| Message ID | 20170505232018.28846-3-matt@nmatt.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Fri, May 05, 2017 at 07:20:18PM -0400, Matt Brown wrote: > 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. > > Acked-by: Serge Hallyn <serge@hallyn.com> > Reviewed-by: Kees Cook <keescook@chromium.org> > Signed-off-by: Matt Brown <matt@nmatt.com> > --- > 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..f7985cf 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 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; Always follow the proper kernel coding style rules, as I don't want to have someone else have to come along and fix up the error you have added here :( checkpatch.pl is your friend, really... And why not do a warning with the device that caused the problem to happen? dev_warn has a ratelimit I think right? "raw" printk messages like this don't help in trying to track down what/who caused the issue. And finally, can userspace see the namespace for the tty? Doesn't things like checkpoint/restore need that in order to properly set the tty connection back up when moving processes? v7? :) thanks, greg k-h
On 5/18/17 9:31 AM, Greg KH wrote: > On Fri, May 05, 2017 at 07:20:18PM -0400, Matt Brown wrote: >> 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. >> >> Acked-by: Serge Hallyn <serge@hallyn.com> >> Reviewed-by: Kees Cook <keescook@chromium.org> >> Signed-off-by: Matt Brown <matt@nmatt.com> >> --- >> 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..f7985cf 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 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; > > Always follow the proper kernel coding style rules, as I don't want to > have someone else have to come along and fix up the error you have added > here :( > > checkpatch.pl is your friend, really... > My bad. Will fix these issues in v7. > And why not do a warning with the device that caused the problem to > happen? dev_warn has a ratelimit I think right? "raw" printk messages > like this don't help in trying to track down what/who caused the issue. > yes <linux/device.h> has dev_warn_ratelimited. I will use that in 7v. > And finally, can userspace see the namespace for the tty? Doesn't > things like checkpoint/restore need that in order to properly set the > tty connection back up when moving processes? This seems like we would need to expose the owner_user_ns of the tty in procfs somewhere. Section 1.7 Documentation/filesystems/proc.txt describes the following files in /proc/tty: Table 1-11: Files in /proc/tty .............................................................................. File Content drivers list of drivers and their usage ldiscs registered line disciplines driver/serial usage statistic and status of single tty lines .............................................................................. The drivers file is the one that gives the most information that we are interested in. However, the current layout combines information about multiple ttys by driver. As I understand it, a single driver may have ttys that span across different owner_user_ns. would it make sense to add a file /proc/tty/ns that would contain the different tty to user namespace mappings? Or is there a better way to do this? I would appreciate any feedback/ideas you have on this. > > v7? :) v7 will be on its way soon. I'm not currently sure how to address the concern of giving things like checkpoint/restore in userland a way to get the owner_user_ns. > > thanks, > > greg k-h > Thanks for the feedback, Matt Brown
diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt index bac23c1..f7985cf 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 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