From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2C318C282D7 for ; Wed, 30 Jan 2019 13:16:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id F19252184D for ; Wed, 30 Jan 2019 13:16:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548854207; bh=pVdtQmgZOyjcqX8J0FTHIyBrj1qymKuhAfoPYHB/Adc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=sWddDaHLb2kNjKPLZyNwaXpxS6ZxoFV/tvXpC8YPN5FJxO0S/0QJy8Mw8m5n6P/q1 uVpDR3r7dW6PnYVWvzBM3OLoFPwVevNZu8f4HC7wWPcGWaPewPPhOReokjA0k6anlQ s2EiwG0sh/WmHWMhaYeOMca9B7HEmiu7WKe9LnXI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731091AbfA3NQp (ORCPT ); Wed, 30 Jan 2019 08:16:45 -0500 Received: from mail.kernel.org ([198.145.29.99]:45448 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730965AbfA3NQp (ORCPT ); Wed, 30 Jan 2019 08:16:45 -0500 Received: from localhost (5356596B.cm-6-7b.dynamic.ziggo.nl [83.86.89.107]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id B2D6A2184D; Wed, 30 Jan 2019 13:16:43 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548854204; bh=pVdtQmgZOyjcqX8J0FTHIyBrj1qymKuhAfoPYHB/Adc=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=zz3ZR2oU+6u3yZtBGHq9wVo4Q2GX8ULA0F4r95qsN09+B1H04a4aJ2re1MfJuTPUL 9VuYg7KqMG5Byrwil88TdRn71BQ1bXgDlVFvCCJBN/01ouyY2YghYk/H20WHJ5X/Lu 77D4km34mFxLWvZZHUNfA5vgbZyIW53Sik2HJWok= Date: Wed, 30 Jan 2019 14:16:41 +0100 From: Greg KH To: "Li,Rongqing" Cc: "jslaby@suse.com" , "linux-kernel@vger.kernel.org" , "gkohli@codeaurora.org" , linux-serial@vger.kernel.org Subject: Re: =?utf-8?B?562U5aSNOiBbUEFUQ0hdW3Y0?= =?utf-8?Q?=5D?= tty: fix race between flush_to_ldisc and tty_open Message-ID: <20190130131641.GA10795@kroah.com> References: <1547803637-29135-1-git-send-email-lirongqing@baidu.com> <20190130101914.GA30587@kroah.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.11.2 (2019-01-07) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jan 30, 2019 at 12:48:42PM +0000, Li,Rongqing wrote: > > > > -----邮件原件----- > > 发件人: linux-kernel-owner@vger.kernel.org > > [mailto:linux-kernel-owner@vger.kernel.org] 代表 Greg KH > > 发送时间: 2019年1月30日 18:19 > > 收件人: Li,Rongqing > > 抄送: jslaby@suse.com; linux-kernel@vger.kernel.org; gkohli@codeaurora.org > > 主题: Re: [PATCH][v4] tty: fix race between flush_to_ldisc and tty_open > > > > On Fri, Jan 18, 2019 at 05:27:17PM +0800, Li RongQing wrote: > > > There still is a race window after the commit b027e2298bd588 > > > ("tty: fix data race between tty_init_dev and flush of buf"), and we > > > encountered this crash issue if receive_buf call comes before tty > > > initialization completes in n_tty_open and > > > tty->driver_data may be NULL. > > > > > > CPU0 CPU1 > > > ---- ---- > > > n_tty_open > > > tty_init_dev > > > tty_ldisc_unlock > > > schedule flush_to_ldisc > > > receive_buf > > > tty_port_default_receive_buf > > > tty_ldisc_receive_buf > > > n_tty_receive_buf_common > > > __receive_buf > > > uart_flush_chars > > > uart_start > > > /*tty->driver_data is NULL*/ > > > tty->ops->open > > > /*init tty->driver_data*/ > > > > > > it can be fixed by extending ldisc semaphore lock in tty_init_dev to > > > driver_data initialized completely after tty->ops->open(), but this > > > will lead to put lock on one function and unlock in some other > > > function, and hard to maintain, so fix this race only by checking > > > tty->driver_data when receiving, and return if tty->driver_data > > > is NULL > > > > > > Signed-off-by: Wang Li > > > Signed-off-by: Zhang Yu > > > Signed-off-by: Li RongQing > > > --- > > > V4: add version information > > > V3: not used ldisc semaphore lock, only checking tty->driver_data with > > > NULL > > > V2: fix building error by EXPORT_SYMBOL tty_ldisc_unlock > > > V1: extend ldisc lock to protect that tty->driver_data is inited > > > > > > drivers/tty/tty_port.c | 3 +++ > > > 1 file changed, 3 insertions(+) > > > > > > diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c index > > > 044c3cbdcfa4..86d0bec38322 100644 > > > --- a/drivers/tty/tty_port.c > > > +++ b/drivers/tty/tty_port.c > > > @@ -31,6 +31,9 @@ static int tty_port_default_receive_buf(struct tty_port > > *port, > > > if (!tty) > > > return 0; > > > > > > + if (!tty->driver_data) > > > + return 0; > > > + > > > > How is this working? What is setting driver_data to NULL to "stop" this race? > > > > > if tty->driver_data is NULL and return, tty_port_default_receive_buf will not step to > uart_start which access tty->driver_data and trigger panic before tty_open, so it can > fix the system panic > > > There's no requirement that a tty driver set this field to NULL when it is "done" > > with the tty device, so I think you are just getting lucky in that your specific > > driver happens to be doing this. > > > > when tty_open is running, tty is allocated by kzalloc in tty_init_dev which called > by tty_open_by_driver, tty is inited to 0 > > > What driver are you testing this against? > > > > 8250 Ok, as this is specific to the uart core, how about this patch instead: diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c index 5c01bb6d1c24..b56a6250df3f 100644 --- a/drivers/tty/serial/serial_core.c +++ b/drivers/tty/serial/serial_core.c @@ -130,6 +130,9 @@ static void uart_start(struct tty_struct *tty) struct uart_port *port; unsigned long flags; + if (!state) + return; + port = uart_port_lock(state, flags); __uart_start(tty); uart_port_unlock(port, flags);