[PATCH] dma-buf: Precheck for a valid dma_fence before acquiring the reference

[Chris Wilson]

Quoting Chris Wilson (2020-02-21 15:23:38)
> Quoting Daniel Vetter (2020-02-21 15:17:24)
> > On Fri, Feb 21, 2020 at 3:38 PM Chris Wilson <chris at chris-wilson.co.uk> wrote:
> > > dma_fence_get_rcu() is used to acquire a reference to under a dma-fence
> > > under racey conditions -- a perfect recipe for a disaster. As we know
> > > the caller may be handling stale memory, use kasan to confirm the
> > > dma-fence, or rather its memory block, is valid before attempting to
> > > acquire a reference. This should help us to more quickly and clearly
> > > identify lost races.
> > 
> > Hm ... I'm a bit lost on the purpose, and what this does. Fences need
> > to be rcu-freed, and I have honestly no idea how kasan treats those.
> > Are we throwing false positives, because kasan thinks the stuff is
> > freed, but we're still accessing it (while the grace period hasn't
> > passed, so anything freed is still guaranteed to be at least in the
> > slab cache somewhere).
> > 
> > I'm not seeing how this catches lost races quicker, since the refcount
> > should get to 0 way before we get to the kfree. So the refcount check
> > on the next line should catch strictly more races than the kasan
> > check.
> 
> It's not about the fence itself, but those pointing to the fence. That's
> where we may find garbage, and by returning NULL the kernel keeps
> working for a bit longer as you try to piece together the race.

Plus given all the inlining, the kasan warning for the refcount is not
that clear about where it is called from, which is a bit of a nuisance,
so an explicit warning was much easier for finding the culprit.
-Chris