Re: [PATCH 00/03] Preventing zero GPU virtual address allocation

Chris Wilson
2015-05-21T09:50:43Z

On Thu, May 21, 2015 at 11:44:11AM +0200, Daniel Vetter wrote:
> On Thu, May 21, 2015 at 09:43:00AM +0100, Chris Wilson wrote:
> > On Thu, May 21, 2015 at 11:08:42AM +0300, David Weinehall wrote:
> > > On Wed, May 20, 2015 at 05:10:58PM +0100, Chris Wilson wrote:
> > > > On Wed, May 20, 2015 at 06:00:43PM +0200, Daniel Vetter wrote:
> > > > > On Wed, May 20, 2015 at 03:14:06PM +0100, Chris Wilson wrote:
> > > > > > On Wed, May 20, 2015 at 03:09:43PM +0100, Chris Wilson wrote:
> > > > > > > On Wed, May 20, 2015 at 04:54:19PM +0300, David Weinehall wrote:
> > > > > > > > This patch series (one patch each for libdrm, the kernel, and beignet)
> > > > > > > > aims to provide a means to add a context-specific means to prevent
> > > > > > > > a mapping to GPU virtual address zero. This is needed at least by
> > > > > > > > Beignet (possibly in other use-cases too, though I don't know of any
> > > > > > > > other) to allow use of address zero to represent NULL.
> > > > > > >
> > > > > > > Urm, you cannot allow absolute addressing period. What happens to the
> > > > > > > object at 0 when the user reads from it or writes to it? You have to
> > > > > > > have an object at 0 for the user's NULL pointer access.
> > > > > >
> > > > > > I'll mollify that: outside of full-ppgtt where you need to share the VM.
> > > > >
> > > > > The description is misleading, the new flag doesn't prevent anything from
> > > > > getting mapped at 0 but only prevents any bo submitted through execbuf on
> > > > > the given context from being bound at address 0. If that would happen
> > > > > compute kernels using NULL checks for some things would fall over.
> > >
> > > Any suggestion for a better description?
> > >
> > > > That does not address the issue that *0 is an untrapped runtime bug that
> > > > allows writing to other objects or reading from them.
> > >
> > > Not exactly sure what you suggest here?
> >
> > That you have an unmitigated security hole in your design.
>
> There's no security issue afaics, there's only a correctness issue. opencl
> kernels must be able to assume that NULL never points to a valid buffer
> address of their own. It might point at other intersting stuff like batch
> buffers or what else, but never about anything created by the ocl client
> itself. This patch achieves that.

It also have just as much risk as reporting EBUSY due to the CL client
trying to use a pinned buffer.

However, it is a security hole because the same process can arrange to
have whatever buffer it likes at 0 then access it through the CL kernel.

> Ofc there's still the issue that anything else mapped can be stomped upon,
> but that's just undefined behaviour in C-spec speak. And there's still the
> isolation issues, but ocl doesn't make any new guarantees there afaik.

I disagree, it is removing the guarantees by providing an absolute
address which can be subverted by an external process.
-Chris

--
Chris Wilson, Intel Open Source Technology Centre