drm/i915: fix race condition UAF in i915_perf_add_config_ioctl

Userspace can guess the id value and try to race oa_config object creation
with config remove, resulting in a use-after-free if we dereference the
object after unlocking the metrics_lock.  For that reason, unlocking the
metrics_lock must be done after we are done dereferencing the object.

Signed-off-by: Min Li <[email protected]>
Fixes: f89823c ("drm/i915/perf: Implement I915_PERF_ADD/REMOVE_CONFIG interface")
Cc: <[email protected]> # v4.14+
Reviewed-by: Andi Shyti <[email protected]>
Reviewed-by: Umesh Nerlige Ramappa <[email protected]>
Signed-off-by: Tvrtko Ursulin <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
[tursulin: Manually added stable tag.]
(cherry picked from commit 49f6f64)
Signed-off-by: Jani Nikula <[email protected]>

Author: Min Li

drivers/gpu/drm/i915/i915_perf.c

@@ -4638,13 +4638,13 @@ int i915_perf_add_config_ioctl(struct drm_device *dev, void *data,
 		err = oa_config->id;
 		goto sysfs_err;
 	}
-
-	mutex_unlock(&perf->metrics_lock);
+	id = oa_config->id;
 
 	drm_dbg(&perf->i915->drm,
 		"Added config %s id=%i\n", oa_config->uuid, oa_config->id);
+	mutex_unlock(&perf->metrics_lock);
 
-	return oa_config->id;
+	return id;
 
 sysfs_err:
 	mutex_unlock(&perf->metrics_lock);