fbcon: Set fb_display[i]->mode to NULL when the mode is released

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

Author: Quanmin Yan

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)
 {

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--;

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) {}