From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io0-f180.google.com (mail-io0-f180.google.com [209.85.223.180]) by kanga.kvack.org (Postfix) with ESMTP id 86AC46B0005 for ; Mon, 18 Jan 2016 04:55:09 -0500 (EST) Received: by mail-io0-f180.google.com with SMTP id 1so497366534ion.1 for ; Mon, 18 Jan 2016 01:55:09 -0800 (PST) Received: from lgeamrelo13.lge.com (LGEAMRELO13.lge.com. [156.147.23.53]) by mx.google.com with ESMTPS id k20si28381875iok.56.2016.01.18.01.55.07 for (version=TLS1 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Mon, 18 Jan 2016 01:55:08 -0800 (PST) From: Junil Lee Subject: [PATCH v4] zsmalloc: fix migrate_zspage-zs_free race condition Date: Mon, 18 Jan 2016 18:55:03 +0900 Message-ID: <1453110903-11394-1-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 | 17 ++++++++++++++++- 1 file changed, 16 insertions(+), 1 deletion(-) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index e7414ce..cb54ce3 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -307,9 +307,24 @@ static void free_handle(struct zs_pool *pool, unsigned long handle) kmem_cache_free(pool->handle_cachep, (void *)handle); } + +/* + * record_obj updates handle's value to free_obj and it shouldn't + * invalidate lock bit(ie, HANDLE_PIN_BIT) of handle, otherwise + * it breaks synchronization using pin_tag(e,g, zs_free) so let's + * keep the lock bit. + */ static void record_obj(unsigned long handle, unsigned long obj) { - *(unsigned long *)handle = obj; + int locked = (*(unsigned long *)handle) & (1 << HANDLE_PIN_BIT); + unsigned long val = obj | locked; + + /* + * WRITE_ONCE could prevent store tearing like below + * *(unsigned long *)handle = free_obj + * *(unsigned long *)handle |= locked; + */ + WRITE_ONCE(*(unsigned long *)handle, val); } /* zpool driver */ -- 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