drm/i915: Check for rq->hwsp validity after acquiring RCU lock

Since we allow removing the timeline map at runtime, there is a risk
that rq->hwsp points into a stale page. To control that risk, we hold
the RCU read lock while reading *rq->hwsp, but we missed a couple of
important barriers. First, the unpinning / removal of the timeline map
must be after all RCU readers into that map are complete, i.e. after an
rcu barrier (in this case courtesy of call_rcu()). Secondly, we must
make sure that the rq->hwsp we are about to dereference under the RCU
lock is valid. In this case, we make the rq->hwsp pointer safe during
i915_request_retire() and so we know that rq->hwsp may become invalid
only after the request has been signaled. Therefore is the request is
not yet signaled when we acquire rq->hwsp under the RCU, we know that
rq->hwsp will remain valid for the duration of the RCU read lock.

This is a very small window that may lead to either considering the
request not completed (causing a delay until the request is checked
again, any wait for the request is not affected) or dereferencing an
invalid pointer.

Fixes: 3adac468 ("drm/i915: Introduce concept of per-timeline (context) HWSP")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: <stable@vger.kernel.org> # v5.1+
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201218122421.18344-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/gt/intel_breadcrumbs.c

@@ -134,11 +134,6 @@ static bool remove_signaling_context(struct intel_breadcrumbs *b,
 	return true;
 }
 
-static inline bool __request_completed(const struct i915_request *rq)
-{
-	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
-}
-
 __maybe_unused static bool
 check_signal_order(struct intel_context *ce, struct i915_request *rq)
 {
@@ -245,7 +240,7 @@ static void signal_irq_work(struct irq_work *work)
 		list_for_each_entry_rcu(rq, &ce->signals, signal_link) {
 			bool release;
 
-			if (!__request_completed(rq))
+			if (!__i915_request_is_complete(rq))
 				break;
 
 			if (!test_and_clear_bit(I915_FENCE_FLAG_SIGNAL,
@@ -380,7 +375,7 @@ static void insert_breadcrumb(struct i915_request *rq)
 	 * straight onto a signaled list, and queue the irq worker for
 	 * its signal completion.
 	 */
-	if (__request_completed(rq)) {
+	if (__i915_request_is_complete(rq)) {
 		irq_signal_request(rq, b);
 		return;
 	}
@@ -468,7 +463,7 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
 	if (release)
 		intel_context_put(ce);
 
-	if (__request_completed(rq))
+	if (__i915_request_is_complete(rq))
 		irq_signal_request(rq, b);
 
 	i915_request_put(rq);

drivers/gpu/drm/i915/gt/intel_timeline.c

@@ -126,6 +126,10 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
 	struct intel_timeline_cacheline *cl =
 		container_of(rcu, typeof(*cl), rcu);
 
+	/* Must wait until after all *rq->hwsp are complete before removing */
+	i915_gem_object_unpin_map(cl->hwsp->vma->obj);
+	__idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
+
 	i915_active_fini(&cl->active);
 	kfree(cl);
 }
@@ -133,11 +137,6 @@ static void __rcu_cacheline_free(struct rcu_head *rcu)
 static void __idle_cacheline_free(struct intel_timeline_cacheline *cl)
 {
 	GEM_BUG_ON(!i915_active_is_idle(&cl->active));
-
-	i915_gem_object_unpin_map(cl->hwsp->vma->obj);
-	i915_vma_put(cl->hwsp->vma);
-	__idle_hwsp_free(cl->hwsp, ptr_unmask_bits(cl->vaddr, CACHELINE_BITS));
-
 	call_rcu(&cl->rcu, __rcu_cacheline_free);
 }
 
@@ -179,7 +178,6 @@ cacheline_alloc(struct intel_timeline_hwsp *hwsp, unsigned int cacheline)
 		return ERR_CAST(vaddr);
 	}
 
-	i915_vma_get(hwsp->vma);
 	cl->hwsp = hwsp;
 	cl->vaddr = page_pack_bits(vaddr, cacheline);
 

drivers/gpu/drm/i915/i915_request.h

@@ -440,7 +440,7 @@ static inline u32 hwsp_seqno(const struct i915_request *rq)
 
 static inline bool __i915_request_has_started(const struct i915_request *rq)
 {
-	return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno - 1);
+	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno - 1);
 }
 
 /**
@@ -471,11 +471,19 @@ static inline bool __i915_request_has_started(const struct i915_request *rq)
  */
 static inline bool i915_request_started(const struct i915_request *rq)
 {
+	bool result;
+
 	if (i915_request_signaled(rq))
 		return true;
 
+	result = true;
+	rcu_read_lock(); /* the HWSP may be freed at runtime */
+	if (likely(!i915_request_signaled(rq)))
 		/* Remember: started but may have since been preempted! */
-	return __i915_request_has_started(rq);
+		result = __i915_request_has_started(rq);
+	rcu_read_unlock();
+
+	return result;
 }
 
 /**
@@ -488,10 +496,16 @@ static inline bool i915_request_started(const struct i915_request *rq)
  */
 static inline bool i915_request_is_running(const struct i915_request *rq)
 {
+	bool result;
+
 	if (!i915_request_is_active(rq))
 		return false;
 
-	return __i915_request_has_started(rq);
+	rcu_read_lock();
+	result = __i915_request_has_started(rq) && i915_request_is_active(rq);
+	rcu_read_unlock();
+
+	return result;
 }
 
 /**
@@ -515,12 +529,25 @@ static inline bool i915_request_is_ready(const struct i915_request *rq)
 	return !list_empty(&rq->sched.link);
 }
 
+static inline bool __i915_request_is_complete(const struct i915_request *rq)
+{
+	return i915_seqno_passed(__hwsp_seqno(rq), rq->fence.seqno);
+}
+
 static inline bool i915_request_completed(const struct i915_request *rq)
 {
+	bool result;
+
 	if (i915_request_signaled(rq))
 		return true;
 
-	return i915_seqno_passed(hwsp_seqno(rq), rq->fence.seqno);
+	result = true;
+	rcu_read_lock(); /* the HWSP may be freed at runtime */
+	if (likely(!i915_request_signaled(rq)))
+		result = __i915_request_is_complete(rq);
+	rcu_read_unlock();
+
+	return result;
 }
 
 static inline void i915_request_mark_complete(struct i915_request *rq)