Messages in this thread | | | Subject | Re: [PATCH v6 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN | From | Matt Brown <> | Date | Fri, 19 May 2017 00:51:29 -0400 |
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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
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