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SubjectRe: general protection fault in fbcon_cursor
From
syzbot has found a reproducer for the following crash on:

HEAD commit: f2850dd5 Merge tag 'kbuild-fixes-v5.6' of git://git.kernel..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=120eaae6e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=735296e4dd620b10
dashboard link: https://syzkaller.appspot.com/bug?extid=6acf28c23c81badd89a7
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=16c8fc09e00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1364c07ee00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+6acf28c23c81badd89a7@syzkaller.appspotmail.com

general protection fault, probably for non-canonical address 0xdffffc0020000001: 0000 [#1] PREEMPT SMP KASAN
KASAN: probably user-memory-access in range [0x0000000100000008-0x000000010000000f]
CPU: 0 PID: 2873 Comm: kworker/0:35 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events console_callback
RIP: 0010:fbcon_cursor+0x114/0x660 drivers/video/fbdev/core/fbcon.c:1387
Code: 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 e6 04 00 00 4d 8b b4 24 a0 03 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <0f> b6 14 02 4c 89 f0 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 ba
RSP: 0018:ffffc90008f7faf0 EFLAGS: 00010203
RAX: dffffc0000000000 RBX: ffff8880a3860000 RCX: ffffffff83f31930
RDX: 0000000020000001 RSI: ffffffff83c5b4ec RDI: ffff88809c3573a0
RBP: ffffc90008f7fb30 R08: ffff88809ec02300 R09: ffffed10147ef884
R10: ffffed10147ef883 R11: ffff8880a3f7c41f R12: ffff88809c357000
R13: ffff8880a385c000 R14: 000000010000000c R15: ffff8880a385c468
FS: 0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000455300 CR3: 00000000a7dad000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
set_cursor drivers/tty/vt/vt.c:908 [inline]
set_cursor+0x1fb/0x280 drivers/tty/vt/vt.c:899
redraw_screen+0x4e1/0x7d0 drivers/tty/vt/vt.c:1013
complete_change_console+0x105/0x3a0 drivers/tty/vt/vt_ioctl.c:1264
change_console+0x19b/0x2c0 drivers/tty/vt/vt_ioctl.c:1389
console_callback+0x3a1/0x400 drivers/tty/vt/vt.c:2824
process_one_work+0xa05/0x17a0 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
Modules linked in:
---[ end trace 10588331f3234462 ]---
RIP: 0010:fbcon_cursor+0x114/0x660 drivers/video/fbdev/core/fbcon.c:1387
Code: 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 e6 04 00 00 4d 8b b4 24 a0 03 00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <0f> b6 14 02 4c 89 f0 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 ba
RSP: 0018:ffffc90008f7faf0 EFLAGS: 00010203
RAX: dffffc0000000000 RBX: ffff8880a3860000 RCX: ffffffff83f31930
RDX: 0000000020000001 RSI: ffffffff83c5b4ec RDI: ffff88809c3573a0
RBP: ffffc90008f7fb30 R08: ffff88809ec02300 R09: ffffed10147ef884
R10: ffffed10147ef883 R11: ffff8880a3f7c41f R12: ffff88809c357000
R13: ffff8880a385c000 R14: 000000010000000c R15: ffff8880a385c468
FS: 0000000000000000(0000) GS:ffff8880ae800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000455300 CR3: 00000000a7dad000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

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