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 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@xxxxxxxxx> --- 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 +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 e774385..2e15dca 100644 --- a/drivers/tty/tty_io.c +++ b/drivers/tty/tty_io.c @@ -2296,11 +2296,17 @@ 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 -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
