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

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Date:   Mon, 11 Nov 2019 18:24:42 +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 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).

Please let me know what you think.

Best regards,

Omer