diff options
author | Jann Horn <jannh@google.com> | 2020-12-03 04:25:05 +0300 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2020-12-04 19:39:58 +0300 |
commit | c8bcd9c5be24fb9e6132e97da5a35e55a83e36b9 (patch) | |
tree | 1b7e3191d3fd63c02d3029b19e45d88d322397df /drivers/tty | |
parent | 54ffccbf053b5b6ca4f6e45094b942fab92a25fc (diff) | |
download | linux-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.c | 7 | ||||
-rw-r--r-- | drivers/tty/tty_jobctrl.c | 44 |
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(¤t->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(¤t->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(¤t->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; } /* |