Re: [syzbot] [mm?] WARNING: bad unlock balance in copy_process

[Vlastimil Babka <vbabka@suse.cz>]

On 9/17/25 22:40, syzbot wrote:
> syzbot has found a reproducer for the following issue on:
> 
> HEAD commit:    6edf2885ebeb Merge branch 'for-next/core' into for-kernelci
> git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-kernelci
> console output: https://syzkaller.appspot.com/x/log.txt?x=16d14c7c580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=b8b6789b42526d72
> dashboard link: https://syzkaller.appspot.com/bug?extid=80cb3cc5c14fad191a10
> compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> userspace arch: arm64
> syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=179d9f62580000
> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=11d14c7c580000
> 
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/c72239eb6d76/disk-6edf2885.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/b67e9820b2be/vmlinux-6edf2885.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/0c4ab7e562f6/Image-6edf2885.gz.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+80cb3cc5c14fad191a10@syzkaller.appspotmail.com
> 
> =====================================
> WARNING: bad unlock balance detected!
> syzkaller #0 Not tainted
> -------------------------------------
> syz.1.48/6865 is trying to release lock (&sighand->siglock) at:
> [<ffff8000803b8634>] spin_unlock include/linux/spinlock.h:391 [inline]
> [<ffff8000803b8634>] copy_process+0x22d4/0x31ec kernel/fork.c:2432

bad_fork_core_free:
        sched_core_free(p);
        spin_unlock(&current->sighand->siglock); <- here

Sebastian, I think it's your 7c4f75a21f63 ("futex: Allow automatic
allocation of process wide futex hash") adding a "goto bad_fork_core_free;"
from a place that doesn't yet have current->sighand->siglock locked?

> but there are no more locks to release!
> 
> other info that might help us debug this:
> 1 lock held by syz.1.48/6865:
>  #0: ffff80008fa00450 (cgroup_threadgroup_rwsem){++++}-{0:0}, at: copy_process+0x2228/0x31ec kernel/fork.c:2274
> 
> stack backtrace:
> CPU: 0 UID: 0 PID: 6865 Comm: syz.1.48 Not tainted syzkaller #0 PREEMPT 
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 06/30/2025
> Call trace:
>  show_stack+0x2c/0x3c arch/arm64/kernel/stacktrace.c:499 (C)
>  __dump_stack+0x30/0x40 lib/dump_stack.c:94
>  dump_stack_lvl+0xd8/0x12c lib/dump_stack.c:120
>  dump_stack+0x1c/0x28 lib/dump_stack.c:129
>  print_unlock_imbalance_bug+0xf4/0xfc kernel/locking/lockdep.c:5298
>  __lock_release kernel/locking/lockdep.c:-1 [inline]
>  lock_release+0x244/0x39c kernel/locking/lockdep.c:5889
>  __raw_spin_unlock include/linux/spinlock_api_smp.h:141 [inline]
>  _raw_spin_unlock+0x24/0x78 kernel/locking/spinlock.c:186
>  spin_unlock include/linux/spinlock.h:391 [inline]
>  copy_process+0x22d4/0x31ec kernel/fork.c:2432
>  kernel_clone+0x1d8/0x84c kernel/fork.c:2605
>  __do_sys_clone kernel/fork.c:2748 [inline]
>  __se_sys_clone kernel/fork.c:2716 [inline]
>  __arm64_sys_clone+0x144/0x1a0 kernel/fork.c:2716
>  __invoke_syscall arch/arm64/kernel/syscall.c:35 [inline]
>  invoke_syscall+0x98/0x2b8 arch/arm64/kernel/syscall.c:49
>  el0_svc_common+0x130/0x23c arch/arm64/kernel/syscall.c:132
>  do_el0_svc+0x48/0x58 arch/arm64/kernel/syscall.c:151
>  el0_svc+0x5c/0x254 arch/arm64/kernel/entry-common.c:744
>  el0t_64_sync_handler+0x84/0x12c arch/arm64/kernel/entry-common.c:763
>  el0t_64_sync+0x198/0x19c arch/arm64/kernel/entry.S:596
> 
> 
> ---
> If you want syzbot to run the reproducer, reply with:
> #syz test: git://repo/address.git branch-or-commit-hash
> If you attach or paste a git patch, syzbot will apply it before testing.