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[PATCH v7 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

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  [PATCH v7 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN Matt Brown <matt@nmatt.com> - 2017-05-29 23:40 +0200
    [PATCH v7 1/2] security: tty: Add owner user namespace to tty_struct Matt Brown <matt@nmatt.com> - 2017-05-29 23:40 +0200

#1652726 — [PATCH v7 0/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

This patchset 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.

# Integration with CRIU:
* The CRIU dev team was contacted about any possible issues that 
  restrict_tiocsti might cause for the CRIU program. Their response was that
  CRIU currently does not utilize the tiocsti ioctl, so it doesn't affect
  them at the moment. They do wish in the future to do checks against
  owner_user_ns, so we will work together on a way to make this information
  available to userspace. In the mean time, The CRIU team is OK with this
  patch moving forward as is.

# Changes since v6:
* fixed style issues
* changed error message to use dev_warn_ratelimited

# Changes since v5:
* added acks/reviews

# Changes since v4:
* fixed typo

# Changes since v3:
* use get_user_ns and put_user_ns to take and drop references to the owner
  user namespace because CONFIG_USER_NS is an option

# Changes since v2:
* take/drop reference to user namespace on tty struct alloc/free to prevent
  use-after-free.

# Changes since v1:
* added owner_user_ns to tty_struct to enable capability checks against
  the namespace that created the tty.
* rewording in different places to make patchset purpose clear
* Added Documentation

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#1652727 — [PATCH v7 1/2] security: tty: Add owner user namespace to tty_struct

This patch adds struct user_namespace *owner_user_ns to the tty_struct.
Then it is set to current_user_ns() in the alloc_tty_struct function.

This is done to facilitate capability checks against the original user
namespace that allocated the tty.

E.g. ns_capable(tty->owner_user_ns,CAP_SYS_ADMIN)

This combined with the use of user namespace's will allow hardening
protections to be built to mitigate container escapes that utilize TTY
ioctls such as TIOCSTI.

See: https://bugzilla.redhat.com/show_bug.cgi?id=1411256

Acked-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Matt Brown <matt@nmatt.com>
---
 drivers/tty/tty_io.c | 2 ++
 include/linux/tty.h  | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e6d1a65..c276814 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -171,6 +171,7 @@ static void free_tty_struct(struct tty_struct *tty)
 	put_device(tty->dev);
 	kfree(tty->write_buf);
 	tty->magic = 0xDEADDEAD;
+	put_user_ns(tty->owner_user_ns);
 	kfree(tty);
 }
 
@@ -3191,6 +3192,7 @@ struct tty_struct *alloc_tty_struct(struct tty_driver *driver, int idx)
 	tty->index = idx;
 	tty_line_name(driver, idx, tty->name);
 	tty->dev = tty_get_device(tty);
+	tty->owner_user_ns = get_user_ns(current_user_ns());
 
 	return tty;
 }
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 1017e904..d902d42 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -12,6 +12,7 @@
 #include <uapi/linux/tty.h>
 #include <linux/rwsem.h>
 #include <linux/llist.h>
+#include <linux/user_namespace.h>
 
 
 /*
@@ -333,6 +334,7 @@ struct tty_struct {
 	/* If the tty has a pending do_SAK, queue it here - akpm */
 	struct work_struct SAK_work;
 	struct tty_port *port;
+	struct user_namespace *owner_user_ns;
 };
 
 /* Each of a tty's open files has private_data pointing to tty_file_private */
-- 
2.10.2

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