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

[Ben Skeggs]

On Thu, Sep 26, 2013 at 12:41 AM, Pasi Kärkkäinen <pasik at 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 at 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 at iki.fi>
>> >>> Tested-by: Pasi Kärkkäinen <pasik at iki.fi>
>> >>> Signed-off-by: Ilia Mirkin <imirkin at alum.mit.edu>
>> >>> Cc: <stable at 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.

Thanks,
Ben.

>
> Thanks,
>
> -- Pasi
>