From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f41.google.com (mail-pa0-f41.google.com [209.85.220.41]) by kanga.kvack.org (Postfix) with ESMTP id B3CD16B0005 for ; Mon, 18 Jan 2016 00:40:40 -0500 (EST) Received: by mail-pa0-f41.google.com with SMTP id ho8so167297389pac.2 for ; Sun, 17 Jan 2016 21:40:40 -0800 (PST) Received: from lgeamrelo12.lge.com (LGEAMRELO12.lge.com. [156.147.23.52]) by mx.google.com with ESMTPS id ml7si238257pab.58.2016.01.17.21.40.29 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Sun, 17 Jan 2016 21:40:39 -0800 (PST) From: 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> MIME-Version: 1.0 Content-Type: text/plain Sender: owner-linux-mm@kvack.org List-ID: To: minchan@kernel.org, ngupta@vflare.org Cc: sergey.senozhatsky.work@gmail.com, akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, vbabka@suse.cz, Junil Lee 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org