drm/i915: Don't touch NULL sg on i915_gem_object_get_pages_gtt() error

commit 2420489b upstream.

On the DMA mapping error path, sg may be NULL (it has already been
marked as the last scatterlist entry), and we should avoid dereferencing
it again.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e2273302 ("drm/i915: avoid leaking DMA mappings")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Imre Deak <imre.deak@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114112930.2033-1-chris@chris-wilson.co.ukReviewed-by: Matthew Auld <matthew.auld@intel.com>
(cherry picked from commit b17993b7)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/i915_gem.c

@@ -2540,7 +2540,7 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 			page = shmem_read_mapping_page(mapping, i);
 			if (IS_ERR(page)) {
 				ret = PTR_ERR(page);
-				goto err_pages;
+				goto err_sg;
 			}
 		}
 #ifdef CONFIG_SWIOTLB
@@ -2583,8 +2583,9 @@ i915_gem_object_get_pages_gtt(struct drm_i915_gem_object *obj)
 
 	return 0;
 
-err_pages:
+err_sg:
 	sg_mark_end(sg);
+err_pages:
 	for_each_sgt_page(page, sgt_iter, st)
 		put_page(page);
 	sg_free_table(st);