linux-kernel - Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in copy_to

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Message-ID: <CAHP4M8Vs8a8u98enuHXaBcC7D4fCZzCOtEq06VnvuPUqhqPK=Q@mail.gmail.com>
Date:   Sun, 7 Nov 2021 02:00:16 +0530
From:   Ajay Garg <ajaygargnsit@...il.com>
To:     Pavel Skripkin <paskripkin@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>, jirislaby@...nel.org,
        kernel@...il.dk, David Laight <David.Laight@...lab.com>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in copy_to_user

> > 2.
> > == Calculate the actual length of kbs, add 1, and then copy those many
> > bytes to user-buffer ==
> >
> > ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
> >                          -EFAULT : 0;
> > =>
> > ret = copy_to_user(user_kdgkb->kb_string, kbs, strlen(kbs) + 1) ?
> >                          -EFAULT : 0;
> >
>
> But isn't strlen(kbs) is guaranteed to be equal to strlcpy() return
> value in this case? As I said in previous emails,
> strlen(func_table[kb_func]) < sizeof(user_kdgkb->kb_string) by design of
> this function.

That's the whole point of the discussion :)

The method "vt_do_kdgkb_ioctl" does not manage "func_table[kb_func]".
Thus, the method does not know whether or not
strlen(func_table[kb_func]) < sizeof(user_kdgkb->kb_string).

The intention is to make the method itself robust, without relying on
any external "chances" :)

>
> Do we need extra strlen() call here? Let's see what more experienced
> people think about it :)

Yep, let's wait for more feedback ..