Re: [PATCH 1/2] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup

From: Thomas Zimmermann <tzimmermann@suse.de>

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: 6a9ee8af344e ("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: <redacted> # v2.6.34+
Link: https://patch.msgid.link/20251105161549.98836-1-tzimmermann@suse.de
(cherry picked from commit eb76d0f5553575599561010f24c277cc5b31d003)
---
  drivers/gpu/drm/drm_fb_helper.c            | 6 ------
  drivers/gpu/drm/i915/display/intel_fbdev.c | 2 --
  drivers/gpu/drm/radeon/radeon_fbdev.c      | 3 ---
  drivers/video/fbdev/core/fbcon.c           | 9 +++++++++
  4 files changed, 9 insertions(+), 11 deletions(-)

diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
index b507c1c008a3e..3915a8b17bc4c 100644
--- a/drivers/gpu/drm/drm_fb_helper.c
+++ b/drivers/gpu/drm/drm_fb_helper.c

@@ -32,7 +32,6 @@
  #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>

@@ -1668,7 +1667,6 @@ static int drm_fb_helper_find_sizes(struct drm_fb_helper *fb_helper,
  static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper)
  {
  	struct drm_client_dev *client = &fb_helper->client;
-	struct drm_device *dev = fb_helper->dev;
  	struct drm_fb_helper_surface_size sizes;
  	int ret;
  

@@ -1687,10 +1685,6 @@ 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;
  }
  

diff --git a/drivers/gpu/drm/i915/display/intel_fbdev.c b/drivers/gpu/drm/i915/display/intel_fbdev.c
index 31d0d695d5671..8bad7e7909f7b 100644
--- a/drivers/gpu/drm/i915/display/intel_fbdev.c
+++ b/drivers/gpu/drm/i915/display/intel_fbdev.c

@@ -36,7 +36,6 @@
  #include <linux/string.h>
  #include <linux/sysrq.h>
  #include <linux/tty.h>
-#include <linux/vga_switcheroo.h>
  
  #include <drm/drm_crtc.h>
  #include <drm/drm_fb_helper.h>

@@ -337,7 +336,6 @@ static int intelfb_create(struct drm_fb_helper *helper,
  	ifbdev->vma_flags = flags;
  
  	intel_runtime_pm_put(&dev_priv->runtime_pm, wakeref);
-	vga_switcheroo_client_fb_set(pdev, info);
  	return 0;
  
  out_unpin:

diff --git a/drivers/gpu/drm/radeon/radeon_fbdev.c b/drivers/gpu/drm/radeon/radeon_fbdev.c
index fb70de29545c6..730efabe05f6b 100644
--- a/drivers/gpu/drm/radeon/radeon_fbdev.c
+++ b/drivers/gpu/drm/radeon/radeon_fbdev.c

@@ -303,7 +303,6 @@ static void radeon_fbdev_client_unregister(struct drm_client_dev *client)
  	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 {

@@ -342,8 +341,6 @@ 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:

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 79525ee90e3b2..57b21b6869231 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c

@@ -64,6 +64,7 @@
  #include <linux/console.h>
  #include <linux/string.h>
  #include <linux/kd.h>
+#include <linux/pci.h>
  #include <linux/slab.h>
  #include <linux/fb.h>
  #include <linux/fbcon.h>

@@ -75,6 +76,7 @@
  #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"

@@ -2909,6 +2911,9 @@ 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), info);
+
  	fbcon_registered_fb[info->node] = NULL;
  	fbcon_num_registered_fb--;
  

@@ -3042,6 +3047,10 @@ 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), NULL);
+
  	return ret;
  }