Re: [PATCH v6 2/2] security: tty: make TIOCSTI ioctl require CAP_SYS_ADMIN

[Greg KH]

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@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> 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..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