Re: [PATCH v4 2/2] tiocsti-restrict : make TIOCSTI ioctl require CAP_SYS_ADMIN

[Serge E. Hallyn]

Quoting Matt Brown (m...@nmatt.com):
> 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: 
> 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:
> 
> 
> 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.
> 
> Signed-off-by: Matt Brown 

Acked-by: Serge Hallyn 

(typo below)

> ---
>  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..c15c660 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 otherprocesses

space between 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

[PATCH v4 2/2] tiocsti-restrict : make TIOCSTI ioctl require CAP_SYS_ADMIN

[Matt Brown]

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: 
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:


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.

Signed-off-by: Matt Brown 
---
 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..c15c660 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 otherprocesses
+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/s