[PATCH 04/16] drm/i915: Fix another another use-after-free in do_switch

[Ben Widawsky <benjamin.widawsky@intel.com>]

See the following for many more details.

commit acc240d41ea1ab9c488a79219fb313b5b46265ae
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Dec 5 15:42:34 2013 +0100

    drm/i915: Fix use-after-free in do_switch

In this case, the issue is only for full PPGTT:
do_switch
  context_unref
    ppgtt_release
      i915_gpu_idle
	switch_to_default
	from changes to default context

This could be backported to the pre do_switch cleanup I did in this
series. However, it's much cleaner and more obvious as a patch on top,
so I'd really like to do this as a post cleanup patch.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index 601a58f..0e6e743 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -745,13 +745,21 @@ static int do_switch_rcs(struct intel_engine_cs *ring,
 		from->obj->dirty = 1;
 		BUG_ON(from->obj->ring != ring);
 
-		/* obj is kept alive until the next request by its active ref */
-		i915_gem_object_ggtt_unpin(from->obj);
 	}
 
 	uninitialized = !to->is_initialized && from == NULL;
 	to->is_initialized = true;
 	do_switch_fini_common(ring, from, to);
+	/* From may have disappeared again after thecontext unref */
+	from = ring->last_context;
+	if (from != NULL) {
+		/* obj is kept alive until the next request by its active ref.
+		 * XXX: The context needs to be unpinned last, or else we risk
+		 * hitting evict/idle on the ppgtt free, which will call back
+		 * into this, and we'll get a double unpin on this context
+		 */
+		i915_gem_object_ggtt_unpin(from->obj);
+	}
 
 	if (uninitialized) {
 		ret = i915_gem_render_state_init(ring);
-- 
2.0.1