6.6-stable review patch. If anyone has any objections, please let me know.
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From: Quanmin Yan yanquanmin1@huawei.com
commit a1f3058930745d2b938b6b4f5bd9630dc74b26b7 upstream.
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 Signed-off-by: Helge Deller deller@gmx.de Cc: stable@vger.kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/video/fbdev/core/fbcon.c | 19 +++++++++++++++++++ drivers/video/fbdev/core/fbmem.c | 1 + include/linux/fbcon.h | 2 ++ 3 files changed, 22 insertions(+)
--- a/drivers/video/fbdev/core/fbcon.c +++ b/drivers/video/fbdev/core/fbcon.c @@ -2824,6 +2824,25 @@ int fbcon_mode_deleted(struct fb_info *i 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) { --- a/drivers/video/fbdev/core/fbmem.c +++ b/drivers/video/fbdev/core/fbmem.c @@ -1030,6 +1030,7 @@ static void do_unregister_framebuffer(st 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--; --- a/include/linux/fbcon.h +++ b/include/linux/fbcon.h @@ -11,6 +11,7 @@ void fbcon_suspended(struct fb_info *inf 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); @@ -31,6 +32,7 @@ static inline void fbcon_suspended(struc 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) {}