linux-kernel - Re: [patch] Staging: tidspbridge: make mmap root-only so it is not a security problem

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Message-Id: <201312011047.30468@pali>
Date:	Sun, 1 Dec 2013 10:47:30 +0100
From:	Pali Rohár <pali.rohar@...il.com>
To:	Greg KH <gregkh@...uxfoundation.org>, Pavel Machek <pavel@....cz>
Cc:	Dan Carpenter <dan.carpenter@...cle.com>,
	Ivajlo Dimitrov <ivo.g.dimitrov.75@...il.com>,
	Ивайло Димитров 
	<freemangordon@....bg>, sre@...g0.de, omar.ramirez@...itl.com,
	tony@...mide.com, felipe.contreras@...il.com, s-anna@...com,
	nm@...com, ohad@...ery.com, stable@...r.kernel.org,
	linux-kernel@...r.kernel.org, nico@...lde.de
Subject: Re: [patch] Staging: tidspbridge: make mmap root-only so it is not a security problem

On Sunday 01 December 2013 04:45:01 Greg KH wrote:
> On Sat, Nov 30, 2013 at 11:58:23PM +0100, Pavel Machek wrote:
> > On Sat 2013-11-30 14:05:53, Greg KH wrote:
> > > On Sat, Nov 30, 2013 at 09:42:37PM +0100, Pavel Machek wrote:
> > > > mmap in tidspbridge is missing range-checks. For now,
> > > > make this interface root-only, so that it does not
> > > > cause security problems.
> > > 
> > > Please fix this properly and don't paper over the real
> > > problem here.
> > 
> > Well, if the driver is left uncompilable, I'm pretty sure it
> > will bitrot.
> > 
> > I do have the hardware, but I'm pretty sure current mailine
> > does not have enough support to boot Maemo, so it is non
> > trivial for me to test changes here.
> > 
> > And yes, I'd like to get N900 to better shape, but there's
> > more urgent work to do there. Such as "make sure N900 still
> > boots once omap moves away from device files".
> > 
> > [It seems like check should be that
> > 
> > vma->vm_pgoff << PAGE_SHIFT >= pdata->phys_mempool_base and
> > vma->vm_end - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT -
> > pdata->phys_mempool_base) <= pdata->phys_mempool_size .
> > 
> > But... this is some kind of DSP coprocessor, and I am not
> > sure if just exposing its address space to untrusted
> > processes is good idea.
> > 
> > Heck, are you sure this is security problem in the first
> > place? Yes, it is unchecked mmap. So what? If the device is
> > 600 root.root, and if the DSP can take over main system,
> > 
> >         if (!capable(CAP_SYS_RAWIO))
> >         
> >                 return -EPERM;
> 
> Will that break userspace?  Who opens and mmaps this device? 
> If you don't know if users do this, how can you say this
> patch isn't going to break things just as much as not having
> the driver there at all?
> 
> thanks,
> 
> greg k-h

Maemo and harmattan userspace using gst-dsp (gstreamer plugin)
for using tidspbridge. It opening /dev/DspBridge and then it calling:

mmap(NULL, seg->size, PROT_READ | PROT_WRITE, MAP_SHARED | 0x2000 /* MAP_LOCKED */, handle, seg->base_pa);

Because it is gstreamer plugin, every application can use it (under non root too).

Can you check if above problematic kernel code is what this gst-dsp mmap calling?

-- 
Pali Rohár
pali.rohar@...il.com

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