linux-kernel - Re: [PATCH] tty: vt: add a bounds checking in vt_do_kdgkb

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Date:   Thu, 8 Sep 2022 10:16:17 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Hangyu Hua <hbh25y@...il.com>
Cc:     jirislaby@...nel.org, changlianzhi@...ontech.com,
        dmitry.torokhov@...il.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: vt: add a bounds checking in vt_do_kdgkb_ioctl()

On Thu, Sep 08, 2022 at 03:54:03PM +0800, Hangyu Hua wrote:
> As array_index_nospec's comments indicate，a bounds checking need to add
> before calling array_index_nospec.
> 
> Signed-off-by: Hangyu Hua <hbh25y@...il.com>
> ---
>  drivers/tty/vt/keyboard.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
> index be8313cdbac3..b9845455df79 100644
> --- a/drivers/tty/vt/keyboard.c
> +++ b/drivers/tty/vt/keyboard.c
> @@ -2067,6 +2067,9 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry __user *user_kdgkb, int perm)
>  	if (get_user(kb_func, &user_kdgkb->kb_func))
>  		return -EFAULT;
>  
> +	if (kb_func >= MAX_NR_FUNC)
> +		return -EFAULT;

Wrong error to return, only ever return that error if you have a memory
fault from copy_to/from_user().

But even then, how can kb_func ever be greater than MAX_NR_FUNC?  And
what is wrong with it being MAX_NR_FUNC?

> +
>  	kb_func = array_index_nospec(kb_func, MAX_NR_FUNC);

Maybe we really don't need this at all, right?

thanks,

greg k-h