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

We found a bug reachable in:

    path    drivers/video/fbdev/core/fb_chrdev.c`, `drivers/video/fbdev/core
    crash   use-after-free in _copy_to_user()
    commit  843bc34db94b

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+c25629c98ba36ebe@bugs.sh

    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/c25629c98ba36ebe/patch.diff
        report.md   https://bugs.sh/b/c25629c98ba36ebe/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@bugs.sh

== Environment =========================================================
    Reproduced on  843bc34db94b
    VM setup       https://bugs.sh/b/c25629c98ba36ebe/run.sh
    config         https://bugs.sh/b/c25629c98ba36ebe/config.gz
    poc            https://bugs.sh/b/c25629c98ba36ebe/repro.c

== Sanitizer Report ====================================================
BUG: KASAN: slab-use-after-free in _copy_to_user (include/linux/instrumented.h:129 include/linux/uaccess.h:201 lib/usercopy.c:24)
Read of size 512 at addr ffff88800dd12800 by task exploit/147
CPU: 1 UID: 0 PID: 147 Comm: exploit Tainted: G        W           7.2.0-rc2+ #6 PREEMPTLAZY
Call Trace:
dump_stack_lvl (lib/dump_stack.c:94 lib/dump_stack.c:120)
print_report (mm/kasan/report.c:378 mm/kasan/report.c:482)
kasan_report (mm/kasan/report.c:595)
kasan_check_range (mm/kasan/generic.c:186 mm/kasan/generic.c:200)
_copy_to_user (include/linux/instrumented.h:129 include/linux/uaccess.h:201 lib/usercopy.c:24)
fb_cmap_to_user (include/linux/uaccess.h:230 drivers/video/fbdev/core/fbcmap.c:211)
do_fb_ioctl (drivers/video/fbdev/core/fb_chrdev.c:114)
__x64_sys_ioctl (fs/ioctl.c:51 fs/ioctl.c:597 fs/ioctl.c:583 fs/ioctl.c:583)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
Allocated by task 1:
kasan_save_stack (mm/kasan/common.c:57)
kasan_save_track (mm/kasan/common.c:78)
__kasan_kmalloc (mm/kasan/common.c:398 mm/kasan/common.c:415)
__kmalloc_noprof (include/linux/kasan.h:263 mm/slub.c:5362 mm/slub.c:5387)
fb_alloc_cmap_gfp (include/linux/slab.h:973 include/linux/slab.h:1290 drivers/video/fbdev/core/fbcmap.c:108)
vfb_probe (drivers/video/fbdev/vfb.c:459)
platform_probe (drivers/base/platform.c:1439)
really_probe (drivers/base/dd.c:628 drivers/base/dd.c:706)
__driver_probe_device (drivers/base/dd.c:868)
driver_probe_device (drivers/base/dd.c:898)
__device_attach_driver (drivers/base/dd.c:1026)
bus_for_each_drv (drivers/base/bus.c:500)
__device_attach (drivers/base/dd.c:1098)
device_initial_probe (drivers/base/dd.c:1153)
bus_probe_device (drivers/base/bus.c:620)
device_add (drivers/base/core.c:3772)
platform_device_add (drivers/base/platform.c:762)
vfb_init (drivers/video/fbdev/vfb.c:525)
do_one_initcall (init/main.c:1347)
kernel_init_freeable (init/main.c:1409 (discriminator 1) init/main.c:1425 (discriminator 1) init/main.c:1445 (discriminator 1) init/main.c:1658 (discriminator 1))
kernel_init (init/main.c:1548)
ret_from_fork (arch/x86/kernel/process.c:158)
ret_from_fork_asm (arch/x86/entry/entry_64.S:245)
Freed by task 144:
kasan_save_stack (mm/kasan/common.c:57)
kasan_save_track (mm/kasan/common.c:78)
kasan_save_free_info (mm/kasan/generic.c:584)
__kasan_slab_free (mm/kasan/common.c:253 mm/kasan/common.c:285)
kfree (include/linux/kasan.h:235 mm/slub.c:2705 mm/slub.c:6405 mm/slub.c:6720)
fb_dealloc_cmap (drivers/video/fbdev/core/fbcmap.c:151)
vfb_remove (drivers/video/fbdev/vfb.c:489)
device_release_driver_internal (drivers/base/dd.c:1349 drivers/base/dd.c:1372)
unbind_store (drivers/base/bus.c:244)
kernfs_fop_write_iter (fs/kernfs/file.c:345)
vfs_write (fs/read_write.c:595 fs/read_write.c:687)
ksys_write (fs/read_write.c:739)
do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:121)
The buggy address belongs to the object at ffff88800dd12800
which belongs to the cache kmalloc-512 of size 512
The buggy address is located 0 bytes inside of
freed 512-byte region [ffff88800dd12800, ffff88800dd12a00)
The buggy address belongs to the physical page:
page dumped because: kasan: bad access detected
Memory state around the buggy address:
ffff88800dd12700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff88800dd12780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

ffff88800dd12800: fa fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb

^
ffff88800dd12880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
ffff88800dd12900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb


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