[PATCH v19 25/30] drm/shmem-helper: Don't free refcounted GEM

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Don't free shmem object if it has pages that are in use at the time of
the GEM's freeing if DRM driver doesn't manage GEM/pages lifetime properly.
This prevents memory corruption due to the use-after-free bug in exchange
to leaking GEM.

Signed-off-by: Dmitry Osipenko <dmitry.osipenko@xxxxxxxxxxxxx>
 drivers/gpu/drm/drm_gem_shmem_helper.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c
index e6e6e693ab95..0d95d723b90d 100644
--- a/drivers/gpu/drm/drm_gem_shmem_helper.c
+++ b/drivers/gpu/drm/drm_gem_shmem_helper.c
@@ -205,9 +205,15 @@ void drm_gem_shmem_free(struct drm_gem_shmem_object *shmem)
 	if (obj->import_attach)
 		drm_prime_gem_destroy(obj, shmem->sgt);
-	drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count));
-	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count));
-	drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count));
+	/*
+	 * Prevent memory corruption caused by the use-after-free bug in a
+	 * case where shmem user erroneously holds reference to pages while
+	 * GEM is freed by leaking the GEM.
+	 */
+	if (drm_WARN_ON(obj->dev, refcount_read(&shmem->vmap_use_count)) ||
+	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_use_count)) ||
+	    drm_WARN_ON(obj->dev, refcount_read(&shmem->pages_pin_count)))
+		return;

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