Re: [PATCH] drm/nouveau: avoid null deref on bad arguments to nouveau_vma_getmap

["Pasi Kärkkäinen" <pasik@iki.fi>]

On Thu, Sep 26, 2013 at 02:48:49AM +1000, Ben Skeggs wrote:
> On Thu, Sep 26, 2013 at 12:41 AM, Pasi Kärkkäinen <pasik@iki.fi> wrote:
> > Hello,
> >
> > On Wed, Sep 04, 2013 at 08:59:13AM +0200, Maarten Lankhorst wrote:
> >> Op 04-09-13 05:41, Ben Skeggs schreef:
> >> > On Thu, Aug 22, 2013 at 5:12 PM, Maarten Lankhorst
> >> > <maarten.lankhorst@canonical.com> wrote:
> >> >> Op 22-08-13 02:10, Ilia Mirkin schreef:
> >> >>> The code expects non-VRAM mem nodes to have a pages list. If that's not
> >> >>> set, it will do a null deref down the line. Warn on that condition and
> >> >>> return an error.
> >> >>>
> >> >>> See https://bugs.freedesktop.org/show_bug.cgi?id=64774
> >> >>>
> >> >>> Reported-by: Pasi Kärkkäinen <pasik@iki.fi>
> >> >>> Tested-by: Pasi Kärkkäinen <pasik@iki.fi>
> >> >>> Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
> >> >>> Cc: <stable@vger.kernel.org> # 3.8+
> >> >>> ---
> >> >>>
> >> >>> I don't exactly understand what's going on, but this is just a
> >> >>> straightforward way to avoid a null deref that you see happens in the
> >> >>> bug. I haven't figured out the root cause of this, but it's getting
> >> >>> well into the "I have no idea how TTM works" space. However this seems
> >> >>> like a bit of defensive programming -- nouveau_vm_map_sg will pass
> >> >>> node->pages as a list down, which will be dereferenced by
> >> >>> nvc0_vm_map_sg. Perhaps the other arguments should make that
> >> >>> dereferencing not happen, but it definitely was happening here, as you
> >> >>> can see in the bug.
> >> >>>
> >> >>> Ben/Maarten, I'll let you judge whether this check is appropriate,
> >> >>> since like I hope I was able to convey above, I'm just not really sure :)
> >> >> Not it really isn't appropriate..
> >> >>
> >> >> You'd have to call call nouveau_vm_map_sg_table instead, the only place that doesn't handle that correctly
> >> >> is where it's not expected to be called.
> >> >>
> >> >> Here, have a completely untested patch to fix things...
> >> >>
> >> >> diff --git a/drivers/gpu/drm/nouveau/nouveau_display.c b/drivers/gpu/drm/nouveau/nouveau_display.c
> >> >> --- a/drivers/gpu/drm/nouveau/nouveau_display.c
> >> >> +++ b/drivers/gpu/drm/nouveau/nouveau_display.c
> >> >> @@ -138,17 +143,26 @@ nouveau_user_framebuffer_create(struct drm_device *dev,
> >> >>  {
> >> >>         struct nouveau_framebuffer *nouveau_fb;
> >> >>         struct drm_gem_object *gem;
> >> >> +       struct nouveau_bo *nvbo;
> >> >>         int ret = -ENOMEM;
> >> >>
> >> >>         gem = drm_gem_object_lookup(dev, file_priv, mode_cmd->handles[0]);
> >> >>         if (!gem)
> >> >>                 return ERR_PTR(-ENOENT);
> >> >>
> >> >> +       nvbo = nouveau_gem_object(gem);
> >> >> +       if (!(nvbo->valid_domains & NOUVEAU_GEM_DOMAIN_VRAM)) {
> >> >> +               nv_warn(nouveau_drm(dev), "Trying to create a fb in vram with"
> >> >> +                       " valid_domains=%08x\n", nvbo->valid_domains);
> >> >> +               ret = -EINVAL;
> >> >> +               goto err_unref;
> >> >> +       }
> >> >> +
> >> > Definitely the right idea, we can't handle this case right now.
> >> > However, we may someday want/need to be able to scan out of system
> >> > memory, so this is the wrong place.
> >> >
> >> > I suspect the correct thing to do (which'll also handle the
> >> > "defensive" part) is to bail in nouveau_bo_move() on attempts to move
> >> > a DMA-BUF backed object into VRAM.
> >> >
> >> > Sound OK?
> >> >
> >> If it has a WARN_ON or something that would be ok, I didn't find any other places that attempt to move buffers to VRAM though, so it's probably harmless.
> >>
> >
> > Ben/Maarten: Are you guys planning to take a look at this and submit another patch, or.. ?
> >
> > I tested the two earlier patches from this thread, and they both fixed the problem (hard kernel crash).
> > I'm hoping this bug could be finally solved in the kernel..
> I shall be looking at it properly once I'm back from XDC next week.
> 

Great, thanks!


-- Pasi

> Thanks,
> Ben.
>