Re: [PATCH] fbcon: Set fb_display[i]->mode to NULL when the mode is released
From: Thomas Zimmermann
Date: Wed Sep 24 2025 - 05:18:14 EST
Hi,
thanks for the report.
Am 23.09.25 um 13:06 schrieb Quanmin Yan:
Recently, we discovered the following issue through syzkaller:
BUG: KASAN: slab-use-after-free in fb_mode_is_equal+0x285/0x2f0
Read of size 4 at addr ff11000001b3c69c by task syz.xxx
...
Call Trace:
<TASK>
dump_stack_lvl+0xab/0xe0
print_address_description.constprop.0+0x2c/0x390
print_report+0xb9/0x280
kasan_report+0xb8/0xf0
fb_mode_is_equal+0x285/0x2f0
fbcon_mode_deleted+0x129/0x180
fb_set_var+0xe7f/0x11d0
do_fb_ioctl+0x6a0/0x750
fb_ioctl+0xe0/0x140
__x64_sys_ioctl+0x193/0x210
do_syscall_64+0x5f/0x9c0
entry_SYSCALL_64_after_hwframe+0x76/0x7e
The issue occurs in the function fb_mode_is_equal(p->mode, mode), I also
noticed that when freeing the memory related to fb_info->modelist, there's
no attempt to set the corresponding fb_display[i]->mode to NULL after
freeing. Based on analysis, the root cause of this bug appears to be that
a certain p->mode has become a wild pointer.
I've identified two code paths for freeing modelist->mode:
1. fb_delete_videomode - removes videomode entry from modelist.
2. fb_destroy_modelist - destroys the entire modelist.
What about fb_new_modelist()? [1] It's called from store_modes() and the
whole logic in the caller seems fragile to me. This could leave p->mode
set when it should be cleared; and vice versa.
[1]
https://elixir.bootlin.com/linux/v6.16.8/source/drivers/video/fbdev/core/fbmem.c#L712
[2]
https://elixir.bootlin.com/linux/v6.16.8/source/drivers/video/fbdev/core/fbsysfs.c#L113
Analysis shows that fb_delete_videomode path should have been fixed in
a previous patch[1]. Therefore, the current bug is likely triggered
through the fb_destroy_modelist path. I've found a reproducible test case:
1. With /dev/fb0 already registered in the system, load a kernel module
to register a new device /dev/fb1;
2. Set fb1's mode to the global fb_display[] array (via FBIOPUT_CON2FBMAP);
3. Switch console from fb to VGA (to allow normal rmmod of the ko);
4. Unload the kernel module - at this point fb1's modelist is freed, leaving
a wild pointer in fb_display[];
5. Trigger the bug via system calls through fb0 attempting to delete a mode
from fb0.
To prevent similar issues from recurring, consider traversing fb_display[]
whenever releasing a mode from fb_info. If the corresponding mode exists
in fb_display[], set its pointer to NULL.
[1] https://lore.kernel.org/all/20210712085544.2828-1-thunder.leizhen@xxxxxxxxxx/
Signed-off-by: Quanmin Yan <yanquanmin1@xxxxxxxxxx>
---
This is my first time working on fb issues. If there are any misunderstandings
in my analysis, I would appreciate corrections from the community.
drivers/video/fbdev/core/fbcon.c | 11 +++++++++++
drivers/video/fbdev/core/modedb.c | 7 +++++++
include/linux/fbcon.h | 2 ++
3 files changed, 20 insertions(+)
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index b062b05f4128..bfbf79d6cd05 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2803,6 +2803,17 @@ int fbcon_mode_deleted(struct fb_info *info,
return found;
}
+void fb_display_clean_videomode(struct fb_videomode *m)
Rather fbcon_delete_mode
+{
+ struct fbcon_display *p;
+
+ for (int i = first_fb_vc; i <= last_fb_vc; i++) {
I think this code also needs to test the fb_info, or it might clear
another display's mode. See [3] for an example
[3]
https://elixir.bootlin.com/linux/v6.16.8/source/drivers/video/fbdev/core/fbcon.c#L2786
+ p = &fb_display[i];
+ if (p->mode == m)
+ p->mode = NULL;
+ }
+}
+
#ifdef CONFIG_VT_HW_CONSOLE_BINDING
static void fbcon_unbind(void)
{
diff --git a/drivers/video/fbdev/core/modedb.c b/drivers/video/fbdev/core/modedb.c
index 53a610948c4a..5a0ee96ebefa 100644
--- a/drivers/video/fbdev/core/modedb.c
+++ b/drivers/video/fbdev/core/modedb.c
@@ -16,6 +16,7 @@
#include <linux/slab.h>
#include <linux/fb.h>
#include <linux/kernel.h>
+#include <linux/fbcon.h>
#undef DEBUG
@@ -1100,6 +1101,7 @@ void fb_delete_videomode(const struct fb_videomode *mode,
modelist = list_entry(pos, struct fb_modelist, list);
m = &modelist->mode;
if (fb_mode_is_equal(m, mode)) {
+ fb_display_clean_videomode(m);
There's only one caller of fb_delete_videomode(). I think this call
should be right before fb_delete_videomode().
list_del(pos);
kfree(pos);
}
@@ -1113,8 +1115,13 @@ void fb_delete_videomode(const struct fb_videomode *mode,
void fb_destroy_modelist(struct list_head *head)
{
struct list_head *pos, *n;
+ struct fb_modelist *modelist;
+ struct fb_videomode *m;
list_for_each_safe(pos, n, head) {
+ modelist = list_entry(pos, struct fb_modelist, list);
+ m = &modelist->mode;
+ fb_display_clean_videomode(m);
Same here: I think fb_destroy_modelist() should only release the mode.
Clearing the fbcon mode should be done by the caller.
Best regards
Thomas
list_del(pos);
kfree(pos);
}
diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
index 81f0e698acbf..2b5e93aeaaff 100644
--- a/include/linux/fbcon.h
+++ b/include/linux/fbcon.h
@@ -18,6 +18,7 @@ void fbcon_suspended(struct fb_info *info);
void fbcon_resumed(struct fb_info *info);
int fbcon_mode_deleted(struct fb_info *info,
struct fb_videomode *mode);
+void fb_display_clean_videomode(struct fb_videomode *m);
void fbcon_new_modelist(struct fb_info *info);
void fbcon_get_requirement(struct fb_info *info,
struct fb_blit_caps *caps);
@@ -38,6 +39,7 @@ static inline void fbcon_suspended(struct fb_info *info) {}
static inline void fbcon_resumed(struct fb_info *info) {}
static inline int fbcon_mode_deleted(struct fb_info *info,
struct fb_videomode *mode) { return 0; }
+static inline void fb_display_clean_videomode(struct fb_videomode *m) {}
static inline void fbcon_new_modelist(struct fb_info *info) {}
static inline void fbcon_get_requirement(struct fb_info *info,
struct fb_blit_caps *caps) {}
--
--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Frankenstrasse 146, 90461 Nuernberg, Germany
GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman
HRB 36809 (AG Nuernberg)