[moderation] [kernel?] KCSAN: data-race in ktime_get_coarse_real_ts64_mg / timekeeping_update_from_shadow

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: b4e07588e743 tracing: tell git to ignore the generated 'un..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15099f16580000
kernel config: https://syzkaller.appspot.com/x/.config?x=ff1bc04bab908995
dashboard link: https://syzkaller.appspot.com/bug?extid=a1d9ae81eab6ce5f53dd
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
CC: [jst...@google.com linux-...@vger.kernel.org sb...@kernel.org tg...@kernel.org]

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/71c6f85eeaf3/disk-b4e07588.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/9da381bc6ca7/vmlinux-b4e07588.xz
kernel image: https://storage.googleapis.com/syzbot-assets/2bd28557b04a/bzImage-b4e07588.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+a1d9ae...@syzkaller.appspotmail.com

==================================================================
BUG: KCSAN: data-race in ktime_get_coarse_real_ts64_mg / timekeeping_update_from_shadow

write to 0xffffffff895d6488 of 304 bytes by interrupt on cpu 1:
timekeeping_update_from_shadow+0x40d/0x440 kernel/time/timekeeping.c:829
__timekeeping_advance+0xa5d/0xc10 kernel/time/timekeeping.c:2532
timekeeping_advance kernel/time/timekeeping.c:2540 [inline]
update_wall_time+0x21/0x50 kernel/time/timekeeping.c:2550
tick_do_update_jiffies64+0x169/0x1c0 kernel/time/tick-sched.c:149
tick_sched_do_timer kernel/time/tick-sched.c:253 [inline]
tick_nohz_handler+0x8d/0x3d0 kernel/time/tick-sched.c:312
__run_hrtimer kernel/time/hrtimer.c:1930 [inline]
__hrtimer_run_queues+0x276/0x4f0 kernel/time/hrtimer.c:1994
hrtimer_interrupt+0x261/0x850 kernel/time/hrtimer.c:2113
local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
__sysvec_apic_timer_interrupt+0x5f/0x1c0 arch/x86/kernel/apic/apic.c:1067
instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1061 [inline]
sysvec_apic_timer_interrupt+0x6f/0x80 arch/x86/kernel/apic/apic.c:1061
asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
arch_atomic64_read arch/x86/include/asm/atomic64_64.h:15 [inline]
raw_atomic64_read include/linux/atomic/atomic-arch-fallback.h:2583 [inline]
raw_atomic_long_read include/linux/atomic/atomic-long.h:38 [inline]
atomic_long_read include/linux/atomic/atomic-instrumented.h:3189 [inline]
find_watchpoint kernel/kcsan/core.c:131 [inline]
check_access kernel/kcsan/core.c:737 [inline]
__tsan_read4+0x2c/0x190 kernel/kcsan/core.c:1024
ns_of_pid include/linux/pid.h:153 [inline]
task_active_pid_ns kernel/pid.c:577 [inline]
pid_vnr+0x3b/0x100 kernel/pid.c:556
scm_set_cred include/net/scm.h:73 [inline]
unix_skb_to_scm net/unix/af_unix.c:2002 [inline]
__unix_dgram_recvmsg+0x5be/0x870 net/unix/af_unix.c:2628
unix_dgram_recvmsg+0x7e/0x90 net/unix/af_unix.c:2676
sock_recvmsg_nosec+0xc2/0xf0 net/socket.c:1137
____sys_recvmsg+0x26f/0x280 net/socket.c:2916
___sys_recvmsg+0x11f/0x3b0 net/socket.c:2960
do_recvmmsg+0x1ef/0x560 net/socket.c:3055
__sys_recvmmsg net/socket.c:3129 [inline]
__do_sys_recvmmsg net/socket.c:3152 [inline]
__se_sys_recvmmsg net/socket.c:3145 [inline]
__x64_sys_recvmmsg+0xe5/0x170 net/socket.c:3145
x64_sys_call+0x80f/0x3020 arch/x86/include/generated/asm/syscalls_64.h:300
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0x12c/0x3b0 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f

read to 0xffffffff895d64f8 of 4 bytes by task 12 on cpu 0:
tk_xtime_coarse kernel/time/timekeeping.c:212 [inline]
ktime_get_coarse_real_ts64_mg+0x89/0x1a0 kernel/time/timekeeping.c:2606
current_time+0x41/0x240 fs/inode.c:2370
need_cmtime_update fs/inode.c:2392 [inline]
file_update_time_flags+0x83/0x2e0 fs/inode.c:2413
file_update_time+0x17/0x20 fs/inode.c:2443
shmem_file_write_iter+0x9c/0xf0 mm/shmem.c:3475
lo_rw_aio+0x67d/0x730 drivers/block/loop.c:-1
do_req_filebacked drivers/block/loop.c:-1 [inline]
loop_handle_cmd drivers/block/loop.c:1925 [inline]
loop_process_work+0x56c/0xac0 drivers/block/loop.c:1960
loop_workfn+0x31/0x40 drivers/block/loop.c:1984
process_one_work kernel/workqueue.c:3302 [inline]
process_scheduled_works+0x4f0/0x9c0 kernel/workqueue.c:3385
worker_thread+0x58a/0x780 kernel/workqueue.c:3466
kthread+0x22a/0x280 kernel/kthread.c:436
ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

value changed: 0x31af5fed -> 0x3247f66d

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 UID: 0 PID: 12 Comm: kworker/u8:0 Not tainted syzkaller #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/18/2026
Workqueue: loop1 loop_workfn
==================================================================


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