linux-kernel - Re: [PATCH v2] comedi: multiq3: sanitize config options in multiq3

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Message-ID: <6b8713a1-4a34-4b0f-80d4-3a6f630c5105@mev.co.uk>
Date: Thu, 23 Oct 2025 14:04:27 +0100
From: Ian Abbott <abbotti@....co.uk>
To: Nikita Zhandarovich <n.zhandarovich@...tech.ru>,
 H Hartley Sweeten <hsweeten@...ionengravers.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org, lvc-project@...uxtesting.org,
 syzbot+7811bb68a317954a0347@...kaller.appspotmail.com
Subject: Re: [PATCH v2] comedi: multiq3: sanitize config options in
 multiq3_attach()

On 23/10/2025 13:53, Nikita Zhandarovich wrote:
> Syzbot identified an issue [1] in multiq3_attach() that induces a
> task timeout due to open() or COMEDI_DEVCONFIG ioctl operations,
> specifically, in the case of multiq3 driver.
> 
> This problem arose when syzkaller managed to craft weird configuration
> options used to specify the number of channels in encoder subdevice.
> If a particularly great number is passed to s->n_chan in
> multiq3_attach() via it->options[2], then multiple calls to
> multiq3_encoder_reset() at the end of driver-specific attach() method
> will be running for minutes, thus blocking tasks and affected devices
> as well.
> 
> While this issue is most likely not too dangerous for real-life
> devices, it still makes sense to sanitize configuration inputs. Enable
> a semi-arbitrary limit on the number of encoder chips to stop this
> behaviour from manifesting.

See comment below about "semi-arbitrary".

> 
> [1] Syzbot crash:
> INFO: task syz.2.19:6067 blocked for more than 143 seconds.
> ...
> Call Trace:
>   <TASK>
>   context_switch kernel/sched/core.c:5254 [inline]
>   __schedule+0x17c4/0x4d60 kernel/sched/core.c:6862
>   __schedule_loop kernel/sched/core.c:6944 [inline]
>   schedule+0x165/0x360 kernel/sched/core.c:6959
>   schedule_preempt_disabled+0x13/0x30 kernel/sched/core.c:7016
>   __mutex_lock_common kernel/locking/mutex.c:676 [inline]
>   __mutex_lock+0x7e6/0x1350 kernel/locking/mutex.c:760
>   comedi_open+0xc0/0x590 drivers/comedi/comedi_fops.c:2868
>   chrdev_open+0x4cc/0x5e0 fs/char_dev.c:414
>   do_dentry_open+0x953/0x13f0 fs/open.c:965
>   vfs_open+0x3b/0x340 fs/open.c:1097
> ...
> 
> Reported-by: syzbot+7811bb68a317954a0347@...kaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7811bb68a317954a0347
> Fixes: 77e01cdbad51 ("Staging: comedi: add multiq3 driver")
> Cc: stable@...r.kernel.org
> Signed-off-by: Nikita Zhandarovich <n.zhandarovich@...tech.ru>
> ---
> v1 -> v2: Lower limit to 8 channels instead of 16 per Ian Abbott's
> <abbotti@....co.uk> suggestion.
> 
>   drivers/comedi/drivers/multiq3.c | 9 +++++++++
>   1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/comedi/drivers/multiq3.c b/drivers/comedi/drivers/multiq3.c
> index 07ff5383da99..08b812332a1a 100644
> --- a/drivers/comedi/drivers/multiq3.c
> +++ b/drivers/comedi/drivers/multiq3.c
> @@ -67,6 +67,11 @@
>   #define MULTIQ3_TRSFRCNTR_OL		0x10	/* xfer CNTR to OL (x and y) */
>   #define MULTIQ3_EFLAG_RESET		0x06	/* reset E bit of flag reg */
>   
> +/*
> + * Semi-arbitrary limit on the number of optional encoder chips
> + */
> +#define MULTIQ3_MAX_ENC_CHIPS		8
> +

That may be slightly confusing because there is a maximum of 4 encoder 
chips, each of which has 2 channels.  Renaming the macro to 
`MULTIQ3_MAX_ENC_CHANS` and adjusting the comment accordingly would be 
better.  Also, the limit isn't really "semi-arbitrary" so I suggest 
removing that part.

>   static void multiq3_set_ctrl(struct comedi_device *dev, unsigned int bits)
>   {
>   	/*
> @@ -312,6 +317,10 @@ static int multiq3_attach(struct comedi_device *dev,
>   	s->insn_read	= multiq3_encoder_insn_read;
>   	s->insn_config	= multiq3_encoder_insn_config;
>   
> +	/* sanity check for number of optional encoders */
> +	if (s->n_chan > MULTIQ3_MAX_ENC_CHIPS)
> +		s->n_chan = MULTIQ3_MAX_ENC_CHIPS;
> +
>   	for (i = 0; i < s->n_chan; i++)
>   		multiq3_encoder_reset(dev, i);
>   

-- 
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