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authorJann Horn <jannh@google.com>2020-12-03 04:25:05 +0300
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2020-12-04 19:39:58 +0300
commitc8bcd9c5be24fb9e6132e97da5a35e55a83e36b9 (patch)
tree1b7e3191d3fd63c02d3029b19e45d88d322397df /drivers/tty
parent54ffccbf053b5b6ca4f6e45094b942fab92a25fc (diff)
downloadlinux-c8bcd9c5be24fb9e6132e97da5a35e55a83e36b9.tar.xz
tty: Fix ->session locking
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>
Diffstat (limited to 'drivers/tty')
-rw-r--r--drivers/tty/tty_io.c7
-rw-r--r--drivers/tty/tty_jobctrl.c44
2 files changed, 37 insertions, 14 deletions
diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index 9f8b9a567b35..56ade99ef99f 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -2897,10 +2897,14 @@ 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);
@@ -2932,6 +2936,7 @@ void __do_SAK(struct tty_struct *tty)
task_unlock(p);
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
+ put_pid(session);
#endif
}
diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index baadeea4a289..aa6d0537b379 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -103,8 +103,8 @@ static void __proc_set_tty(struct tty_struct *tty)
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 *tty, struct tty_struct *real_tty, pid_t
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 *tty, struct tty_struct *real_tty, pid_t
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;
}
/*