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 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 303CAC6377A for ; Thu, 15 Jul 2021 19:52:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 20D54613C4 for ; Thu, 15 Jul 2021 19:52:36 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1345559AbhGOTyy (ORCPT ); Thu, 15 Jul 2021 15:54:54 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:42388 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S245474AbhGOTg7 (ORCPT ); Thu, 15 Jul 2021 15:36:59 -0400 From: John Ogness DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1626377644; 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=zgaJtSBLF4nBRlm25z1Xt7502dBDKcFRJ4X31C1mM5s=; b=eJkvKqnKfND+7+60SM5Qw8K6Xoqcv9BwQ4nVMkXEv7ae17tWLl2wWFanTP7CRNIbNicAwc PTlUygsGHpNl9hgGFJyucA0oqSmESSaKTC5l4wdH5GMKbLalJ6fNzZtlpTEpmmSOIrbmlf 7tsLBAIRi20+oftAiCD8mefbEZqeiwQ4HtHSpzRcbrh/OFkyDwfBgU6uu6LtuLPDh1Hmxf WrGa8ql+IGzEyT3a9AYwnTbC3Umjs2MVLLlPzdqceAo/OYXxt0e79JNjayxwdpG3G/i8pZ d5PKDuS+LuonKCllr/YtDTdOncrsKbZfMFnM8SrzdOB/rANcCBbqrhRoCNTJAA== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1626377644; 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=zgaJtSBLF4nBRlm25z1Xt7502dBDKcFRJ4X31C1mM5s=; b=27dl5hFdon8v+HG4rLKWf+PTckeEUKpkPpvaoTnRd7OeAmbRWxYsEyq5rzstjvfaMyKACB nizmuswaVih2/NAQ== To: Petr Mladek Cc: Sergey Senozhatsky , Steven Rostedt , Thomas Gleixner , linux-kernel@vger.kernel.org Subject: [PATCH printk v4 6/6] printk: syslog: close window between wait and read Date: Thu, 15 Jul 2021 21:39:59 +0206 Message-Id: <20210715193359.25946-7-john.ogness@linutronix.de> In-Reply-To: <20210715193359.25946-1-john.ogness@linutronix.de> References: <20210715193359.25946-1-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 | 55 +++++++++++++++++++++++++++--------------- 1 file changed, 36 insertions(+), 19 deletions(-) diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c index 99160d0dafd6..d11612687323 100644 --- a/kernel/printk/printk.c +++ b/kernel/printk/printk.c @@ -1480,12 +1480,14 @@ static u64 find_first_fitting_seq(u64 start_seq, u64 max_seq, size_t size, return seq; } +/* The caller is responsible for making sure @size is greater than 0. */ static int syslog_print(char __user *buf, int size) { struct printk_info info; struct printk_record r; char *text; int len = 0; + u64 seq; text = kmalloc(CONSOLE_LOG_MAX, GFP_KERNEL); if (!text) @@ -1493,15 +1495,35 @@ static int syslog_print(char __user *buf, int size) prb_rec_init_rd(&r, &info, text, CONSOLE_LOG_MAX); - while (size > 0) { + mutex_lock(&syslog_lock); + + /* + * Wait for the @syslog_seq record to be available. @syslog_seq may + * change while waiting. + */ + do { + seq = syslog_seq; + + mutex_unlock(&syslog_lock); + len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL)); + mutex_lock(&syslog_lock); + + if (len) + goto out; + } while (syslog_seq != seq); + + /* + * Copy records that fit into the buffer. The above cycle makes sure + * that the first record is always available. + */ + do { size_t n; size_t skip; + int err; - mutex_lock(&syslog_lock); - if (!prb_read_valid(prb, syslog_seq, &r)) { - mutex_unlock(&syslog_lock); + if (!prb_read_valid(prb, syslog_seq, &r)) break; - } + if (r.info->seq != syslog_seq) { /* message is gone, move to next valid one */ syslog_seq = r.info->seq; @@ -1528,12 +1550,15 @@ static int syslog_print(char __user *buf, int size) syslog_partial += n; } else n = 0; - mutex_unlock(&syslog_lock); if (!n) break; - if (copy_to_user(buf, text + skip, n)) { + mutex_unlock(&syslog_lock); + err = copy_to_user(buf, text + skip, n); + mutex_lock(&syslog_lock); + + if (err) { if (!len) len = -EFAULT; break; @@ -1542,8 +1567,9 @@ static int syslog_print(char __user *buf, int size) len += n; size -= n; buf += n; - } - + } while (size); +out: + mutex_unlock(&syslog_lock); kfree(text); return len; } @@ -1614,7 +1640,6 @@ 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 +1657,6 @@ 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 +1723,7 @@ 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