[BrownBag] drm/i915/gtt: Neuter the deferred unbind callback from gen6_ppgtt_cleanup

Chris Wilson
2019-05-24T06:45:49Z

Having deferred the vma destruction to a worker where we can acquire the
struct_mutex, we have to avoid chasing back into the now destroyed
ppgtt. The pd_vma is special in having a custom unbind function to scan
for unused pages despite the VMA itself being notionally part of the
GGTT. As such, we need to disable that callback to avoid a
use-after-free.

This unfortunately blew up so early during boot that CI declared the
machine unreachable as opposed to being the major failure it was. Oops.

Fixes: d3622099c76f ("drm/i915/gtt: Always acquire struct_mutex for gen6_ppgtt_cleanup")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
---
drivers/gpu/drm/i915/i915_gem_gtt.c | 28 ++++++++++++++++++++++++++++
1 file changed, 28 insertions(+)

diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 8d8a4b0ad4d9..266baa11df64 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1847,6 +1847,33 @@ static void gen6_ppgtt_cleanup_work(struct work_struct *wrk)
kfree(work);
}

+static int nop_set_pages(struct i915_vma *vma)
+{
+ return -ENODEV;
+}
+
+static void nop_clear_pages(struct i915_vma *vma)
+{
+}
+
+static int nop_bind(struct i915_vma *vma,
+ enum i915_cache_level cache_level,
+ u32 unused)
+{
+ return -ENODEV;
+}
+
+static void nop_unbind(struct i915_vma *vma)
+{
+}
+
+static const struct i915_vma_ops nop_vma_ops = {
+ .set_pages = nop_set_pages,
+ .clear_pages = nop_clear_pages,
+ .bind_vma = nop_bind,
+ .unbind_vma = nop_unbind,
+};
+
static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
{
struct gen6_hw_ppgtt *ppgtt = to_gen6_ppgtt(i915_vm_to_ppgtt(vm));
@@ -1855,6 +1882,7 @@ static void gen6_ppgtt_cleanup(struct i915_address_space *vm)
/* FIXME remove the struct_mutex to bring the locking under control */
INIT_WORK(&work->base, gen6_ppgtt_cleanup_work);
work->vma = ppgtt->vma;
+ work->vma->ops = &nop_vma_ops;
schedule_work(&work->base);

gen6_ppgtt_free_pd(ppgtt);
--
2.20.1