[PATCH 5.4 12/54] tty: Fix ->session locking

Greg Kroah-Hartman
2020-12-10T14:39:49Z

From: Jann Horn <jannh@google.com>

commit c8bcd9c5be24fb9e6132e97da5a35e55a83e36b9 upstream.

Currently, locking of ->session is very inconsistent; most places
protect it using the legacy tty mutex, but disassociate_ctty(),
__do_SAK(), tiocspgrp() and tiocgsid() don't.
Two of the writers hold the ctrl_lock (because they already need it for
->pgrp), but __proc_set_tty() doesn't do that yet.

On a PREEMPT=y system, an unprivileged user can theoretically abuse
this broken locking to read 4 bytes of freed memory via TIOCGSID if
tiocgsid() is preempted long enough at the right point. (Other things
might also go wrong, especially if root-only ioctls are involved; I'm
not sure about that.)

Change the locking on ->session such that:

- tty_lock() is held by all writers: By making disassociate_ctty()
hold it. This should be fine because the same lock can already be
taken through the call to tty_vhangup_session().
The tricky part is that we need to shorten the area covered by
siglock to be able to take tty_lock() without ugly retry logic; as
far as I can tell, this should be fine, since nothing in the
signal_struct is touched in the `if (tty)` branch.
- ctrl_lock is held by all writers: By changing __proc_set_tty() to
hold the lock a little longer.
- All readers that aren't holding tty_lock() hold ctrl_lock: By
adding locking to tiocgsid() and __do_SAK(), and expanding the area
covered by ctrl_lock in tiocspgrp().

Cc: stable@kernel.org
Signed-off-by: Jann Horn <jannh@google.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/tty/tty_io.c | 7 ++++++-
drivers/tty/tty_jobctrl.c | 44 +++++++++++++++++++++++++++++++-------------
include/linux/tty.h | 4 ++++
3 files changed, 41 insertions(+), 14 deletions(-)

--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2894,10 +2894,14 @@ void __do_SAK(struct tty_struct *tty)
struct task_struct *g, *p;
struct pid *session;
int i;
+ unsigned long flags;

if (!tty)
return;
- session = tty->session;
+
+ spin_lock_irqsave(&tty->ctrl_lock, flags);
+ session = get_pid(tty->session);
+ spin_unlock_irqrestore(&tty->ctrl_lock, flags);

tty_ldisc_flush(tty);

@@ -2929,6 +2933,7 @@ void __do_SAK(struct tty_struct *tty)
task_unlock(p);
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
+ put_pid(session);
#endif
}

--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -103,8 +103,8 @@ static void __proc_set_tty(struct tty_st
put_pid(tty->session);
put_pid(tty->pgrp);
tty->pgrp = get_pid(task_pgrp(current));
- spin_unlock_irqrestore(&tty->ctrl_lock, flags);
tty->session = get_pid(task_session(current));
+ spin_unlock_irqrestore(&tty->ctrl_lock, flags);
if (current->signal->tty) {
tty_debug(tty, "current tty %s not NULL!!\n",
current->signal->tty->name);
@@ -293,20 +293,23 @@ void disassociate_ctty(int on_exit)
spin_lock_irq(&current->sighand->siglock);
put_pid(current->signal->tty_old_pgrp);
current->signal->tty_old_pgrp = NULL;
-
tty = tty_kref_get(current->signal->tty);
+ spin_unlock_irq(&current->sighand->siglock);
+
if (tty) {
unsigned long flags;
+
+ tty_lock(tty);
spin_lock_irqsave(&tty->ctrl_lock, flags);
put_pid(tty->session);
put_pid(tty->pgrp);
tty->session = NULL;
tty->pgrp = NULL;
spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+ tty_unlock(tty);
tty_kref_put(tty);
}

- spin_unlock_irq(&current->sighand->siglock);
/* Now clear signal->tty under the lock */
read_lock(&tasklist_lock);
session_clear_tty(task_session(current));
@@ -477,14 +480,19 @@ static int tiocspgrp(struct tty_struct *
return -ENOTTY;
if (retval)
return retval;
- if (!current->signal->tty ||
- (current->signal->tty != real_tty) ||
- (real_tty->session != task_session(current)))
- return -ENOTTY;
+
if (get_user(pgrp_nr, p))
return -EFAULT;
if (pgrp_nr < 0)
return -EINVAL;
+
+ spin_lock_irq(&real_tty->ctrl_lock);
+ if (!current->signal->tty ||
+ (current->signal->tty != real_tty) ||
+ (real_tty->session != task_session(current))) {
+ retval = -ENOTTY;
+ goto out_unlock_ctrl;
+ }
rcu_read_lock();
pgrp = find_vpid(pgrp_nr);
retval = -ESRCH;
@@ -494,12 +502,12 @@ static int tiocspgrp(struct tty_struct *
if (session_of_pgrp(pgrp) != task_session(current))
goto out_unlock;
retval = 0;
- spin_lock_irq(&real_tty->ctrl_lock);
put_pid(real_tty->pgrp);
real_tty->pgrp = get_pid(pgrp);
- spin_unlock_irq(&real_tty->ctrl_lock);
out_unlock:
rcu_read_unlock();
+out_unlock_ctrl:
+ spin_unlock_irq(&real_tty->ctrl_lock);
return retval;
}

@@ -511,20 +519,30 @@ out_unlock:
*
* Obtain the session id of the tty. If there is no session
* return an error.
- *
- * Locking: none. Reference to current->signal->tty is safe.
*/
static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
+ unsigned long flags;
+ pid_t sid;
+
/*
* (tty == real_tty) is a cheap way of
* testing if the tty is NOT a master pty.
*/
if (tty == real_tty && current->signal->tty != real_tty)
return -ENOTTY;
+
+ spin_lock_irqsave(&real_tty->ctrl_lock, flags);
if (!real_tty->session)
- return -ENOTTY;
- return put_user(pid_vnr(real_tty->session), p);
+ goto err;
+ sid = pid_vnr(real_tty->session);
+ spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
+
+ return put_user(sid, p);
+
+err:
+ spin_unlock_irqrestore(&real_tty->ctrl_lock, flags);
+ return -ENOTTY;
}

/*
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -306,6 +306,10 @@ struct tty_struct {
struct termiox *termiox; /* May be NULL for unsupported */
char name[64];
struct pid *pgrp; /* Protected by ctrl lock */
+ /*
+ * Writes protected by both ctrl lock and legacy mutex, readers must use
+ * at least one of them.
+ */
struct pid *session;
unsigned long flags;
int count;