[BUG] drivers/video: use-after-free in fb_videomode_to_var()

[co]

We found a bug reachable in:

    path    drivers/video/fbdev
    crash   use-after-free in fb_videomode_to_var()
    commit  3cd8b194bf34 ("Merge tag 'v7.1-rc-part1-smbdirect-fixes' of git://git.samba.org/ksmbd")

Config, environment, the sanitizer report and a C reproducer follow.

== Notes ===============================================================
    If you patch the bug based on our artifacts, a tag would be
    appreciated:

Reported-by: co+ed9902f2e44184d9@xxxxxxx

    Everything in this mail is validated by the reproducer below.

    We also hold an LLM-generated root-cause analysis and a candidate
    patch. The patch passes an A/B test: the same reproducer panics the
    unpatched kernel and runs clean on the patched one. Neither has had
    human review, so both still require validation before you send or
    apply them. Available on:

        patch.diff  https://bugs.sh/b/ed9902f2e44184d9/patch.diff
        report.md   https://bugs.sh/b/ed9902f2e44184d9/report.md

    This is an open science project. The code and the full set of PoCs
    are not public at this moment, as we intend to disclose our findings
    in an ethical way.

    Happy to test patches. Complaints and suggestions about our work
    are welcome at:

        cedalion@xxxxxxx

== Environment =========================================================
    Reproduced on  3cd8b194bf34 ("Merge tag 'v7.1-rc-part1-smbdirect-fixes' of git://git.samba.org/ksmbd")
    VM setup       https://bugs.sh/b/ed9902f2e44184d9/run.sh
    config         https://bugs.sh/b/ed9902f2e44184d9/config.gz
    poc            https://bugs.sh/b/ed9902f2e44184d9/repro.c

== Sanitizer Report ====================================================
BUG: KASAN: slab-use-after-free in fb_videomode_to_var (drivers/video/fbdev/core/modedb.c:906)
Read of size 4 at addr ffff88801057471c by task kworker/1:2/113
CPU: 1 UID: 0 PID: 113 Comm: kworker/1:2 Not tainted 7.0.0+ #1 PREEMPT(lazy)
Workqueue: events console_callback
Call Trace:
dump_stack_lvl (lib/dump_stack.c:123)
print_report (mm/kasan/report.c:379 mm/kasan/report.c:482)
kasan_report (mm/kasan/report.c:597)
fb_videomode_to_var (drivers/video/fbdev/core/modedb.c:906)
display_to_var (drivers/video/fbdev/core/fbcon.c:1000)
fbcon_switch (drivers/video/fbdev/core/fbcon.c:2190)
redraw_screen (drivers/tty/vt/vt.c:995)
complete_change_console (drivers/tty/vt/vt_ioctl.c:1149)
change_console (drivers/tty/vt/vt_ioctl.c:1263)
console_callback (drivers/tty/vt/vt.c:3320)
process_one_work (kernel/workqueue.c:3307)
worker_thread (kernel/workqueue.c:3379 (discriminator 2) kernel/workqueue.c:3466 (discriminator 2))
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:164)
ret_from_fork_asm (arch/x86/entry/entry_64.S:255)
Allocated by task 12:
kasan_save_stack (mm/kasan/common.c:58)
kasan_save_track (mm/kasan/common.c:70 (discriminator 1) mm/kasan/common.c:79 (discriminator 1))
__kasan_kmalloc (mm/kasan/common.c:398 mm/kasan/common.c:415)
fb_add_videomode (./include/linux/slab.h:950 drivers/video/fbdev/core/modedb.c:1073)
dlfb_setup_modes (drivers/video/fbdev/udlfb.c:1316 (discriminator 2))
dlfb_usb_probe (drivers/video/fbdev/udlfb.c:1685)
usb_probe_interface (drivers/usb/core/driver.c:397)
really_probe (drivers/base/dd.c:631 drivers/base/dd.c:709)
__driver_probe_device (drivers/base/dd.c:851)
driver_probe_device (drivers/base/dd.c:882)
__device_attach_driver (drivers/base/dd.c:1010)
bus_for_each_drv (drivers/base/bus.c:500)
__device_attach (drivers/base/dd.c:1083)
device_initial_probe (drivers/base/dd.c:1136)
bus_probe_device (drivers/base/bus.c:615)
device_add (drivers/base/core.c:3698)
usb_new_device (drivers/usb/core/hub.c:2696)
hub_event (drivers/usb/core/hub.c:5568 drivers/usb/core/hub.c:5707 drivers/usb/core/hub.c:5871 drivers/usb/core/hub.c:5953)
process_one_work (kernel/workqueue.c:3307)
worker_thread (kernel/workqueue.c:3379 (discriminator 2) kernel/workqueue.c:3466 (discriminator 2))
kthread (kernel/kthread.c:436)
ret_from_fork (arch/x86/kernel/process.c:164)
ret_from_fork_asm (arch/x86/entry/entry_64.S:255)
Freed by task 173:
kasan_save_stack (mm/kasan/common.c:58)
kasan_save_track (mm/kasan/common.c:70 (discriminator 1) mm/kasan/common.c:79 (discriminator 1))
kasan_save_free_info (mm/kasan/generic.c:587 (discriminator 1))
__kasan_slab_free (mm/kasan/common.c:287)
kfree (mm/slub.c:6246 (discriminator 3) mm/slub.c:6561 (discriminator 3))
fb_destroy_modelist (drivers/video/fbdev/core/modedb.c:1116 (discriminator 3))
dlfb_setup_modes (drivers/video/fbdev/udlfb.c:1268)
edid_store (drivers/video/fbdev/udlfb.c:1456)
sysfs_kf_bin_write (fs/sysfs/file.c:164)
kernfs_fop_write_iter (fs/kernfs/file.c:88 fs/kernfs/file.c:356)
vfs_write (fs/read_write.c:596 (discriminator 1) fs/read_write.c:688 (discriminator 1))
ksys_write (fs/read_write.c:741)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 (discriminator 1) arch/x86/entry/syscall_64.c:94 (discriminator 1))
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
The buggy address belongs to the object at ffff888010574700
which belongs to the cache kmalloc-96 of size 96
The buggy address is located 28 bytes inside of


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