Re: [PATCH 6.6.y] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup

[Thomas Zimmermann <tzimmermann@suse.de>]

Hi,

thanks for backporting.

You also have to remove the calls to vga_switcheroo_client_fb_set() from 
these files

https://elixir.bootlin.com/linux/v6.6/source/drivers/gpu/drm/i915/display/intel_fbdev.c 

https://elixir.bootlin.com/linux/v6.6/source/drivers/gpu/drm/radeon/radeon_fbdev.c 


Best regards
Thomas

Am 01.12.25 um 23:51 schrieb Sasha Levin:
> From: Thomas Zimmermann <tzimmermann@suse.de>
>
> [ Upstream commit eb76d0f5553575599561010f24c277cc5b31d003 ]
>
> 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
> [ drm_fb_helper_unregister_info() lacks vga_switcheroo code ]
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> ---
>   drivers/gpu/drm/drm_fb_helper.c  | 7 -------
>   drivers/video/fbdev/core/fbcon.c | 9 +++++++++
>   2 files changed, 9 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_fb_helper.c b/drivers/gpu/drm/drm_fb_helper.c
> index b507c1c008a3e..3891837a78414 100644
> --- a/drivers/gpu/drm/drm_fb_helper.c
> +++ b/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>
> @@ -1668,7 +1666,6 @@ 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 +1684,6 @@ 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/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 78a5b22c8d150..8b2c3065c0c26 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"
> @@ -2913,6 +2915,9 @@ 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--;
>   
> @@ -3046,6 +3051,10 @@ 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;
>   }
>   

-- 
--
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)