[PATCH 5.15.y] fbcon: Set fb_display[i]->mode to NULL when the mode is released

[Johnny Hao]

From: Quanmin Yan <yanquanmin1@xxxxxxxxxx>

[ Upstream commit a1f3058930745d2b938b6b4f5bd9630dc74b26b7 ]

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@xxxxxxxxxx>
Reviewed-by: Thomas Zimmermann <tzimmermann@xxxxxxx>
Signed-off-by: Helge Deller <deller@xxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
[ The context change is due to the commit 2c0c19b681d5
("fbdev: fbmem: Fix double free of 'fb_info->pixmap.addr'") in v5.16
which is irrelevant to the logic of this patch. ]
Signed-off-by: Johnny Hao <johnny_haocn@xxxxxxxx>
---
 drivers/video/fbdev/core/fbcon.c | 20 ++++++++++++++++++++
 drivers/video/fbdev/core/fbmem.c |  2 ++
 include/linux/fbcon.h            |  2 ++
 3 files changed, 24 insertions(+)

diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 8922595cc491..ede993b3fbd4 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -2820,6 +2820,26 @@ int fbcon_mode_deleted(struct fb_info *info,
 	return found;
 }
 
+static void fbcon_delete_mode(struct fb_videomode *m)
+{
+	struct fbcon_display *p;
+	int i;
+
+	for (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 3b52ddfe0350..03a7a7e2a670 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1750,6 +1750,8 @@ static void do_unregister_framebuffer(struct fb_info *fb_info)
 	if (fb_info->pixmap.addr &&
 	    (fb_info->pixmap.flags & FB_PIXMAP_DEFAULT))
 		kfree(fb_info->pixmap.addr);
+
+	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 2382dec6d6ab..fb0fc2736b80 100644
--- a/include/linux/fbcon.h
+++ b/include/linux/fbcon.h
@@ -11,6 +11,7 @@ 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);
@@ -31,6 +32,7 @@ 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) {}
-- 
2.34.1