linux-kernel - Re: [PATCH] HID: uhid: refactor deprecated strncpy

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Message-Id: <98d981a1-4e4c-4173-b8eb-09b4245bca60@app.fastmail.com>
Date:   Fri, 15 Sep 2023 09:36:23 +0200
From:   "David Rheinsberg" <david@...dahead.eu>
To:     "Kees Cook" <keescook@...omium.org>,
        "Justin Stitt" <justinstitt@...gle.com>
Cc:     "Jiri Kosina" <jikos@...nel.org>,
        "Benjamin Tissoires" <benjamin.tissoires@...hat.com>,
        linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org,
        "David Herrmann" <dh.herrmann@...il.com>
Subject: Re: [PATCH] HID: uhid: refactor deprecated strncpy

Hi

On Fri, Sep 15, 2023, at 7:13 AM, Kees Cook wrote:
>> -	/* @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);
>
> ev->u.create2 is:
> struct uhid_create2_req {
>         __u8 name[128];
>         __u8 phys[64];
>         __u8 uniq[64];
> 	...
>
> hid is:
> struct hid_device { /* device report descriptor */
> 	...
>         char name[128]; /* Device name */
>         char phys[64]; /* Device physical location */
>         char uniq[64]; /* Device unique identifier (serial #) */
>
> So these "min" calls are redundant -- it wants to copy at most 1 less so
> it can be %NUL terminated. Which is what strscpy() already does. And
> source and dest are the same size, so we can't over-read source if it
> weren't terminated (since strscpy won't overread like strlcpy).

I *really* think we should keep the `min` calls. The compiler should already optimize them away, as both arguments are compile-time constants. There is no inherent reason why source and target are equal in size. Yes, it is unlikely to change, but I don't understand why we would want to implicitly rely on it, rather than make the compiler verify it for us. And `struct hid_device` is very much allowed to change in the future.

As an alternative, you can use BUILD_BUG_ON() and verify both are equal in length.

Thanks
David