rdev->pdev, NULL);
 		drm_helper_force_disable_all(dev);
 		drm_fb_helper_unregister_info(fb_helper);
 	} else {
@@ -325,7 +323,6 @@ static int radeon_fbdev_client_hotplug(struct drm_client_dev *client)
 {
 	struct drm_fb_helper *fb_helper = drm_fb_helper_from_client(client);
 	struct drm_device *dev = client->dev;
-	struct radeon_device *rdev = dev->dev_private;
 	int ret;
 
 	if (dev->fb_helper)
@@ -342,8 +339,6 @@ static int radeon_fbdev_client_hotplug(struct drm_client_dev *client)
 	if (ret)
 		goto err_drm_fb_helper_fini;
 
-	vga_switcheroo_client_fb_set(rdev->pdev, fb_helper->info);
-
 	return 0;
 
 err_drm_fb_helper_fini:

drivers/video/fbdev/core/fbcon.c

@@ -65,6 +65,7 @@
 #include <linux/string.h>
 #include <linux/kd.h>
 #include <linux/panic.h>
+#include <linux/pci.h>
 #include <linux/printk.h>
 #include <linux/slab.h>
 #include <linux/fb.h>
@@ -77,6 +78,7 @@
 #include <linux/interrupt.h>
 #include <linux/crc32.h> /* For counting font checksums */
 #include <linux/uaccess.h>
+#include <linux/vga_switcheroo.h>
 #include <asm/irq.h>
 
 #include "fbcon.h"
@@ -2894,6 +2896,9 @@ void fbcon_fb_unregistered(struct fb_info *info)
 
 	console_lock();
 
+	if (info->device && dev_is_pci(info->device))
+		vga_switcheroo_client_fb_set(to_pci_dev(info->device), NULL);
+
 	fbcon_registered_fb[info->node] = NULL;
 	fbcon_num_registered_fb--;
 
@@ -3027,6 +3032,10 @@ static int do_fb_registered(struct fb_info *info)
 		}
 	}
 
+	/* Set the fb info for vga_switcheroo clients. Does nothing otherwise. */
+	if (info->device && dev_is_pci(info->device))
+		vga_switcheroo_client_fb_set(to_pci_dev(info->device), info);
+
 	return ret;
 }