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tty: fix data race in tty_ldisc_ref_wait()

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tty_ldisc_ref_wait() checks tty->ldisc under tty->ldisc_sem.
But if ldisc==NULL it releases them sem and reloads
tty->ldisc without holding the sem. This is wrong and
can lead to returning non-NULL ldisc without protection.

Don't reload tty->ldisc second time.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: syzkaller@googlegroups.com
Cc: linux-kernel@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Dmitry Vyukov 2017-03-04 13:46:12 +01:00 committed by Greg Kroah-Hartman
parent 5362544beb
commit a4a3e06114
1 changed files with 5 additions and 2 deletions
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7
drivers/tty/tty_ldisc.c
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@ -271,10 +271,13 @@ const struct file_operations tty_ldiscs_proc_fops = {
struct tty_ldisc *tty_ldisc_ref_wait(struct tty_struct *tty)
{
struct tty_ldisc *ld;
ldsem_down_read(&tty->ldisc_sem, MAX_SCHEDULE_TIMEOUT);
if (!tty->ldisc)
ld = tty->ldisc;
if (!ld)
ldsem_up_read(&tty->ldisc_sem);
return tty->ldisc;
return ld;
}
EXPORT_SYMBOL_GPL(tty_ldisc_ref_wait);