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SubjectKASAN: vmalloc-out-of-bounds Write in drm_fb_helper_dirty_work

syzbot found the following crash on:

HEAD commit: 62104694 Merge branch 'parisc-5.5-2' of git://git.kernel.o..
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:

BUG: KASAN: vmalloc-out-of-bounds in memcpy include/linux/string.h:380
BUG: KASAN: vmalloc-out-of-bounds in drm_fb_helper_dirty_blit_real
drivers/gpu/drm/drm_fb_helper.c:399 [inline]
BUG: KASAN: vmalloc-out-of-bounds in drm_fb_helper_dirty_work+0x44c/0x780
Write of size 4064 at addr ffffc9000adea020 by task kworker/2:2/8685

CPU: 2 PID: 8685 Comm: kworker/2:2 Not tainted 5.5.0-rc2-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 04/01/2014
Workqueue: events drm_fb_helper_dirty_work
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x197/0x210 lib/dump_stack.c:118
print_address_description.constprop.0.cold+0x5/0x30b mm/kasan/report.c:374
__kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
kasan_report+0x12/0x20 mm/kasan/common.c:639
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
memcpy+0x38/0x50 mm/kasan/common.c:126
memcpy include/linux/string.h:380 [inline]
drm_fb_helper_dirty_blit_real drivers/gpu/drm/drm_fb_helper.c:399 [inline]
drm_fb_helper_dirty_work+0x44c/0x780 drivers/gpu/drm/drm_fb_helper.c:428
process_one_work+0x9af/0x1740 kernel/workqueue.c:2264
worker_thread+0x98/0xe40 kernel/workqueue.c:2410
kthread+0x361/0x430 kernel/kthread.c:255
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Memory state around the buggy address:
ffffc9000ade9f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffffc9000ade9f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffffc9000adea000: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
ffffc9000adea080: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
ffffc9000adea100: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9

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