From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759651AbdDSFU0 (ORCPT ); Wed, 19 Apr 2017 01:20:26 -0400 Received: from mail-io0-f173.google.com ([209.85.223.173]:33021 "EHLO mail-io0-f173.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1759579AbdDSFUW (ORCPT ); Wed, 19 Apr 2017 01:20:22 -0400 MIME-Version: 1.0 In-Reply-To: <20170419045813.GA17990@mail.hallyn.com> References: <20170419034526.18565-1-matt@nmatt.com> <20170419045813.GA17990@mail.hallyn.com> From: Kees Cook Date: Tue, 18 Apr 2017 22:20:20 -0700 X-Google-Sender-Auth: iKcuAbZCMk2Py8lRE_3BYVV0FZM Message-ID: Subject: Re: [PATCH] make TIOCSTI ioctl require CAP_SYS_ADMIN To: "Serge E. Hallyn" , Matt Brown Cc: James Morris , Greg KH , Jiri Slaby , Andrew Morton , Jann Horn , "kernel-hardening@lists.openwall.com" , linux-security-module , LKML Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Apr 18, 2017 at 9:58 PM, Serge E. Hallyn wrote: > On Tue, Apr 18, 2017 at 11:45:26PM -0400, Matt Brown wrote: >> This patch reproduces GRKERNSEC_HARDEN_TTY functionality from the grsecurity >> project in-kernel. >> >> This will create the Kconfig SECURITY_TIOCSTI_RESTRICT and the corresponding >> sysctl kernel.tiocsti_restrict that, when activated, restrict all TIOCSTI >> ioctl calls from non CAP_SYS_ADMIN users. >> >> Possible effects on userland: >> >> There could be a few user programs that would be effected by this >> change. >> See: >> notable programs are: agetty, csh, xemacs and tcsh >> >> However, I still believe that this change is worth it given that the >> Kconfig defaults to n. This will be a feature that is turned on for the > > It's not worthless, but note that for instance before this was fixed > in lxc, this patch would not have helped with escapes from privileged > containers. > >> same reason that people activate it when using grsecurity. Users of this >> opt-in feature will realize that they are choosing security over some OS >> features like unprivileged TIOCSTI ioctls, as should be clear in the >> Kconfig help message. >> >> Threat Model/Patch Rational: >> >> >From grsecurity's config for GRKERNSEC_HARDEN_TTY. >> >> | There are very few legitimate uses for this functionality and it >> | has made vulnerabilities in several 'su'-like programs possible in >> | the past. Even without these vulnerabilities, it provides an >> | attacker with an easy mechanism to move laterally among other >> | processes within the same user's compromised session. >> >> So if one process within a tty session becomes compromised it can follow >> that additional processes, that are thought to be in different security >> boundaries, can be compromised as a result. When using a program like su >> or sudo, these additional processes could be in a tty session where TTY file >> descriptors are indeed shared over privilege boundaries. >> >> This is also an excellent writeup about the issue: >> >> >> Signed-off-by: Matt Brown Thanks for working on this! I think it'll be nice to have available. >> --- >> drivers/tty/tty_io.c | 4 ++++ >> include/linux/tty.h | 2 ++ >> kernel/sysctl.c | 12 ++++++++++++ >> security/Kconfig | 13 +++++++++++++ >> 4 files changed, 31 insertions(+) >> >> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c >> index e6d1a65..31894e8 100644 >> --- a/drivers/tty/tty_io.c >> +++ b/drivers/tty/tty_io.c >> @@ -2296,11 +2296,15 @@ static int tty_fasync(int fd, struct file *filp, int on) >> * FIXME: may race normal receive processing >> */ >> >> +int tiocsti_restrict = IS_ENABLED(CONFIG_SECURITY_TIOCSTI_RESTRICT); >> + >> static int tiocsti(struct tty_struct *tty, char __user *p) >> { >> char ch, mbz = 0; >> struct tty_ldisc *ld; >> >> + if (tiocsti_restrict && !capable(CAP_SYS_ADMIN)) >> + return -EPERM; I wonder if it might be worth adding a pr_warn_ratelimited() here to help people identify either programs that want to use this feature or actual attacks? >> if ((current->signal->tty != tty) && !capable(CAP_SYS_ADMIN)) >> return -EPERM; >> if (get_user(ch, p)) >> diff --git a/include/linux/tty.h b/include/linux/tty.h >> index 1017e904..7011102 100644 >> --- a/include/linux/tty.h >> +++ b/include/linux/tty.h >> @@ -342,6 +342,8 @@ struct tty_file_private { >> struct list_head list; >> }; >> >> +extern int tiocsti_restrict; >> + >> /* tty magic number */ >> #define TTY_MAGIC 0x5401 >> >> diff --git a/kernel/sysctl.c b/kernel/sysctl.c >> index acf0a5a..68d1363 100644 >> --- a/kernel/sysctl.c >> +++ b/kernel/sysctl.c >> @@ -67,6 +67,7 @@ >> #include >> #include >> #include >> +#include >> >> #include >> #include >> @@ -833,6 +834,17 @@ static struct ctl_table kern_table[] = { >> .extra2 = &two, >> }, >> #endif >> +#if defined CONFIG_TTY >> + { >> + .procname = "tiocsti_restrict", >> + .data = &tiocsti_restrict, Since this is a new sysctl, it'll need to get documented in Documentation/sysctl/kernel.txt as part of this patch. >> + .maxlen = sizeof(int), >> + .mode = 0644, >> + .proc_handler = proc_dointvec_minmax_sysadmin, >> + .extra1 = &zero, >> + .extra2 = &one, >> + }, >> +#endif >> { >> .procname = "ngroups_max", >> .data = &ngroups_max, >> diff --git a/security/Kconfig b/security/Kconfig >> index 3ff1bf9..7d13331 100644 >> --- a/security/Kconfig >> +++ b/security/Kconfig >> @@ -18,6 +18,19 @@ config SECURITY_DMESG_RESTRICT >> >> If you are unsure how to answer this question, answer N. >> >> +config SECURITY_TIOCSTI_RESTRICT > > This is an odd way to name this. Shouldn't the name reflect that it > is setting the default, rather than enabling the feature? This is modeled after SECURITY_DMESG_RESTRICT. I think the Kconfig name is fine (given the other one), but it'd be worth maybe reorganizing the commit message to say "this introduces tiocsti_restrict sysctl, whose default is controlled via CONFIG_SECURITY_TIOCSTI_RESTRICT" or similar. Right now the commit message doesn't, I don't think, make clear what the config does. > > Besides that, I'm ok with the patch. > >> + bool "Restrict unprivileged use of tiocsti command injection" >> + default n >> + help >> + This enforces restrictions on unprivileged users injecting commands >> + into other processes which share a tty session using the TIOCSTI >> + ioctl. This option makes TIOCSTI use require CAP_SYS_ADMIN. >> + >> + If this option is not selected, no restrictions will be enforced >> + unless the tiocsti_restrict sysctl is explicitly set to (1). >> + >> + If you are unsure how to answer this question, answer N. >> + >> config SECURITY >> bool "Enable different security models" >> depends on SYSFS >> -- >> 2.10.2 -Kees -- Kees Cook Pixel Security