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

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Message-ID: <1305bb43-b4bf-e129-af6e-957d1f30f269@gmail.com>
Date:   Wed, 10 Nov 2021 10:37:03 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Jiri Slaby <jirislaby@...nel.org>,
        Ajay Garg <ajaygargnsit@...il.com>
Cc:     Andy Shevchenko <andy.shevchenko@...il.com>,
        Greg KH <gregkh@...uxfoundation.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

On 11/10/21 08:22, Jiri Slaby wrote:
> I don't quite understand what the problem is. Provided I wrote the code,
> is there something wrong with this commit (and its explanation), in
> particular?
> 
> commit 6ca03f90527e499dd5e32d6522909e2ad390896b
> Author: Jiri Slaby <jirislaby@...nel.org>
> Date:   Mon Oct 19 10:55:16 2020 +0200
> 
>       vt: keyboard, simplify vt_kdgkbsent
> 
>       Use 'strlen' of the string, add one for NUL terminator and simply do
>       'copy_to_user' instead of the explicit 'for' loop. This makes the
>       KDGKBSENT case more compact.
> 
>       The only thing we need to take care about is NULL 'func_table[i]'. Use
>       an empty string in that case.
> 
>       The original check for overflow could never trigger as the func_buf
>       strings are always shorter or equal to 'struct kbsentry's.
> 
> thanks,
> 

As I said in my few previous emails, I don't see any bugs/problems in 
current code.

Ajay wants to be safe and he thinks, that relying on fact, that 
strlen(func_table[kb_func]) < sizeof(user_kdgkb->kb_string) is not good 
approach, since it's external for vt_do_kdgkb_ioctl. (I hope, I've 
explained his idea in the right way)



With regards,
Pavel Skripkin