Re: KASAN: vmalloc-out-of-bounds Write in bitfill_aligned

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    fb33c651 Linux 5.6-rc6
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17dacd55e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9f894bd92023de02
dashboard link: https://syzkaller.appspot.com/bug?extid=e5fd3e65515b48c02a30
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11b8ca75e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=114800e5e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+e5fd3e65515b48c02a30@xxxxxxxxxxxxxxxxxxxxxxxxx

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BUG: KASAN: vmalloc-out-of-bounds in bitfill_aligned drivers/video/fbdev/core/sysfillrect.c:54 [inline]
BUG: KASAN: vmalloc-out-of-bounds in bitfill_aligned+0x34b/0x410 drivers/video/fbdev/core/sysfillrect.c:25
Write of size 8 at addr ffffc90009621000 by task syz-executor767/9337

CPU: 3 PID: 9337 Comm: syz-executor767 Not tainted 5.6.0-rc6-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+0x5/0x315 mm/kasan/report.c:374
 __kasan_report.cold+0x1a/0x32 mm/kasan/report.c:506
 kasan_report+0xe/0x20 mm/kasan/common.c:641
 bitfill_aligned drivers/video/fbdev/core/sysfillrect.c:54 [inline]
 bitfill_aligned+0x34b/0x410 drivers/video/fbdev/core/sysfillrect.c:25
 sys_fillrect+0x415/0x7a0 drivers/video/fbdev/core/sysfillrect.c:291
 drm_fb_helper_sys_fillrect+0x1c/0x190 drivers/gpu/drm/drm_fb_helper.c:719
 bit_clear_margins+0x2d5/0x4a0 drivers/video/fbdev/core/bitblit.c:232
 fbcon_clear_margins+0x1de/0x240 drivers/video/fbdev/core/fbcon.c:1379
 fbcon_switch+0xd1b/0x1740 drivers/video/fbdev/core/fbcon.c:2361
 redraw_screen+0x2a8/0x770 drivers/tty/vt/vt.c:1008
 fbcon_modechanged+0x5bd/0x780 drivers/video/fbdev/core/fbcon.c:2998
 fbcon_update_vcs+0x3a/0x50 drivers/video/fbdev/core/fbcon.c:3045
 fb_set_var+0xad0/0xd40 drivers/video/fbdev/core/fbmem.c:1056
 do_fb_ioctl+0x390/0x7d0 drivers/video/fbdev/core/fbmem.c:1109
 fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1185
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x11a/0x180 fs/ioctl.c:763
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x6f/0xb0 fs/ioctl.c:770
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x433d29
Code: c4 18 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 0f 83 eb da fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff33d61508 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004002e0 RCX: 0000000000433d29
RDX: 00000000200001c0 RSI: 0000000000004601 RDI: 0000000000000003
RBP: 00000000006b2018 R08: 0000000000000000 R09: 00000000004002e0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401bc0
R13: 0000000000401c50 R14: 0000000000000000 R15: 0000000000000000


Memory state around the buggy address:
 ffffc90009620f00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffffc90009620f80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffc90009621000: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
                   ^
 ffffc90009621080: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
 ffffc90009621100: f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9 f9
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