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BUG: KASAN: vmalloc-out-of-bounds in sys_imageblit+0xff5/0x1e40
Write of size 8 at addr ffffc900049a9000 by task syz.0.756/8443

CPU: 0 UID: 0 PID: 8443 Comm: syz.0.756 Tainted: G             L      syzkaller #0 PREEMPT(full) 
Tainted: [L]=SOFTLOCKUP
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150
 print_report+0xba/0x230
 kasan_report+0x117/0x150
 sys_imageblit+0xff5/0x1e40
 drm_fbdev_shmem_defio_imageblit+0x2c/0x110
 soft_cursor+0xa0e/0xc40
 bit_cursor+0x167a/0x1eb0
 hide_cursor+0xd3/0x490
 do_blank_screen+0x298/0x790
 fbcon_fb_blanked+0x2ac/0x420
 do_fb_ioctl+0x50a/0x800
 __se_sys_ioctl+0xfc/0x170
 do_syscall_64+0x170/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8758d9e019
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 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 73 01 c3 48 c7 c1 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f8759ba4028 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f8759026090 RCX: 00007f8758d9e019
RDX: 0000000000000002 RSI: 0000000000004611 RDI: 0000000000000009
RBP: 00007f8758e3500c R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8759026128 R14: 00007f8759026090 R15: 00007ffeb7d2e118
 </TASK>

The buggy address belongs to a vmalloc virtual mapping
Memory state around the buggy address:
 ffffc900049a8f00: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc900049a8f80: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
>ffffc900049a9000: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
                   ^
 ffffc900049a9080: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
 ffffc900049a9100: f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8 f8
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