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

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Message-ID: <9eafae1f-d9f0-298d-cf20-212865d0becc@gmail.com>
Date:   Sat, 6 Nov 2021 14:23:51 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     Ajay Garg <ajaygargnsit@...il.com>, linux-serial@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: vt: keyboard: do not copy an extra-byte in
 copy_to_user

Hi, Ajay!

On 11/6/21 12:20, Ajay Garg wrote:
> Both (statically-allocated) "user_kdgkb->kb_string" and
> (dynamically-allocated) "kbs" are of length "len", so we must
> not copy more than "len" bytes.
> 
> Signed-off-by: Ajay Garg <ajaygargnsit@...il.com>
> ---
>   drivers/tty/vt/keyboard.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index c7fbbcdcc346..dfef7de8a057 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -2070,7 +2070,7 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
>   		len = strlcpy(kbs, func_table[kb_func] ? : "", len);

		^^^^^^^^^

len is reinitialized here, i.e len passed to kmalloc and len passed to 
copy_to_user() can be different.

strlcpy() returns strlen() of source string (2nd argument), that's why 
we need +1 here to pass null byte to user.

Am I missing something?


>   		spin_unlock_irqrestore(&func_buf_lock, flags);
>   
> -		ret = copy_to_user(user_kdgkb->kb_string, kbs, len + 1) ?
> +		ret = copy_to_user(user_kdgkb->kb_string, kbs, len) ?
>   			-EFAULT : 0;
>   
>   		break;
> 


With regards,
Pavel Skripkin