linux-kernel - Re: [PATCH] tty/vt: Fix possible deadlock in input_inject

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Message-ID: <ef5a5364-94c1-452c-9c97-81f66246088a@kylinos.cn>
Date: Wed, 1 Oct 2025 18:42:10 +0800
From: pengyu <pengyu@...inos.cn>
To: Dmitry Torokhov <dmitry.torokhov@...il.com>
Cc: gregkh@...uxfoundation.org, jirislaby@...nel.org, legion@...nel.org,
 mingo@...nel.org, myrrhperiwinkle@...labs.xyz, tglx@...utronix.de,
 changlianzhi@...ontech.com, linux-kernel@...r.kernel.org,
 linux-serial@...r.kernel.org,
 syzbot+79c403850e6816dc39cf@...kaller.appspotmail.com,
 jiangshanlai@...il.com, tj@...nel.org
Subject: Re: [PATCH] tty/vt: Fix possible deadlock in input_inject_event

在 2025/9/29 12:54, Dmitry Torokhov 写道:
> 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.
> 

Hi,

I noticed that besides do_wait, there are many places in the kernel 
where read_lock(tasklist_lock) is used without disabling interrupts. 
Addressing this solely through tasklist_lock may not fully resolve the 
issue.

This involves tasklist_lock, evdev_write, and various input device 
drivers. The only approach I can think of is to move functions like 
input_[inject]_event in the input drivers out of the interrupt context. 
This could affect many code paths, so I plan to start by modifying the 
keyboard code first.

-- 
Thanks,
Yu Peng