linux-kernel - Re: [PATCH] media:usb:cpia2: Properly check framebuffer mmap offsets

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Date:   Mon, 11 Nov 2019 18:53:23 +0000
From:   Omer Shalev <omerdeshalev@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Richard Fontana <rfontana@...hat.com>,
        Allison Randal <allison@...utok.net>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media:usb:cpia2: Properly check framebuffer mmap offsets

On Mon, Nov 11, 2019 at 05:29:07PM +0100, Greg Kroah-Hartman wrote:
> On Mon, Nov 11, 2019 at 06:24:42PM +0000, Omer Shalev wrote:
> > On Mon, Nov 11, 2019 at 12:46:15PM +0100, Greg Kroah-Hartman wrote:
> > > On Sat, Nov 09, 2019 at 12:39:43PM +0100, Hans Verkuil wrote:
> > > > Hi Greg,
> > > > 
> > > > On 11/8/19 9:49 PM, Greg Kroah-Hartman wrote:
> > > > > On Fri, Nov 08, 2019 at 09:50:36PM +0000, Omer Shalev wrote:
> > > > >> The cpai2 driver's mmap implementation wasn't properly check for all
> > > > >> possible offset values. Given a huge offset value , the calculation
> > > > >> start_offset + size can wrap around to a low value and pass the check
> > > > > 
> > > > > I thought we checked that in the core of the kernel now, to keep all
> > > > > drivers from not having to do this type of thing (as they obviously all
> > > > > forgot to.)  Why is this still needed here as well?
> > > > 
> > > > Where is that checked in the core? I couldn't find anything, but I might
> > > > have been looking in the wrong place.
> > > 
> > > Sorry, took me a while to find it.  Look at be83bbf80682 ("mmap:
> > > introduce sane default mmap limits") as I think this should handle the
> > > problem already.
> > > 
> > > thanks,
> > > 
> > > greg k-h
> > 
> > Thanks Greg. But All other drivers I've seen implement it like that: if(size > total_size || offset >
> > total_size - size). Which I think, is a better way to write this code, and generally more
> > secure. Plus, no extra code is needed (just changing this line).
> 
> The point of the above commit that is in the tree is that no driver has
> to do this check at all, it's already been done before the driver ever
> gets called, right?
> 
> So yes, there's lots of history of drivers doing the check themselves
> (and getting it wrong as you point out), but that should not matter
> anymore.
> 
> Can you verify that your change isn't even needed due to the above
> mentioned core check for valid values?
> 
> thanks,
> 
> greg k-h

Yes I got it , and thanks again. I think that programmatically , its
better to write that this way, And therefore I suggested this patch. 

thanks,

Omer