Re: [PATCH] tty/vt: Fix possible deadlock in input_inject_event

[Dmitry Torokhov]

Hi,

On Sun, Sep 28, 2025 at 09:08:19PM +0800, pengyu wrote:
> syzkaller testing revealed a potential deadlock involving keyboard 
> handling:
> 
> CPU0                       CPU1                      CPU2
> ----                       ----                      ----
> read_lock(tasklist_lock);  evdev_write
>                           input_inject_event     write_lock(tasklist_lock);
>                          lock(&dev->event_lock);
>                         read_lock(tasklist_lock);
> <Interrupt>
> kbd_bh() / kd_sound_helper()
> input_inject_event
> lock(&dev->event_lock); // Deadlock risk
> 
> The deadlock occurs because:
> 1. Both kbd_bh and kd_sound_helper run in interrupt context
> 2. tasklist_lock is interrupt-unsafe
> 3. When evdev_write holds both dev->event_lock and tasklist_lock,
>    interrupt context attempts to acquire dev->event_lock create deadlock 
>    risks
> 
> Convert both kbd_bh and kd_sound_helper to use workqueues. This moves
> input_inject_event execution to process context, where it's safe to 
> acquire locks that may be held by code using interrupt-unsafe locks.

So if we ignore the input code and instead look at the send_sigio()
(which input core ends up calling) and do_wait() we see that
send_sigio() disables interrupts and takes the owner's spinlock
before taking the tasklist_lock, while do_wait() takes the tasklist_lock
first, without disabling interrupts. This is root of the issue as far as
I can tell and no amount of changes to the keyboard handler (which is
just happens to be in the middle) will not solve for all potential cases
and code paths. 

I believe either do_exit() or send_sigio() have to be changed to fix
this properly. 

Thanks.

-- 
Dmitry