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=-15.3 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,INVALID_DATE_TZ_ABSURD,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,URIBL_BLOCKED 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 77150C48BDF for ; Thu, 24 Jun 2021 11:12:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 613AA613CE for ; Thu, 24 Jun 2021 11:12:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232467AbhFXLOZ (ORCPT ); Thu, 24 Jun 2021 07:14:25 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:44090 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232429AbhFXLOM (ORCPT ); Thu, 24 Jun 2021 07:14:12 -0400 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1624533112; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=e+YEXzJlGfG0hu3d0X2DfAVFyIJLvZiTDn2Uh6d5r70=; b=Jlu0jTJtfR592f90KKjCJZ/7l6QnnoTzS2nosqX85JewLe8Tb120FxBo84qqFwHHmFr3T9 eXlsO4mIkctDRAs3NzvmWQ0a/2+/KxbjeD3iKCyJz7ZOWXRoAZ/hp7TgOfqx4aEoV3oP07 yukAt6DHtbZT07sAjEUECzyXFM/I/fdIuJSOzIao69HE/ddVbWdoGHyk3J3FHb20Wh6ST+ 3QpT2w6e5bd7FNmyRb3wWgbwGfODwCJH18Qyv/v1p3SvUlh+r/rYpZXK2h9H/8K2gw45AU 0sVHR1nwl/1lOmhyto9MuooFxb80NTRcOtoFeT3Oa8pmq6yDkitavjdABY8CtQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1624533112; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=e+YEXzJlGfG0hu3d0X2DfAVFyIJLvZiTDn2Uh6d5r70=; b=0ZbYGpFfO+4cfCc9g02bGA2DWWs5EplFt4aNq+AHHbTLR6UEVa45TceufUsm2+iseDpSAx PTIUXt9HjY0RVmDA== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH printk v3 6/6] printk: syslog: close window between wait and read Date: Thu, 24 Jun 2021 13:17:48 +0206 Message-Id: <20210624111148.5190-7-john.ogness@linutronix.de> In-Reply-To: <20210624111148.5190-1-john.ogness@linutronix.de> References: <20210624111148.5190-1-john.ogness@linutronix.de> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Syslog's SYSLOG_ACTION_READ is supposed to block until the next syslog record can be read, and then it should read that record. However, because @syslog_lock is not held between waking up and reading the record, another reader could read the record first, thus causing SYSLOG_ACTION_READ to return with a value of 0, never having read _anything_. By holding @syslog_lock between waking up and reading, it can be guaranteed that SYSLOG_ACTION_READ blocks until it successfully reads a syslog record (or a real error occurs). Signed-off-by: John Ogness --- kernel/printk/printk.c | 50 +++++++++++++++++++++++++++++++----------- 1 file changed, 37 insertions(+), 13 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 90954cb5a0ab..4737804d6c6d 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1486,6 +1486,7 @@ static int syslog_print(char __user *buf, int size) struct printk_record r; char *text; int len = 0; + u64 seq; text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL); if (!text) @@ -1493,11 +1494,38 @@ static int syslog_print(char __user *buf, int size) prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX); - while (size > 0) { + /* Get a consistent copy of @syslog_seq. */ + mutex_lock(&syslog_lock); + seq = syslog_seq; + mutex_unlock(&syslog_lock); + + /* Wait for the @syslog_seq record to be available. */ + for (;;) { + len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL)); + if (len) + goto out; + + /* + * @syslog_seq may have changed while waiting. If so, wait + * for the new @syslog_seq record. + */ + + mutex_lock(&syslog_lock); + if (syslog_seq == seq) + break; + seq = syslog_seq; + mutex_unlock(&syslog_lock); + } + + /* + * @syslog_lock is held when entering the read loop to prevent + * another reader from modifying @syslog_seq. + */ + + for (;;) { size_t n; size_t skip; - mutex_lock(&syslog_lock); if (!prb_read_valid(prb, syslog_seq, &r)) { mutex_unlock(&syslog_lock); break; @@ -1542,8 +1570,13 @@ static int syslog_print(char __user *buf, int size) len += n; size -= n; buf += n; - } + if (!size) + break; + + mutex_lock(&syslog_lock); + } +out: kfree(text); return len; } @@ -1614,7 +1647,6 @@ int do_syslog(int type, char __user *buf, int len, int source) bool clear = false; static int saved_console_loglevel = LOGLEVEL_DEFAULT; int error; - u64 seq; error = check_syslog_permissions(type, source); if (error) @@ -1632,15 +1664,6 @@ int do_syslog(int type, char __user *buf, int len, int source) return 0; if (!access_ok(buf, len)) return -EFAULT; - - /* Get a consistent copy of @syslog_seq. */ - mutex_lock(&syslog_lock); - seq = syslog_seq; - mutex_unlock(&syslog_lock); - - error = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL)); - if (error) - return error; error = syslog_print(buf, len); break; /* Read/clear last kernel messages */ @@ -1707,6 +1730,7 @@ int do_syslog(int type, char __user *buf, int len, int source) } else { bool time = syslog_partial ? syslog_time : printk_time; unsigned int line_count; + u64 seq; prb_for_each_info(syslog_seq, prb, seq, &info, &line_count) { -- 2.20.1