On 5/18/17 9:31 AM, Greg KH wrote:

On Fri, May 05, 2017 at 07:20:18PM -0400, Matt Brown wrote:

quoted

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:

quoted

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 <redacted>
Signed-off-by: Matt Brown <redacted>
---
 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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