From patchwork Mon Jan 21 16:26:42 2019 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Greg Kroah-Hartman X-Patchwork-Id: 10774179 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 443D2139A for ; Mon, 21 Jan 2019 16:33:12 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 331FD2A594 for ; Mon, 21 Jan 2019 16:33:12 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 25A792A742; Mon, 21 Jan 2019 16:33:12 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from mother.openwall.net (mother.openwall.net [195.42.179.200]) by mail.wl.linuxfoundation.org (Postfix) with SMTP id 35CDF2A594 for ; Mon, 21 Jan 2019 16:33:10 +0000 (UTC) Received: (qmail 14202 invoked by uid 550); 21 Jan 2019 16:33:08 -0000 Mailing-List: contact kernel-hardening-help@lists.openwall.com; run by ezmlm Precedence: bulk List-Post: List-Help: List-Unsubscribe: List-Subscribe: List-ID: Delivered-To: mailing list kernel-hardening@lists.openwall.com Delivered-To: moderator for kernel-hardening@lists.openwall.com Received: (qmail 7574 invoked from network); 21 Jan 2019 16:26:57 -0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1548088005; bh=ivb6FET/75tes04EFLig2SXP8yPgwTA8LdPoxQ1sc1k=; h=Date:From:To:Cc:Subject:From; b=EIorTJ7LmMh/xD6sndMoVqZxTENVXX5tdxnO5g357ymmuRi13Fvw7m2AFI3labh6c G+2UxsoZ5aG/jcWIiXlijNbvOOVIlgFSSvIl9/gCi11+l2FAywFCPRhTugxnwAEh8S rDTtpKG5wNmSWNhYPQMihNgveXCSInBfzMKfB+sk= Date: Mon, 21 Jan 2019 17:26:42 +0100 From: Greg Kroah-Hartman To: kernel-hardening@lists.openwall.com, linux-kernel@vger.kernel.org, linux-serial@vger.kernel.org Cc: Jiri Slaby Subject: [PATCH] tty: ldisc: add sysctl to prevent autoloading of ldiscs Message-ID: <20190121162642.GA2944@kroah.com> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.11.2 (2019-01-07) X-Virus-Scanned: ClamAV using ClamSMTP By default, the kernel will automatically load the module of any line dicipline that is asked for. As this sometimes isn't the safest thing to do, provide a sysctl to disable this feature. By default, we set this to 'y' as that is the historical way that Linux has worked, and we do not want to break working systems. But in the future, perhaps this can default to 'n' to prevent this functionality. Signed-off-by: Greg Kroah-Hartman Reviewed-by: Theodore Ts'o --- Every once in a while people are really surprised that any line discipline can be loaded by any user. Despite the ability to just disable the modules from the modules.conf file, sometimes it is good to provide protection from within the kernel instead of relying on a module configuration file. So that's why I wrote this patch up. Anyone have any objections to it. It keeps the functionality that we have today by default, but allows admins to disable the auto-loading if they want to at runtime. drivers/tty/Kconfig | 24 +++++++++++++++++++++ drivers/tty/tty_io.c | 3 +++ drivers/tty/tty_ldisc.c | 47 +++++++++++++++++++++++++++++++++++++++++ 3 files changed, 74 insertions(+) diff --git a/drivers/tty/Kconfig b/drivers/tty/Kconfig index 0840d27381ea..e0a04bfc873e 100644 --- a/drivers/tty/Kconfig +++ b/drivers/tty/Kconfig @@ -441,4 +441,28 @@ config VCC depends on SUN_LDOMS help Support for Sun logical domain consoles. + +config LDISC_AUTOLOAD + bool "Automatically load TTY Line Disciplines" + default y + help + Historically the kernel has always automatically loaded any + line discipline that is in a kernel module when a user asks + for it to be loaded with the TIOCSETD ioctl, or through other + means. This is not always the best thing to do on systems + where you know you will not be using some of the more + "ancient" line disciplines, so prevent the kernel from doing + this unless the request is coming from a process with the + CAP_SYS_MODULE permissions. + + Say 'Y' here if you trust your userspace users to do the right + thing, or if you have only provided the line disciplines that + you know you will be using, or if you wish to continue to use + the traditional method of on-demand loading of these modules + by any user. + + This functionality can be changed at runtime with the + dev.tty.ldisc_autoload sysctl, this configuration option will + only set the default value of this functionality. + endif # TTY diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c index 21ffcce16927..5fa250157025 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -513,6 +513,8 @@ static const struct file_operations hung_up_tty_fops = { static DEFINE_SPINLOCK(redirect_lock); static struct file *redirect; +extern void tty_sysctl_init(void); + /** * tty_wakeup - request more data * @tty: terminal @@ -3483,6 +3485,7 @@ void console_sysfs_notify(void) */ int __init tty_init(void) { + tty_sysctl_init(); cdev_init(&tty_cdev, &tty_fops); if (cdev_add(&tty_cdev, MKDEV(TTYAUX_MAJOR, 0), 1) || register_chrdev_region(MKDEV(TTYAUX_MAJOR, 0), 1, "/dev/tty") < 0) diff --git a/drivers/tty/tty_ldisc.c b/drivers/tty/tty_ldisc.c index 45eda69b150c..e38f104db174 100644 --- a/drivers/tty/tty_ldisc.c +++ b/drivers/tty/tty_ldisc.c @@ -156,6 +156,13 @@ static void put_ldops(struct tty_ldisc_ops *ldops) * takes tty_ldiscs_lock to guard against ldisc races */ +#if defined(CONFIG_LDISC_AUTOLOAD) + #define INITIAL_AUTOLOAD_STATE 1 +#else + #define INITIAL_AUTOLOAD_STATE 0 +#endif +static int tty_ldisc_autoload = INITIAL_AUTOLOAD_STATE; + static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc) { struct tty_ldisc *ld; @@ -170,6 +177,8 @@ static struct tty_ldisc *tty_ldisc_get(struct tty_struct *tty, int disc) */ ldops = get_ldops(disc); if (IS_ERR(ldops)) { + if (!capable(CAP_SYS_MODULE) && !tty_ldisc_autoload) + return ERR_PTR(-EPERM); request_module("tty-ldisc-%d", disc); ldops = get_ldops(disc); if (IS_ERR(ldops)) @@ -845,3 +854,41 @@ void tty_ldisc_deinit(struct tty_struct *tty) tty_ldisc_put(tty->ldisc); tty->ldisc = NULL; } + +static int zero; +static int one = 1; +static struct ctl_table tty_table[] = { + { + .procname = "ldisc_autoload", + .data = &tty_ldisc_autoload, + .maxlen = sizeof(tty_ldisc_autoload), + .mode = 0644, + .proc_handler = proc_dointvec, + .extra1 = &zero, + .extra2 = &one, + }, + { } +}; + +static struct ctl_table tty_dir_table[] = { + { + .procname = "tty", + .mode = 0555, + .child = tty_table, + }, + { } +}; + +static struct ctl_table tty_root_table[] = { + { + .procname = "dev", + .mode = 0555, + .child = tty_dir_table, + }, + { } +}; + +void tty_sysctl_init(void) +{ + register_sysctl_table(tty_root_table); +}