[BUG] drivers/video: out-of-bounds in tile_putcs()

We found a bug reachable in:

    path    drivers/video/fbdev/core/tileblit.c`, `drivers/video/fbdev/core
    crash   out-of-bounds in tile_putcs()
    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+1875dd47169bd757@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/1875dd47169bd757/patch.diff
        report.md   https://bugs.sh/b/1875dd47169bd757/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/1875dd47169bd757/run.sh
    config         https://bugs.sh/b/1875dd47169bd757/config.gz
    poc            https://bugs.sh/b/1875dd47169bd757/repro.c

== Sanitizer Report ====================================================
BUG: KASAN: slab-out-of-bounds in tile_putcs (drivers/video/fbdev/core/tileblit.c:69)
Write of size 4 at addr ffff88800c622000 by task exploit/143
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)
tile_putcs (drivers/video/fbdev/core/tileblit.c:69)
fbcon_putcs (drivers/video/fbdev/core/fbcon.c:1392 (discriminator 3))
do_update_region (drivers/tty/vt/vt.c:656)
redraw_screen (drivers/tty/vt/vt.c:1008)
fbcon_blank (drivers/video/fbdev/core/fbcon.c:2315)
do_unblank_screen (drivers/tty/vt/vt.c:4756 drivers/tty/vt/vt.c:4724)
vt_ioctl (drivers/tty/vt/vt_ioctl.c:276 drivers/tty/vt/vt_ioctl.c:381 drivers/tty/vt/vt_ioctl.c:743)
tty_ioctl (drivers/tty/tty_io.c:2792)
__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 143:
__kmalloc_cache_noprof (include/linux/kasan.h:263 mm/slub.c:5515)
do_register_framebuffer (include/linux/slab.h:969 drivers/video/fbdev/core/fbmem.c:502)
register_framebuffer (drivers/video/fbdev/core/fbmem.c:602)
ark_pci_probe (drivers/video/fbdev/arkfb.c:1051)
local_pci_probe (drivers/pci/pci-driver.c:332)
pci_device_probe (drivers/pci/pci-driver.c:394 drivers/pci/pci-driver.c:455 drivers/pci/pci-driver.c:489)
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)
__driver_attach (drivers/base/dd.c:1292)
bus_for_each_dev (drivers/base/bus.c:383)
new_id_store (drivers/pci/pci-driver.c:249)
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 ffff88800c620000
which belongs to the cache kmalloc-8k of size 8192
The buggy address is located 0 bytes to the right of


---
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