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

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Message-ID: <65c45951-08ba-26bb-f96b-3d4442b1d4d4@gmail.com>
Date:   Sat, 6 Nov 2021 23:48:30 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Ajay Garg <ajaygargnsit@...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

On 11/6/21 23:44, Ajay Garg wrote:
>> >
>> > 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).
>> >
>>
>> It manages. The code under `case KDSKBSENT:` sets func_table[] entries
>> via vt_kdskbsent().
>>
>> kbs = strndup_user(..., sizeof(user_kdgkb->kb_string));
>>
>> is used to allocate buffer for the func_table[] entry. That's my main
>> point :)
> 
> func_table is set in vt_kdskbent, which itself is external.
> 
> More importantly, vt_kdskbent is handled in case KDSKBSENT:, while the
> strlcpy issue we are dealing with is in case KDGKBSENT:
> In case KDGKBSENT, following are managed :
> 
>                  ssize_t len = sizeof(user_kdgkb->kb_string);
>                  kbs = kmalloc(len, GFP_KERNEL);
> 
> while func_table[kb_func] is external entity here, so no assumption
> ought to be made for it, just my 2 cents though :)
> 
> Anyhow, really, it is the maintainers' choice now :), since there
> isn't a burning (compilation/runtime) issue.
> 

I fully agree here, it's maintainer's choice. Let's sit down and wait 
what experienced people thing about this :)

I've just wanted to explain my idea better to exclude possible 
misunderstanding.

Thanks



With regards,
Pavel Skripkin