Re: [RFC] kretprobe: Prevent triggering kretprobe from within kprobe_flush_task

[Naveen N. Rao]

Hi Jiri,

Jiri Olsa wrote:
> hi,
> Ziqian reported lockup when adding retprobe on _raw_spin_lock_irqsave.
> My test was also able to trigger lockdep output:
>
>  ============================================
>  WARNING: possible recursive locking detected
>  5.6.0-rc6+ #6 Not tainted
>  --------------------------------------------
>  sched-messaging/2767 is trying to acquire lock:
>  ffffffff9a492798 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_hash_lock+0x52/0xa0
>
>  but task is already holding lock:
>  ffffffff9a491a18 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_trampoline+0x0/0x50
>
>  other info that might help us debug this:
>   Possible unsafe locking scenario:
>
>         CPU0
>         ----
>    lock(&(kretprobe_table_locks[i].lock));
>    lock(&(kretprobe_table_locks[i].lock));
>
>   *** DEADLOCK ***
>
>   May be due to missing lock nesting notation
>
>  1 lock held by sched-messaging/2767:
>   #0: ffffffff9a491a18 (&(kretprobe_table_locks[i].lock)){-.-.}, at: kretprobe_trampoline+0x0/0x50
>
>  stack backtrace:
>  CPU: 3 PID: 2767 Comm: sched-messaging Not tainted 5.6.0-rc6+ #6
>  Call Trace:
>   dump_stack+0x96/0xe0
>   __lock_acquire.cold.57+0x173/0x2b7
>   ? native_queued_spin_lock_slowpath+0x42b/0x9e0
>   ? lockdep_hardirqs_on+0x590/0x590
>   ? __lock_acquire+0xf63/0x4030
>   lock_acquire+0x15a/0x3d0
>   ? kretprobe_hash_lock+0x52/0xa0
>   _raw_spin_lock_irqsave+0x36/0x70
>   ? kretprobe_hash_lock+0x52/0xa0
>   kretprobe_hash_lock+0x52/0xa0
>   trampoline_handler+0xf8/0x940
>   ? kprobe_fault_handler+0x380/0x380
>   ? find_held_lock+0x3a/0x1c0
>   kretprobe_trampoline+0x25/0x50
>   ? lock_acquired+0x392/0xbc0
>   ? _raw_spin_lock_irqsave+0x50/0x70
>   ? __get_valid_kprobe+0x1f0/0x1f0
>   ? _raw_spin_unlock_irqrestore+0x3b/0x40
>   ? finish_task_switch+0x4b9/0x6d0
>   ? __switch_to_asm+0x34/0x70
>   ? __switch_to_asm+0x40/0x70
>
> The code within the kretprobe handler checks for probe reentrancy,
> so we won't trigger any _raw_spin_lock_irqsave probe in there.
>
> The problem is in outside kprobe_flush_task, where we call:
>
>   kprobe_flush_task
>     kretprobe_table_lock
>       raw_spin_lock_irqsave
>         _raw_spin_lock_irqsave
>
> where _raw_spin_lock_irqsave triggers the kretprobe and installs
> kretprobe_trampoline handler on _raw_spin_lock_irqsave return.
>
> The kretprobe_trampoline handler is then executed with already
> locked kretprobe_table_locks, and first thing it does is to
> lock kretprobe_table_locks ;-) the whole lockup path like:
>
>   kprobe_flush_task
>     kretprobe_table_lock
>       raw_spin_lock_irqsave
>         _raw_spin_lock_irqsave ---> probe triggered, kretprobe_trampoline installed
>
>         ---> kretprobe_table_locks locked
>
>         kretprobe_trampoline
>           trampoline_handler
>             kretprobe_hash_lock(current, &head, &flags);  <--- deadlock
>
> The change below sets current_kprobe in kprobe_flush_task, so the probe
> recursion protection check is hit and the probe is never set. It seems
> to fix the deadlock.

Good analysis!

> I'm not sure this is the best fix, any ideas are welcome ;-)

I think this is a good way to address this issue.

> thanks,
> jirka
>
>
> ---
>  kernel/kprobes.c | 6 ++++++
>  1 file changed, 6 insertions(+)
>
> diff --git a/kernel/kprobes.c b/kernel/kprobes.c
> index 2625c241ac00..b13247cae752 100644
> --- a/kernel/kprobes.c
> +++ b/kernel/kprobes.c
> @@ -1236,6 +1236,10 @@ __releases(hlist_lock)
>  }
>  NOKPROBE_SYMBOL(kretprobe_table_unlock);
>  
> +static struct kprobe kretprobe_dummy = {
> +        .addr = (void *)kretprobe_trampoline,
> +};
> +

Perhaps a more meaningful name, say, kprobe_flush_task_protect ?

>  /*
>   * This function is called from finish_task_switch when task tk becomes dead,
>   * so that we can recycle any function-return probe instances associated
> @@ -1256,12 +1260,14 @@ void kprobe_flush_task(struct task_struct *tk)
>  	INIT_HLIST_HEAD(&empty_rp);
>  	hash = hash_ptr(tk, KPROBE_HASH_BITS);
>  	head = &kretprobe_inst_table[hash];
> +	__this_cpu_write(current_kprobe, &kretprobe_dummy);
>  	kretprobe_table_lock(hash, &flags);
>  	hlist_for_each_entry_safe(ri, tmp, head, hlist) {
>  		if (ri->task == tk)
>  			recycle_rp_inst(ri, &empty_rp);
>  	}
>  	kretprobe_table_unlock(hash, &flags);
> +	__this_cpu_write(current_kprobe, NULL);

I would move this to the end of the function to also cover the below 
code. kprobe_flush_task() is marked NOKPROBE, so it is good to prevent 
probe handling in the below code too.

>  	hlist_for_each_entry_safe(ri, tmp, &empty_rp, hlist) {
>  		hlist_del(&ri->hlist);
>  		kfree(ri);


- Naveen