[PATCH] drm/gem: fix up flink name create race

[Daniel Vetter]

On Wed, Jul 24, 2013 at 11:02:02AM +0200, Daniel Vetter wrote:
> On Wed, Jul 24, 2013 at 8:04 AM, Daniel Vetter <daniel.vetter at ffwll.ch> wrote:
> > This is the 2nd attempt, I've always been a bit dissatisified with the
> > tricky nature of the first one:
> >
> > http://lists.freedesktop.org/archives/dri-devel/2012-July/025451.html
> >
> > The issue is that the flink ioctl can race with calling gem_close on
> > the last gem handle. In that case we'll end up with a zero handle
> > count, but an flink name (and it's corresponding reference). Which
> > results in a neat space leak.
> >
> > In my first attempt I've solved this by rechecking the handle count.
> > But fundamentally the issue is that ->handle_count isn't your usual
> > refcount - it can be resurrected from 0 among other things.
> >
> > For those special beasts atomic_t often suggest way more ordering that
> > it actually guarantees. To prevent being tricked by those hairy
> > semantics take the easy way out and simply protect the handle with the
> > existing dev->object_name_lock.
> >
> > With that change implemented it's dead easy to fix the flink vs. gem
> > close reace: When we try to create the name we simply have to check
> > whether there's still officially a gem handle around and if not refuse
> > to create the flink name. Since the handle count decrement and flink
> > name destruction is now also protected by that lock the reace is gone
> > and we can't ever leak the flink reference again.
> >
> > Outside of the drm core only the exynos driver looks at the handle
> > count, and tbh I have no idea why (it's just for debug dmesg output
> > luckily).
> >
> > I've considered inlining the drm_gem_object_handle_free, but I plan to
> > add more name-like things (like the exported dma_buf) to this scheme,
> > so it's clearer to leave the handle freeing in its own function.
> >
> > This is exercised by the new gem_flink_race i-g-t testcase, which on
> > my snb leaks gem objects at a rate of roughly 1k objects/s.
> 
> That's actually incorrect since the leak I've found is just a race in
> the drm/i915 object tracking. So I need to go back to the drawing
> board and figure out which are the ghosts and which the dragons here.
> 
> I've turned that testcase into an exercise for "drm/gem: completely
> close gem_open vs. gem_close races", but that race only results in
> userspace seeing different flink names for the same object. And that
> only happens if userspace is racy already.
> 
> For this patch here I still think there's an issue, but I seriously
> need to restart my brain first and flush out the bogons with some
> coffee before I try again ;-)

Ok, I've written a second subtest now, and the race is indeed there and
I've managed to leak a few objects. It's rather hard to hit though, I get
a leak for roughly ever 1M attempts to provoke the race. But I'm rather
convinced now that the leak is indeed there ;-)

So I think the patch as-is stands as correct and required to block off
evil usespace.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch