linux-kernel - Re: [PATCH 1/2] misc: c2port: core: Make copying name from userspace more secure

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Message-ID: <9f10500a-cfd7-bcbe-7b8e-edd49ab4d43c@enneenne.com>
Date:   Mon, 2 Nov 2020 15:31:01 +0100
From:   Rodolfo Giometti <giometti@...eenne.com>
To:     Lee Jones <lee.jones@...aro.org>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>
Cc:     David Laight <David.Laight@...lab.com>,
        "arnd@...db.de" <arnd@...db.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Eurotech S.p.A" <info@...otech.it>,
        Geert Uytterhoeven <geert+renesas@...der.be>
Subject: Re: [PATCH 1/2] misc: c2port: core: Make copying name from userspace
 more secure

On 02/11/2020 14:47, Lee Jones wrote:
> On Mon, 02 Nov 2020, gregkh@...uxfoundation.org wrote:
> 
>> On Mon, Nov 02, 2020 at 12:43:01PM +0000, Lee Jones wrote:
>>> On Mon, 02 Nov 2020, gregkh@...uxfoundation.org wrote:
>>>
>>>> On Mon, Nov 02, 2020 at 11:49:03AM +0000, Lee Jones wrote:
>>>>> On Mon, 02 Nov 2020, David Laight wrote:
>>>>>
>>>>>> From: Lee Jones
>>>>>>> Sent: 02 November 2020 11:12
>>>>>>>
>>>>>>> strncpy() may not provide a NUL terminator, which means that a 1-byte
>>>>>>> leak would be possible *if* this was ever copied to userspace.  Ensure
>>>>>>> the buffer will always be NUL terminated by using the kernel's
>>>>>>> strscpy() which a) uses the destination (instead of the source) size
>>>>>>> as the bytes to copy and b) is *always* NUL terminated.
>>>>>>>
>>>>>>> Cc: Rodolfo Giometti <giometti@...eenne.com>
>>>>>>> Cc: "Eurotech S.p.A" <info@...otech.it>
>>>>>>> Reported-by: Geert Uytterhoeven <geert+renesas@...der.be>
>>>>>>> Acked-by: Arnd Bergmann <arnd@...db.de>
>>>>>>> Signed-off-by: Lee Jones <lee.jones@...aro.org>
>>>>>>> ---
>>>>>>>  drivers/misc/c2port/core.c | 2 +-
>>>>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>
>>>>>>> diff --git a/drivers/misc/c2port/core.c b/drivers/misc/c2port/core.c
>>>>>>> index 80d87e8a0bea9..b96444ec94c7e 100644
>>>>>>> --- a/drivers/misc/c2port/core.c
>>>>>>> +++ b/drivers/misc/c2port/core.c
>>>>>>> @@ -923,7 +923,7 @@ struct c2port_device *c2port_device_register(char *name,
>>>>>>>  	}
>>>>>>>  	dev_set_drvdata(c2dev->dev, c2dev);
>>>>>>>
>>>>>>> -	strncpy(c2dev->name, name, C2PORT_NAME_LEN - 1);
>>>>>>> +	strscpy(c2dev->name, name, sizeof(c2dev->name));
>>>>>>
>>>>>> strscpy() doesn't zero fill so if the memory isn't zeroed
>>>>>> and a 'blind' copy to user of the structure is done
>>>>>> then more data is leaked.
>>>>>>
>>>>>> strscpy() may be better, but rational isn't right.
>>>>>
>>>>> The original patch zeroed the data too, but I was asked to remove that
>>>>> part [0].  In your opinion, should it be reinstated?
>>>>>
>>>>> [0] https://lore.kernel.org/patchwork/patch/1272290/
>>>>
>>>> Just keep the kzalloc() part of the patch, this portion makes no sense
>>>> to me.
>>>
>>> Can do.
>>>
>>>> But if you REALLY want to get it correct, call dev_set_name()
>>>> instead please, as that is what it is there for.
>>>
>>> The line above isn't setting the 'struct device' name.  It looks as
>>> though 'struct c2port' has it's own member, also called 'name'.  As to
>>> how they differ, I'm not currently aware.  Nor do I wish to mess
>>> around with the semantics all that much.
>>>
>>> Going with suggestion #1.
>>
>> As the "device" already has a name, I suggest just getting rid of this
>> name field anyway, no need for duplicates.
> 
> That definitely goes against the point I made above:
> 
>  "Nor do I wish to mess around with the semantics all that much."
> 
> It looks as though the device name 'c2port%d' varies greatly to the
> requested name 'uc'.  I don't have enough knowledge of how user-
> space expects to use the provided sysfs entries to be able to
> competently merge/decide which of these should be kept and which to
> discard.
> 
> Hopefully one of the authors/maintainers are reading this and can come
> up with an acceptable solution.

User-space usage can change its behavior so, please, consider the best solution
from the kernel space point-of-view. :)

Ciao,

Rodolfo

-- 
GNU/Linux Solutions                  e-mail: giometti@...eenne.com
Linux Device Driver                          giometti@...ux.it
Embedded Systems                     phone:  +39 349 2432127
UNIX programming                     skype:  rodolfo.giometti