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

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Message-ID: <YqNVcppyBXU8mQ/Z@kroah.com>
Date:   Fri, 10 Jun 2022 16:30:10 +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 10, 2022 at 07:12:02PM +0530, Shreenidhi Shedi wrote:
> On 10/06/22 6:58 pm, Greg KH wrote:
> > 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
> 
> Thanks for the review. Assume lp_count is less than LP_NO now and we enter the for loop
> and for some reason for loop exits after i reaching the value LP_NO

Wait, how can that happen?  That's what I am saying, the loop will never
reach that value from what I can tell.

Yes, this whole thing should be moved to something more sane like an
idr structure, but as-is, it seems correct to me.

Have you tested the code with that many devices to see if it really can
overflow?

thanks,

greg k-h