[PATCH 04/68] 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.

v2: There was a bug in the original patch where the ring->last_context
was set too early. I am not sure how this wasn't being hit when I sent
this previously.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
---
 drivers/gpu/drm/i915/i915_gem_context.c | 24 +++++++++++++++++-------
 1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem_context.c b/drivers/gpu/drm/i915/i915_gem_context.c
index c9aa3e6..0ce8fc9 100644
--- a/drivers/gpu/drm/i915/i915_gem_context.c
+++ b/drivers/gpu/drm/i915/i915_gem_context.c
@@ -609,14 +609,18 @@ mi_set_context(struct intel_engine_cs *ring,
 	return ret;
 }
 
-static void do_switch_fini_common(struct intel_engine_cs *ring,
-				  struct intel_context *from,
-				  struct intel_context *to)
+static struct intel_context *do_switch_fini_common(struct intel_engine_cs *ring,
+						   struct intel_context *from,
+						   struct intel_context *to)
 {
+	struct intel_context *ret;
 	if (likely(from))
 		i915_gem_context_unreference(from);
 	i915_gem_context_reference(to);
+	ret = ring->last_context;
 	ring->last_context = to;
+
+	return ret;
 }
 
 static int do_switch_xcs(struct intel_engine_cs *ring,
@@ -762,14 +766,20 @@ static int do_switch_rcs(struct intel_engine_cs *ring,
 		 */
 		from->legacy_hw_ctx.rcs_state->dirty = 1;
 		BUG_ON(from->legacy_hw_ctx.rcs_state->ring != ring);
-
-		/* obj is kept alive until the next request by its active ref */
-		i915_gem_object_ggtt_unpin(from->legacy_hw_ctx.rcs_state);
 	}
 
 	uninitialized = !to->legacy_hw_ctx.initialized && from == NULL;
 	to->legacy_hw_ctx.initialized = true;
-	do_switch_fini_common(ring, from, to);
+	/* From may have disappeared again after the context unref */
+	from = do_switch_fini_common(ring, from, to);
+	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->legacy_hw_ctx.rcs_state);
+	}
 
 	if (uninitialized) {
 		ret = i915_gem_render_state_init(ring);
-- 
2.0.4