From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751029AbcARFkS (ORCPT ); Mon, 18 Jan 2016 00:40:18 -0500 Received: from LGEAMRELO12.lge.com ([156.147.23.52]:55978 "EHLO lgeamrelo12.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750796AbcARFkO (ORCPT ); Mon, 18 Jan 2016 00:40:14 -0500 X-Original-SENDERIP: 156.147.1.125 X-Original-MAILFROM: junil0814.lee@lge.com X-Original-SENDERIP: 165.244.98.150 X-Original-MAILFROM: junil0814.lee@lge.com X-Original-SENDERIP: 10.168.177.123 X-Original-MAILFROM: junil0814.lee@lge.com From: Junil Lee To: , CC: sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, vbabka@suse.cz, Junil Lee Subject: [PATCH v3] zsmalloc: fix migrate_zspage-zs_free race condition Date: Mon, 18 Jan 2016 14:39:56 +0900 Message-ID: <1453095596-44055-1-git-send-email-junil0814.lee@lge.com> X-Mailer: git-send-email 2.6.2 X-MIMETrack: Itemize by SMTP Server on LGEKRMHUB06/LGE/LG Group(Release 8.5.3FP6|November 21, 2013) at 2016/01/18 14:40:11, Serialize by Router on LGEKRMHUB06/LGE/LG Group(Release 8.5.3FP6|November 21, 2013) at 2016/01/18 14:40:11, Serialize complete at 2016/01/18 14:40:11 MIME-Version: 1.0 Content-Type: text/plain Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org record_obj() in migrate_zspage() does not preserve handle's HANDLE_PIN_BIT, set by find_aloced_obj()->trypin_tag(), and implicitly (accidentally) un-pins the handle, while migrate_zspage() still performs an explicit unpin_tag() on the that handle. This additional explicit unpin_tag() introduces a race condition with zs_free(), which can pin that handle by this time, so the handle becomes un-pinned. Schematically, it goes like this: CPU0 CPU1 migrate_zspage find_alloced_obj trypin_tag set HANDLE_PIN_BIT zs_free() pin_tag() obj_malloc() -- new object, no tag record_obj() -- remove HANDLE_PIN_BIT set HANDLE_PIN_BIT unpin_tag() -- remove zs_free's HANDLE_PIN_BIT The race condition may result in a NULL pointer dereference: Unable to handle kernel NULL pointer dereference at virtual address 00000000 CPU: 0 PID: 19001 Comm: CookieMonsterCl Tainted: PC is at get_zspage_mapping+0x0/0x24 LR is at obj_free.isra.22+0x64/0x128 Call trace: [] get_zspage_mapping+0x0/0x24 [] zs_free+0x88/0x114 [] zram_free_page+0x64/0xcc [] zram_slot_free_notify+0x90/0x108 [] swap_entry_free+0x278/0x294 [] free_swap_and_cache+0x38/0x11c [] unmap_single_vma+0x480/0x5c8 [] unmap_vmas+0x44/0x60 [] exit_mmap+0x50/0x110 [] mmput+0x58/0xe0 [] do_exit+0x320/0x8dc [] do_group_exit+0x44/0xa8 [] get_signal+0x538/0x580 [] do_signal+0x98/0x4b8 [] do_notify_resume+0x14/0x5c Fix the race by removing explicit unpin_tag() from migrate_zspage(). Signed-off-by: Junil Lee --- mm/zsmalloc.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index e7414ce..0acfa20 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -1635,8 +1635,8 @@ static int migrate_zspage(struct zs_pool *pool, struct size_class *class, free_obj = obj_malloc(d_page, class, handle); zs_object_copy(free_obj, used_obj, class); index++; + /* This also effectively unpins the handle */ record_obj(handle, free_obj); - unpin_tag(handle); obj_free(pool, class, used_obj); } -- 2.6.2