drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++ drivers/video/fbdev/core/fbmem.c | 1 + include/linux/fbcon.h | 2 ++ 3 files changed, 22 insertions(+)
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
Based on experimentation and analysis, during framebuffer unregistration,
only the memory of fb_info->modelist is freed, without setting the
corresponding fb_display[i]->mode to NULL for the freed modes. This leads
to UAF issues during subsequent accesses. Here's an example of reproduction
steps:
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.
Add a check in do_unregister_framebuffer(): if the mode to be freed exists
in fb_display[], set the corresponding mode pointer to NULL.
Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
---
Changes from v1
(https://lore.kernel.org/all/20250923110608.3385083-1-yanquanmin1@huawei.com/)
- Focus on fixing the issue specifically in the framebuffer unregistration
path, as other paths do not encounter this problem.
- Adjusted according to Thomas's suggestions.
drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++
drivers/video/fbdev/core/fbmem.c | 1 +
include/linux/fbcon.h | 2 ++
3 files changed, 22 insertions(+)
diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 96cc9b389246..9bd3c3814b5c 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2810,6 +2810,25 @@ int fbcon_mode_deleted(struct fb_info *info,
return found;
}
+static void fbcon_delete_mode(struct fb_videomode *m)
+{
+ struct fbcon_display *p;
+
+ for (int i = first_fb_vc; i <= last_fb_vc; i++) {
+ p = &fb_display[i];
+ if (p->mode == m)
+ p->mode = NULL;
+ }
+}
+
+void fbcon_delete_modelist(struct list_head *head)
+{
+ struct fb_modelist *modelist;
+
+ list_for_each_entry(modelist, head, list)
+ fbcon_delete_mode(&modelist->mode);
+}
+
#ifdef CONFIG_VT_HW_CONSOLE_BINDING
static void fbcon_unbind(void)
{
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 53f1719b1ae1..eff757ebbed1 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -544,6 +544,7 @@ static void do_unregister_framebuffer(struct fb_info *fb_info)
fb_info->pixmap.addr = NULL;
}
+ fbcon_delete_modelist(&fb_info->modelist);
fb_destroy_modelist(&fb_info->modelist);
registered_fb[fb_info->node] = NULL;
num_registered_fb--;
diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
index 81f0e698acbf..f206370060e1 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 fbcon_delete_modelist(struct list_head *head);
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 fbcon_delete_modelist(struct list_head *head) {}
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) {}
--
2.43.0On 10/10/25 10:16, Quanmin Yan wrote: > 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 > > Based on experimentation and analysis, during framebuffer unregistration, > only the memory of fb_info->modelist is freed, without setting the > corresponding fb_display[i]->mode to NULL for the freed modes. This leads > to UAF issues during subsequent accesses. Here's an example of reproduction > steps: > 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. > > Add a check in do_unregister_framebuffer(): if the mode to be freed exists > in fb_display[], set the corresponding mode pointer to NULL. > > Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com> > --- > drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++ > drivers/video/fbdev/core/fbmem.c | 1 + > include/linux/fbcon.h | 2 ++ > 3 files changed, 22 insertions(+) applied. Thanks! Helge
Am 10.10.25 um 10:16 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
>
> Based on experimentation and analysis, during framebuffer unregistration,
> only the memory of fb_info->modelist is freed, without setting the
> corresponding fb_display[i]->mode to NULL for the freed modes. This leads
> to UAF issues during subsequent accesses. Here's an example of reproduction
> steps:
> 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.
>
> Add a check in do_unregister_framebuffer(): if the mode to be freed exists
> in fb_display[], set the corresponding mode pointer to NULL.
>
> Signed-off-by: Quanmin Yan <yanquanmin1@huawei.com>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Thanks for fixing the bug.
> ---
>
> Changes from v1
> (https://lore.kernel.org/all/20250923110608.3385083-1-yanquanmin1@huawei.com/)
> - Focus on fixing the issue specifically in the framebuffer unregistration
> path, as other paths do not encounter this problem.
> - Adjusted according to Thomas's suggestions.
>
> drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++
> drivers/video/fbdev/core/fbmem.c | 1 +
> include/linux/fbcon.h | 2 ++
> 3 files changed, 22 insertions(+)
>
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 96cc9b389246..9bd3c3814b5c 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -2810,6 +2810,25 @@ int fbcon_mode_deleted(struct fb_info *info,
> return found;
> }
>
> +static void fbcon_delete_mode(struct fb_videomode *m)
> +{
> + struct fbcon_display *p;
> +
> + for (int i = first_fb_vc; i <= last_fb_vc; i++) {
> + p = &fb_display[i];
> + if (p->mode == m)
> + p->mode = NULL;
> + }
> +}
> +
> +void fbcon_delete_modelist(struct list_head *head)
> +{
> + struct fb_modelist *modelist;
> +
> + list_for_each_entry(modelist, head, list)
> + fbcon_delete_mode(&modelist->mode);
> +}
> +
> #ifdef CONFIG_VT_HW_CONSOLE_BINDING
> static void fbcon_unbind(void)
> {
> diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
> index 53f1719b1ae1..eff757ebbed1 100644
> --- a/drivers/video/fbdev/core/fbmem.c
> +++ b/drivers/video/fbdev/core/fbmem.c
> @@ -544,6 +544,7 @@ static void do_unregister_framebuffer(struct fb_info *fb_info)
> fb_info->pixmap.addr = NULL;
> }
>
> + fbcon_delete_modelist(&fb_info->modelist);
> fb_destroy_modelist(&fb_info->modelist);
> registered_fb[fb_info->node] = NULL;
> num_registered_fb--;
> diff --git a/include/linux/fbcon.h b/include/linux/fbcon.h
> index 81f0e698acbf..f206370060e1 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 fbcon_delete_modelist(struct list_head *head);
> 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 fbcon_delete_modelist(struct list_head *head) {}
> 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)
© 2016 - 2025 Red Hat, Inc.