KASAN: vmalloc-out-of-bounds Read in drm_fb_helper_dirty_work

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    7e0165b2 Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1619eb1ee00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=1b59a3066828ac4c
dashboard link: https://syzkaller.appspot.com/bug?extid=5d11928e253121e6c196
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:
Reported-by: syzbot+5d11928e253121e6c196 at syzkaller.appspotmail.com

BUG: KASAN: vmalloc-out-of-bounds in memcpy include/linux/string.h:380  
[inline]
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  
drivers/gpu/drm/drm_fb_helper.c:428
Read of size 4096 at addr ffffc90008bc1000 by task kworker/1:3/17225

CPU: 1 PID: 17225 Comm: kworker/1:3 Not tainted 5.5.0-rc2-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS  
rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 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+0x24/0x50 mm/kasan/common.c:125
  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:
  ffffc90008bc0f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  ffffc90008bc0f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffffc90008bc1000: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
                    ^
  ffffc90008bc1080: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
  ffffc90008bc1100: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
==================================================================


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