Sasha Levin
2025-Dec-01 22:51 UTC
[PATCH 6.6.y] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup
From: Thomas Zimmermann <tzimmermann at 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 at 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 at redhat.com>
Acked-by: Alex Deucher <alexander.deucher at amd.com>
Cc: dri-devel at lists.freedesktop.org
Cc: nouveau at lists.freedesktop.org
Cc: amd-gfx at lists.freedesktop.org
Cc: linux-fbdev at vger.kernel.org
Cc: <stable at vger.kernel.org> # v2.6.34+
Link: https://patch.msgid.link/20251105161549.98836-1-tzimmermann at suse.de
[ drm_fb_helper_unregister_info() lacks vga_switcheroo code ]
Signed-off-by: Sasha Levin <sashal at 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;
}
--
2.51.0
Thomas Zimmermann
2025-Dec-02 07:42 UTC
[PATCH 6.6.y] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon setup
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 at 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 at 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 at redhat.com> > Acked-by: Alex Deucher <alexander.deucher at amd.com> > Cc: dri-devel at lists.freedesktop.org > Cc: nouveau at lists.freedesktop.org > Cc: amd-gfx at lists.freedesktop.org > Cc: linux-fbdev at vger.kernel.org > Cc: <stable at vger.kernel.org> # v2.6.34+ > Link: https://patch.msgid.link/20251105161549.98836-1-tzimmermann at suse.de > [ drm_fb_helper_unregister_info() lacks vga_switcheroo code ] > Signed-off-by: Sasha Levin <sashal at 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 N?rnberg, Germany, www.suse.com GF: Jochen Jaser, Andrew McDonald, Werner Knoblich, (HRB 36809, AG N?rnberg)