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

[Thomas Zimmermann <tzimmermann@suse.de>]

Please ignore. This patch has been merged already.

Am 20.01.26 um 09:28 schrieb Thomas Zimmermann:
> 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: <stable@vger.kernel.org> # 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;
>   }
>   

-- 
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstr. 146, 90461 Nürnberg, Germany, www.suse.com
GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG Nürnberg)