CVE-2025-40323: fbcon: Set fb_display[i]->mode to NULL when the mode is released

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

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.

The Linux kernel CVE team has assigned CVE-2025-40323 to this issue.


Affected and fixed versions
===========================

	Fixed in 6.1.159 with commit 4ac18f0e6a6d599ca751c4cd98e522afc8e3d4eb
	Fixed in 6.6.117 with commit 468f78276a37f4c6499385a4ce28f4f57be6655d
	Fixed in 6.12.58 with commit c079d42f70109512eee49123a843be91d8fa133f
	Fixed in 6.17.8 with commit de89d19f4f30d9a8de87b9d08c1bd35cb70576d8
	Fixed in 6.18 with commit a1f3058930745d2b938b6b4f5bd9630dc74b26b7

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-40323
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	drivers/video/fbdev/core/fbcon.c
	drivers/video/fbdev/core/fbmem.c
	include/linux/fbcon.h


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/4ac18f0e6a6d599ca751c4cd98e522afc8e3d4eb
	https://git.kernel.org/stable/c/468f78276a37f4c6499385a4ce28f4f57be6655d
	https://git.kernel.org/stable/c/c079d42f70109512eee49123a843be91d8fa133f
	https://git.kernel.org/stable/c/de89d19f4f30d9a8de87b9d08c1bd35cb70576d8
	https://git.kernel.org/stable/c/a1f3058930745d2b938b6b4f5bd9630dc74b26b7