KASAN: vmalloc-out-of-bounds Write in sys_imageblit

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    435faf5c Merge tag 'riscv-for-linus-5.8-mw0' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1519d83e100000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3dbb617b9c2a5bdf
dashboard link: https://syzkaller.appspot.com/bug?extid=26dc38a00dc05118a4e6
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17fbd189100000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+26dc38a00dc05118a4e6 at syzkaller.appspotmail.com

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BUG: KASAN: vmalloc-out-of-bounds in fast_imageblit drivers/video/fbdev/core/sysimgblt.c:229 [inline]
BUG: KASAN: vmalloc-out-of-bounds in sys_imageblit+0x1181/0x1240 drivers/video/fbdev/core/sysimgblt.c:275
Write of size 4 at addr ffffc90009c1b040 by task syz-executor.0/9196

CPU: 2 PID: 9196 Comm: syz-executor.0 Not tainted 5.7.0-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS rel-1.12.0-59-gc9ba5276e321-prebuilt.qemu.org 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0x5/0x413 mm/kasan/report.c:383
 __kasan_report mm/kasan/report.c:513 [inline]
 kasan_report.cold+0x1f/0x37 mm/kasan/report.c:530
 fast_imageblit drivers/video/fbdev/core/sysimgblt.c:229 [inline]
 sys_imageblit+0x1181/0x1240 drivers/video/fbdev/core/sysimgblt.c:275
 drm_fb_helper_sys_imageblit+0x1c/0x180 drivers/gpu/drm/drm_fb_helper.c:747
 bit_putcs_unaligned drivers/video/fbdev/core/bitblit.c:139 [inline]
 bit_putcs+0x8d0/0xd60 drivers/video/fbdev/core/bitblit.c:188
 fbcon_putcs+0x345/0x3f0 drivers/video/fbdev/core/fbcon.c:1362
 do_update_region+0x398/0x630 drivers/tty/vt/vt.c:683
 invert_screen+0x2a7/0x600 drivers/tty/vt/vt.c:800
 highlight drivers/tty/vt/selection.c:57 [inline]
 clear_selection drivers/tty/vt/selection.c:84 [inline]
 clear_selection+0x55/0x70 drivers/tty/vt/selection.c:80
 vc_do_resize+0xff3/0x1370 drivers/tty/vt/vt.c:1230
 fbcon_do_set_font+0x4a0/0x950 drivers/video/fbdev/core/fbcon.c:2608
 fbcon_set_font+0x732/0x870 drivers/video/fbdev/core/fbcon.c:2705
 con_font_set drivers/tty/vt/vt.c:4571 [inline]
 con_font_op+0xd65/0x1160 drivers/tty/vt/vt.c:4636
 vt_ioctl+0x1740/0x2640 drivers/tty/vt/vt_ioctl.c:980
 tty_ioctl+0xedc/0x1440 drivers/tty/tty_io.c:2656
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x11a/0x180 fs/ioctl.c:771
 __do_sys_ioctl fs/ioctl.c:780 [inline]
 __se_sys_ioctl fs/ioctl.c:778 [inline]
 __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:778
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x49/0xb3
RIP: 0033:0x45c879
Code: 1d b2 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 0f 83 eb b1 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fb46427fc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 000000000074bf00 RCX: 000000000045c879
RDX: 0000000020000080 RSI: 0000000000004b72 RDI: 0000000000000003
RBP: 00000000006fae40 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fb4642806d4
R13: 000000000000037a R14: 00000000006ecfc0 R15: 00000000004ace26


Memory state around the buggy address:
 ffffc90009c1af00: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
 ffffc90009c1af80: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
>ffffc90009c1b000: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
                                           ^
 ffffc90009c1b080: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
 ffffc90009c1b100: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
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