linux-kernel - Re: [PATCH 1/2] char: lp: ensure that index has not exceeded LP

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Date:   Fri, 10 Jun 2022 15:28:04 +0200
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Shreenidhi Shedi <yesshedi@...il.com>
Cc:     arnd@...db.de, linux-kernel@...r.kernel.org,
        Shreenidhi Shedi <sshedi@...are.com>
Subject: Re: [PATCH 1/2] char: lp: ensure that index has not exceeded LP_NO

On Fri, Jun 03, 2022 at 06:30:39PM +0530, Shreenidhi Shedi wrote:
> From: Shreenidhi Shedi <sshedi@...are.com>
> 
> After finishing the loop, index value can be equal to LP_NO and lp_table
> array is of size LP_NO, so this can end up in accessing an out of bound
> address in lp_register function.
> 
> Signed-off-by: Shreenidhi Shedi <sshedi@...are.com>
> ---
>  drivers/char/lp.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/lp.c b/drivers/char/lp.c
> index 0e22e3b0a..d474d02b6 100644
> --- a/drivers/char/lp.c
> +++ b/drivers/char/lp.c
> @@ -972,7 +972,7 @@ static void lp_attach(struct parport *port)
>  			if (port_num[i] == -1)
>  				break;
> 
> -		if (!lp_register(i, port))
> +		if (i < LP_NO && !lp_register(i, port))
>  			lp_count++;

How can this ever be needed?  Look at the check further up for the check
of lp_count which prevents this from every going too large.

So how can an address be accessed out of bound here?

thanks,

greg k-h