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

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Message-ID: <9717b429-597f-7778-c880-94361bcdee7f@gmail.com>
Date:   Sat, 6 Nov 2021 23:34:01 +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 23:30, Ajay Garg wrote:
>> > 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).
> 

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 :)




With regards,
Pavel Skripkin