linux-kernel - Re: [PATCH] comedi: zero-init data in do_insn

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Message-ID: <a7ab6827-771b-48b4-98b2-4789d78e19e0@mev.co.uk>
Date: Fri, 25 Jul 2025 11:04:22 +0100
From: Ian Abbott <abbotti@....co.uk>
To: "Lecomte, Arnaud" <contact@...aud-lcm.com>, hsweeten@...ionengravers.com,
 gregkh@...uxfoundation.org,
 syzbot+a5e45f768aab5892da5d@...kaller.appspotmail.com
Cc: jannh@...gle.com, linux-kernel@...r.kernel.org,
 syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH] comedi: zero-init data in do_insn_ioctl

On 25/07/2025 10:43, Lecomte, Arnaud wrote:
> Hi Ian,
> Thanks for the feedback, I can send the revision of the patch by the end 
> of the day if it is fine for you.

Actually, I'm just building a (hopefully) fixed version at the moment. 
I'll add you to the Cc list for it.

> On 25/07/2025 10:41, Ian Abbott wrote:
>> On 24/07/2025 22:04, Arnaud Lecomte wrote:
>>> KMSAN reported a kernel-infoleak when copying instruction data back to
>>> userspace in do_insnlist_ioctl(). The issue occurs because allocated
>>> memory buffers weren't properly initialized (not
>>> zero initialized)  before being copied to
>>> userspace, potentially leaking kernel memory.
>>>
>>> Reported-by: syzbot+a5e45f768aab5892da5d@...kaller.appspotmail.com
>>> Closes: https://syzkaller.appspot.com/bug?extid=a5e45f768aab5892da5d
>>> Tested-by: syzbot+a5e45f768aab5892da5d@...kaller.appspotmail.com
>>> Signed-off-by: Arnaud Lecomte <contact@...aud-lcm.com>
>>> ---
>>>   drivers/comedi/comedi_fops.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/drivers/comedi/comedi_fops.c b/drivers/comedi/comedi_fops.c
>>> index c83fd14dd7ad..15fee829d14c 100644
>>> --- a/drivers/comedi/comedi_fops.c
>>> +++ b/drivers/comedi/comedi_fops.c
>>> @@ -1636,7 +1636,7 @@ static int do_insn_ioctl(struct comedi_device 
>>> *dev,
>>>           n_data = MAX_SAMPLES;
>>>       }
>>>   -    data = kmalloc_array(n_data, sizeof(unsigned int), GFP_KERNEL);
>>> +    data = kcalloc(n_data, sizeof(unsigned int), GFP_KERNEL);
>>>       if (!data) {
>>>           ret = -ENOMEM;
>>>           goto error;
>>
>> I thought my commit 46d8c744136c ("comedi: Fix initialization of data 
>> for instructions that write to subdevice" would have fixed this as 
>> long as all the driver code was playing nicely, but it seems I was 
>> mistaken.
>>
>> I don't think it is necessary to use kcalloc().  The buffer already 
>> gets initialized (partly by `copy_from_user()`) when `insn->insn & 
>> INSN_MASK_WRITE` is non-zero, so it just needs to be initialized when 
>> `insn->insn & INSN_MASK_WRITE` is zero too.
>>
>> There is nearly identical code in `do_insnlist_ioctl()` that needs 
>> fixing, so it would be better to fix both at the same time.
>>


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