drm/i915: Protect debugfs per_file_stats with RCU lock

If we make sure we grab a strong reference to each object as we dump it,
we can reduce the locks outside of our iterators to an rcu_read_lock.

This should prevent errors like:
[ 2138.371911] BUG: KASAN: use-after-free in per_file_stats+0x43/0x380 [i915]
[ 2138.371924] Read of size 8 at addr ffff888223651000 by task cat/8293

[ 2138.371947] CPU: 0 PID: 8293 Comm: cat Not tainted 5.3.0-rc6-CI-Custom_4352+ #1
[ 2138.371953] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./J4205-ITX, BIOS P1.40 07/14/2017
[ 2138.371959] Call Trace:
[ 2138.371974]  dump_stack+0x7c/0xbb
[ 2138.372099]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372108]  print_address_description+0x73/0x3a0
[ 2138.372231]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372352]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372362]  __kasan_report+0x14e/0x192
[ 2138.372489]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372502]  kasan_report+0xe/0x20
[ 2138.372625]  per_file_stats+0x43/0x380 [i915]
[ 2138.372751]  ? i915_panel_show+0x110/0x110 [i915]
[ 2138.372761]  idr_for_each+0xa7/0x160
[ 2138.372773]  ? idr_get_next_ul+0x110/0x110
[ 2138.372782]  ? do_raw_spin_lock+0x10a/0x1d0
[ 2138.372923]  print_context_stats+0x264/0x510 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903062133.27360-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_debugfs.c

@@ -237,6 +237,9 @@ static int per_file_stats(int id, void *ptr, void *data)
 	struct file_stats *stats = data;
 	struct i915_vma *vma;
 
+	if (!kref_get_unless_zero(&obj->base.refcount))
+		return 0;
+
 	stats->count++;
 	stats->total += obj->base.size;
 	if (!atomic_read(&obj->bind_count))
@@ -284,6 +287,7 @@ static int per_file_stats(int id, void *ptr, void *data)
 	}
 	spin_unlock(&obj->vma.lock);
 
+	i915_gem_object_put(obj);
 	return 0;
 }
 
@@ -313,10 +317,12 @@ static void print_context_stats(struct seq_file *m,
 				    i915_gem_context_lock_engines(ctx), it) {
 			intel_context_lock_pinned(ce);
 			if (intel_context_is_pinned(ce)) {
+				rcu_read_lock();
 				if (ce->state)
 					per_file_stats(0,
 						       ce->state->obj, &kstats);
 				per_file_stats(0, ce->ring->vma->obj, &kstats);
+				rcu_read_unlock();
 			}
 			intel_context_unlock_pinned(ce);
 		}
@@ -328,9 +334,9 @@ static void print_context_stats(struct seq_file *m,
 			struct task_struct *task;
 			char name[80];
 
-			spin_lock(&file->table_lock);
+			rcu_read_lock();
 			idr_for_each(&file->object_idr, per_file_stats, &stats);
-			spin_unlock(&file->table_lock);
+			rcu_read_unlock();
 
 			rcu_read_lock();
 			task = pid_task(ctx->pid ?: file->pid, PIDTYPE_PID);