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

Signed-off-by: Matt Brown <redacted>
---
 drivers/tty/tty_io.c | 1 +
 include/linux/tty.h  | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index e6d1a65..e774385 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c

@@ -3191,6 +3191,7 @@ 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 = current_user_ns();