[v6.6] WARNING in vkms_get_vblank_timestamp

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: c2603c511feb Linux 6.6.93
git tree: linux-6.6.y
console output: https://syzkaller.appspot.com/x/log.txt?x=13fc6370580000
kernel config: https://syzkaller.appspot.com/x/.config?x=486bade17c8c30b9
dashboard link: https://syzkaller.appspot.com/bug?extid=cf3d745dbef25999ba26
compiler: Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/8754f950a6e7/disk-c2603c51.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3b19332dbf63/vmlinux-c2603c51.xz
kernel image: https://storage.googleapis.com/syzbot-assets/cb245e836038/bzImage-c2603c51.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+cf3d74...@syzkaller.appspotmail.com

------------[ cut here ]------------
WARNING: CPU: 0 PID: 16684 at drivers/gpu/drm/vkms/vkms_crtc.c:103 vkms_get_vblank_timestamp+0x188/0x1d0 drivers/gpu/drm/vkms/vkms_crtc.c:103
Modules linked in:
CPU: 0 PID: 16684 Comm: syz.1.3359 Not tainted 6.6.93-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
RIP: 0010:vkms_get_vblank_timestamp+0x188/0x1d0 drivers/gpu/drm/vkms/vkms_crtc.c:103
Code: 89 d8 48 c1 e8 03 42 80 3c 28 00 74 08 48 89 df e8 1d fc ac fc 4c 89 33 b0 01 5b 41 5c 41 5d 41 5e 41 5f 5d c3 e8 78 56 54 fc <0f> 0b eb ea 44 89 f9 80 e1 07 80 c1 03 38 c1 0f 8c ba fe ff ff 4c
RSP: 0018:ffffc9000fa572c8 EFLAGS: 00010287
RAX: ffffffff85314068 RBX: ffffc9000fa57440 RCX: 0000000000080000
RDX: ffffc90014962000 RSI: 000000000002d838 RDI: 000000000002d839
RBP: 1ffff92001f4ae88 R08: ffffc90003c0a000 R09: 0000000000000000
R10: ffffc90003c0a000 R11: 0000000000000000 R12: 0000012bfbcb1980
R13: dffffc0000000000 R14: ffff88801fb90000 R15: 0000012bfbcb1980
FS: 00007ff18c9306c0(0000) GS:ffff8880b8e00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000001b2cd10ff8 CR3: 000000007f884000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
drm_crtc_get_last_vbltimestamp drivers/gpu/drm/drm_vblank.c:877 [inline]
drm_crtc_next_vblank_start+0x21f/0x450 drivers/gpu/drm/drm_vblank.c:1012
set_fence_deadline drivers/gpu/drm/drm_atomic_helper.c:1583 [inline]
drm_atomic_helper_wait_for_fences+0x263/0x8b0 drivers/gpu/drm/drm_atomic_helper.c:1630
drm_atomic_helper_commit+0x5b5/0xae0 drivers/gpu/drm/drm_atomic_helper.c:2059
drm_atomic_commit+0x24c/0x2a0 drivers/gpu/drm/drm_atomic.c:1513
drm_client_modeset_commit_atomic+0x621/0x760 drivers/gpu/drm/drm_client_modeset.c:1057
drm_client_modeset_commit_locked+0xcb/0x4c0 drivers/gpu/drm/drm_client_modeset.c:1160
pan_display_atomic drivers/gpu/drm/drm_fb_helper.c:1386 [inline]
drm_fb_helper_pan_display+0x3e7/0xbd0 drivers/gpu/drm/drm_fb_helper.c:1446
fb_pan_display+0x39f/0x690 drivers/video/fbdev/core/fbmem.c:731
bit_update_start+0x4d/0x1e0 drivers/video/fbdev/core/bitblit.c:389
fbcon_switch+0x1425/0x1ec0 drivers/video/fbdev/core/fbcon.c:2172
redraw_screen+0x561/0xe80 drivers/tty/vt/vt.c:969
do_fb_ioctl+0x6f0/0x740 drivers/video/fbdev/core/fb_chrdev.c:92
vfs_ioctl fs/ioctl.c:51 [inline]
__do_sys_ioctl fs/ioctl.c:871 [inline]
__se_sys_ioctl+0xfd/0x170 fs/ioctl.c:857
do_syscall_x64 arch/x86/entry/common.c:51 [inline]
do_syscall_64+0x55/0xb0 arch/x86/entry/common.c:81
entry_SYSCALL_64_after_hwframe+0x68/0xd2
RIP: 0033:0x7ff18bb8e929
Code: ff ff 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 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ff18c930038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007ff18bdb6080 RCX: 00007ff18bb8e929
RDX: 0000200000000040 RSI: 0000000000004601 RDI: 000000000000000b
RBP: 00007ff18bc10b39 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007ff18bdb6080 R15: 00007ffe13d27f68
</TASK>


---
This report is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzk...@googlegroups.com.

syzbot will keep track of this issue. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.

If the report is already addressed, let syzbot know by replying with:
#syz fix: exact-commit-title

If you want to overwrite report's subsystems, reply with:
#syz set subsystems: new-subsystem
(See the list of subsystem names on the web dashboard)

If the report is a duplicate of another one, reply with:
#syz dup: exact-subject-of-another-report

If you want to undo deduplication, reply with:
#syz undup