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FAILED: patch "[PATCH] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon" failed to apply to 5.15-stable tree

PATCH [email protected] 1 participant 1 message Dec 1, 2025

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Stable branch DRM drivers DRM misc Framebuffer layer +2 more subsystems
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The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x eb76d0f5553575599561010f24c277cc5b31d003
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2025120121-cruncher-risotto-7ffa@gregkh' --subject-prefix 'PATCH 5.15.y' HEAD^..

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From eb76d0f5553575599561010f24c277cc5b31d003 Mon Sep 17 00:00:00 2001
From: Thomas Zimmermann <[email protected]>
Date: Wed, 5 Nov 2025 17:14:49 +0100
Subject: [PATCH] drm, fbcon, vga_switcheroo: Avoid race condition in fbcon
setup

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 <[email protected]>
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 <[email protected]>
Acked-by Alex Deucher <[email protected]>
Cc [email protected]
Cc [email protected]
Cc [email protected]
Cc [email protected]
Cc <[email protected]> # v2.6.34+
Link https://patch.msgid.link/[email protected] open here
drivers/gpu/drm/drm_fb_helper.c +0 -14
index 11a5b60cb9ce..0b3ee008523d 100644
@@ -31,9 +31,7 @@
 
 #include <linux/console.h>
 #include <linux/export.h>
-#include <linux/pci.h>
 #include <linux/sysrq.h>
-#include <linux/vga_switcheroo.h>
 
 #include <drm/drm_atomic.h>
 #include <drm/drm_drv.h>

@@ -566,11 +564,6 @@ EXPORT_SYMBOL(drm_fb_helper_release_info);
  */
 void drm_fb_helper_unregister_info(struct drm_fb_helper *fb_helper)
 {
-	struct fb_info *info = fb_helper->info;
-	struct device *dev = info->device;
-
-	if (dev_is_pci(dev))
-		vga_switcheroo_client_fb_set(to_pci_dev(dev), NULL);
 	unregister_framebuffer(fb_helper->info);
 }
 EXPORT_SYMBOL(drm_fb_helper_unregister_info);

@@ -1632,7 +1625,6 @@ 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;
-	struct fb_info *info;
 	int ret;
 
 	if (drm_WARN_ON(dev, !dev->driver->fbdev_probe))

@@ -1653,12 +1645,6 @@ static int drm_fb_helper_single_fb_probe(struct drm_fb_helper *fb_helper)
 
 	strcpy(fb_helper->fb->comm, "[fbcon]");
 
-	info = fb_helper->info;
-
-	/* Set the fb info for vgaswitcheroo clients. Does nothing otherwise. */
-	if (dev_is_pci(info->device))
-		vga_switcheroo_client_fb_set(to_pci_dev(info->device), info);
-
 	return 0;
 }
drivers/video/fbdev/core/fbcon.c +9 -0
index 9bd3c3814b5c..e7e07eb2142e 100644
@@ -66,6 +66,7 @@
 #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>

@@ -78,6 +79,7 @@
 #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"

@@ -2899,6 +2901,9 @@ 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--;
 

@@ -3032,6 +3037,10 @@ 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;
 }