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

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Message-ID: <9e3047a9-ad29-ab83-670b-4d28e6ec6dbf@gmail.com>
Date:   Mon, 8 Nov 2021 14:58:04 +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 11/8/21 11:59, Ajay Garg wrote:
> Dropping all further discussions on this thread, as a RFC for a new
> string-copy method has been posted at :
> https://lore.kernel.org/linux-hardening/CAHP4M8U=0aTHgfREGJpSboV6J4X+E3Y6+H_kb-PvXxDKtV=n-g@mail.gmail.com/T/#t
> 
> which, if accepted, will make the clients' lives a lot easier.
> 

Honestly, I can't get what you are trying to achieve with new string 
function.

If caller knows, that there is no possible overflow, it can omit bounds 
checking (like in vt_do_kdgkb_ioctl). If caller needs return value equal 
to destination length it can use strscpy().

There is a bunch of str*cpy() functions and every month I see new 
conversations between them on ML. As Andy said it's really chaos. These 
conversation are needed, of course, from security point of view, but 
lib/string is already big. It contains functions for every possible 
scenario, caller just needs to pick right one.

I might be too dumb in this topic, so it's just my IMHO, since I am on 
CC list.

With regards,
Pavel Skripkin