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Date: Mon, 24 Apr 2017 08:59:07 -0500
From: "Serge E. Hallyn" <serge@...lyn.com>
To: Matt Brown <matt@...tt.com>
Cc: serge@...lyn.com, jmorris@...ei.org, gregkh@...uxfoundation.org,
jslaby@...e.com, corbet@....net, keescook@...omium.org,
akpm@...ux-foundation.org, jannh@...gle.com,
kernel-hardening@...ts.openwall.com,
linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org
Subject: Re: [PATCH v4 2/2] tiocsti-restrict : make TIOCSTI ioctl require
CAP_SYS_ADMIN
Quoting Matt Brown (matt@...tt.com):
> This introduces the tiocsti_restrict sysctl, whose default is controlled via
> CONFIG_SECURITY_TIOCSTI_RESTRICT. When activated, this control restricts
> all TIOCSTI ioctl calls from non CAP_SYS_ADMIN users.
>
> This patch depends on patch 1/2
>
> This patch was inspired from GRKERNSEC_HARDEN_TTY.
>
> This patch would have prevented
> https://bugzilla.redhat.com/show_bug.cgi?id=1411256 under the following
> conditions:
> * non-privileged container
> * container run inside new user namespace
>
> Possible effects on userland:
>
> There could be a few user programs that would be effected by this
> change.
> See: <https://codesearch.debian.net/search?q=ioctl%5C%28.*TIOCSTI>
> 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
> 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:
> <http://www.halfdog.net/Security/2012/TtyPushbackPrivilegeEscalation/>
>
> When user namespaces are in use, the check for the capability
> CAP_SYS_ADMIN is done against the user namespace that originally opened
> the tty.
>
> Signed-off-by: Matt Brown <matt@...tt.com>
Acked-by: Serge Hallyn <serge@...lyn.com>
(typo below)
> ---
> Documentation/sysctl/kernel.txt | 21 +++++++++++++++++++++
> drivers/tty/tty_io.c | 6 ++++++
> include/linux/tty.h | 2 ++
> kernel/sysctl.c | 12 ++++++++++++
> security/Kconfig | 13 +++++++++++++
> 5 files changed, 54 insertions(+)
>
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index bac23c1..c15c660 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -89,6 +89,7 @@ show up in /proc/sys/kernel:
> - sysctl_writes_strict
> - tainted
> - threads-max
> +- tiocsti_restrict
> - unknown_nmi_panic
> - watchdog
> - watchdog_thresh
> @@ -987,6 +988,26 @@ available RAM pages threads-max is reduced accordingly.
>
> ==============================================================
>
> +tiocsti_restrict:
> +
> +This toggle indicates whether unprivileged users are prevented
> +from using the TIOCSTI ioctl to inject commands into otherprocesses
space between other processes
> +which share a tty session.
> +
> +When tiocsti_restrict is set to (0) there are no restrictions(accept
> +the default restriction of only being able to injection commands into
> +one's own tty). When tiocsti_restrict is set to (1), users must
> +have CAP_SYS_ADMIN to use the TIOCSTI ioctl.
> +
> +When user namespaces are in use, the check for the capability
> +CAP_SYS_ADMIN is done against the user namespace that originally
> +opened the tty.
> +
> +The kernel config option CONFIG_SECURITY_TIOCSTI_RESTRICT sets the
> +default value of tiocsti_restrict.
> +
> +==============================================================
> +
> unknown_nmi_panic:
>
> The value in this file affects behavior of handling NMI. When the
> diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
> index c276814..fe68d14 100644
> --- a/drivers/tty/tty_io.c
> +++ b/drivers/tty/tty_io.c
> @@ -2297,11 +2297,17 @@ 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 && !ns_capable(tty->owner_user_ns,CAP_SYS_ADMIN)) {
> + pr_warn_ratelimited("TIOCSTI ioctl call blocked for non-privileged process\n");
> + return -EPERM;
> + }
> 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 d902d42..2fd7f49 100644
> --- a/include/linux/tty.h
> +++ b/include/linux/tty.h
> @@ -344,6 +344,8 @@ 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 <linux/kexec.h>
> #include <linux/bpf.h>
> #include <linux/mount.h>
> +#include <linux/tty.h>
>
> #include <linux/uaccess.h>
> #include <asm/processor.h>
> @@ -833,6 +834,17 @@ static struct ctl_table kern_table[] = {
> .extra2 = &two,
> },
> #endif
> +#if defined CONFIG_TTY
> + {
> + .procname = "tiocsti_restrict",
> + .data = &tiocsti_restrict,
> + .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
> + 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
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