linux-kernel - Re: [PATCH] Revert "HID: uhid: use strlcpy() instead of strncpy()"

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Date:   Wed, 14 Nov 2018 17:09:42 -0600
From:   Kees Cook <keescook@...omium.org>
To:     David Herrmann <dh.herrmann@...il.com>
Cc:     Laura Abbott <labbott@...hat.com>,
        linux-input <linux-input@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Xiongfeng Wang <xiongfeng.wang@...aro.org>
Subject: Re: [PATCH] Revert "HID: uhid: use strlcpy() instead of strncpy()"

On Wed, Nov 14, 2018 at 9:40 AM, Laura Abbott <labbott@...hat.com> wrote:
> On 11/14/18 5:16 AM, David Herrmann wrote:
>>
>> This reverts commit 336fd4f5f25157e9e8bd50e898a1bbcd99eaea46.
>>
>> Please note that `strlcpy()` does *NOT* do what you think it does.
>> strlcpy() *ALWAYS* reads the full input string, regardless of the
>> 'length' parameter. That is, if the input is not zero-terminated,
>> strlcpy() will *READ* beyond input boundaries. It does this, because it
>> always returns the size it *would* copy if the target was big enough,
>> not the truncated size it actually copied.
>>
>> The original code was perfectly fine. The hid device is
>> zero-initialized and the strncpy() functions copied up to n-1
>> characters. The result is always zero-terminated this way.
>>
>> This is the third time someone tried to replace strncpy with strlcpy in
>> this function, and gets it wrong. I now added a comment that should at
>> least make people reconsider.
>>
>
> Can we switch to strscpy instead? This will quiet gcc and avoid the
> issues with strlcpy.

Yes please: it looks like these strings are expected to be NUL
terminated, so strscpy() without the "- 1" and min() logic would be
the correct solution here. If @hid is already zero, then this would
just be:

       strscpy(hid->name, ev->u.create2.name, sizeof(hid->name));
       strscpy(hid->phys, ev->u.create2.phys, sizeof(hid->phys));
       strscpy(hid->uniq, ev->u.create2.uniq, sizeof(hid->uniq));

If they are NOT NUL terminated, then keep using strncpy() but mark the
fields in the struct with the __nonstring attribute.

-Kees

>
>
>> Signed-off-by: David Herrmann <dh.herrmann@...il.com>
>> ---
>>   drivers/hid/uhid.c | 13 +++++++------
>>   1 file changed, 7 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/hid/uhid.c b/drivers/hid/uhid.c
>> index fefedc0b4dc6..0dfdd0ac7120 100644
>> --- a/drivers/hid/uhid.c
>> +++ b/drivers/hid/uhid.c
>> @@ -496,12 +496,13 @@ static int uhid_dev_create2(struct uhid_device
>> *uhid,
>>                 goto err_free;
>>         }
>>   -     len = min(sizeof(hid->name), sizeof(ev->u.create2.name));
>> -       strlcpy(hid->name, ev->u.create2.name, len);
>> -       len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys));
>> -       strlcpy(hid->phys, ev->u.create2.phys, len);
>> -       len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq));
>> -       strlcpy(hid->uniq, ev->u.create2.uniq, len);
>> +       /* @hid is zero-initialized, strncpy() is correct, strlcpy() not
>> */
>> +       len = min(sizeof(hid->name), sizeof(ev->u.create2.name)) - 1;
>> +       strncpy(hid->name, ev->u.create2.name, len);
>> +       len = min(sizeof(hid->phys), sizeof(ev->u.create2.phys)) - 1;
>> +       strncpy(hid->phys, ev->u.create2.phys, len);
>> +       len = min(sizeof(hid->uniq), sizeof(ev->u.create2.uniq)) - 1;
>> +       strncpy(hid->uniq, ev->u.create2.uniq, len);
>>         hid->ll_driver = &uhid_hid_driver;
>>         hid->bus = ev->u.create2.bus;
>>
>



-- 
Kees Cook