[syzbot] [kernel?] KMSAN: uninit-value in hrtimer_wakeup

[syzbot] [kernel?] KMSAN: uninit-value in hrtimer_wakeup

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit:    bf61759db409 Merge tag 'sched_ext-for-6.16-rc6-fixes' of g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1693938c580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5227c65742122bdd
dashboard link: https://syzkaller.appspot.com/bug?extid=e84a763987edd173d82f
compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/c9e297bb553f/disk-bf61759d.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/7f2d1bf53414/vmlinux-bf61759d.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9a4f67426eab/bzImage-bf61759d.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e84a763987edd173d82f@syzkaller.appspotmail.com

=====================================================
BUG: KMSAN: uninit-value in hrtimer_wakeup+0xcb/0xd0 kernel/time/hrtimer.c:1997
 hrtimer_wakeup+0xcb/0xd0 kernel/time/hrtimer.c:1997
 __run_hrtimer kernel/time/hrtimer.c:1761 [inline]
 __hrtimer_run_queues+0x556/0xd80 kernel/time/hrtimer.c:1825
 hrtimer_interrupt+0x456/0xb80 kernel/time/hrtimer.c:1887
 local_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1039 [inline]
 __sysvec_apic_timer_interrupt+0xa7/0x420 arch/x86/kernel/apic/apic.c:1056
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1050 [inline]
 sysvec_apic_timer_interrupt+0x7f/0x90 arch/x86/kernel/apic/apic.c:1050
 asm_sysvec_apic_timer_interrupt+0x1f/0x30 arch/x86/include/asm/idtentry.h:702
 memmove+0x28/0x1b0 arch/x86/lib/memmove_64.S:45
 ext4_xattr_set_entry+0x1928/0x3440 fs/ext4/xattr.c:1773
 ext4_xattr_block_set+0xc82/0x5010 fs/ext4/xattr.c:2020
 ext4_xattr_set_handle+0x2a44/0x2c00 fs/ext4/xattr.c:2447
 ext4_xattr_set+0x2ff/0x5b0 fs/ext4/xattr.c:2549
 ext4_xattr_trusted_set+0x51/0x70 fs/ext4/xattr_trusted.c:38
 __vfs_setxattr+0x742/0x850 fs/xattr.c:200
 __vfs_setxattr_noperm+0x224/0xad0 fs/xattr.c:234
 __vfs_setxattr_locked+0x448/0x490 fs/xattr.c:295
 vfs_setxattr+0x27f/0x640 fs/xattr.c:321
 do_setxattr fs/xattr.c:636 [inline]
 filename_setxattr+0x3a4/0xcc0 fs/xattr.c:665
 path_setxattrat+0x734/0x820 fs/xattr.c:713
 __do_sys_lsetxattr fs/xattr.c:754 [inline]
 __se_sys_lsetxattr fs/xattr.c:750 [inline]
 __x64_sys_lsetxattr+0x103/0x1c0 fs/xattr.c:750
 x64_sys_call+0x2464/0x3db0 arch/x86/include/generated/asm/syscalls_64.h:190
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xd9/0x210 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Local variable t created at:
 hrtimer_nanosleep+0x48/0x480 kernel/time/hrtimer.c:2142
 common_nsleep+0x118/0x160 kernel/time/posix-timers.c:1353

CPU: 0 UID: 0 PID: 17784 Comm: syz.6.3366 Not tainted 6.16.0-rc6-syzkaller-00279-gbf61759db409 #0 PREEMPT(none) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
=====================================================


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Re: [syzbot] [kernel?] KMSAN: uninit-value in hrtimer_wakeup

[Thomas Gleixner]

On Thu, Jul 24 2025 at 03:44, syzbot wrote:
> HEAD commit:    bf61759db409 Merge tag 'sched_ext-for-6.16-rc6-fixes' of g..
> git tree:       upstream
> console output: https://syzkaller.appspot.com/x/log.txt?x=1693938c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=5227c65742122bdd
> dashboard link: https://syzkaller.appspot.com/bug?extid=e84a763987edd173d82f
> compiler:       Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c9e297bb553f/disk-bf61759d.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/7f2d1bf53414/vmlinux-bf61759d.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/9a4f67426eab/bzImage-bf61759d.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+e84a763987edd173d82f@syzkaller.appspotmail.com
>
> =====================================================
> BUG: KMSAN: uninit-value in hrtimer_wakeup+0xcb/0xd0 kernel/time/hrtimer.c:1997
>  hrtimer_wakeup+0xcb/0xd0 kernel/time/hrtimer.c:1997

...

> Local variable t created at:
>  hrtimer_nanosleep+0x48/0x480 kernel/time/hrtimer.c:2142
>  common_nsleep+0x118/0x160 kernel/time/posix-timers.c:1353

This makes absolutely no sense. hrtimer_nanosleep() initializes the
local variable

      struct hrtimer_sleeper t;

in two steps:

      hrtimer_setup_sleeper_on_stack(&t, clockid, mode);
      hrtimer_set_expires_range_ns(&t.timer, rqtp, current->timer_slack_ns);
      
and the complaint in hrtimer_wakeup() is:

 1989 static enum hrtimer_restart hrtimer_wakeup(struct hrtimer *timer)
 1990 {
 1991         struct hrtimer_sleeper *t =
 1992                 container_of(timer, struct hrtimer_sleeper, timer);
 1993         struct task_struct *task = t->task;
 1994 
 1995         t->task = NULL;
 1996         if (task)
 1997                 wake_up_process(task);    <---- here
 1998 
 1999         return HRTIMER_NORESTART;
 2000 }

t->task was initialized:

 2027 static void __hrtimer_setup_sleeper(struct hrtimer_sleeper *sl,
 2028                                     clockid_t clock_id, enum hrtimer_mode mode)
 2029 {
 ...
 2054         __hrtimer_setup(&sl->timer, hrtimer_wakeup, clock_id, mode);
 2055         sl->task = current;              <---- here
 2056 }

This code hasn't changed in a very long time. Looks like KMSAN is confused...