tty_open can return to userspace holding tty_mutex

__tty_open could return (to userspace) holding the tty_mutex thanks to a
regression introduced by 4a2b5fddd53b80efcb3266ee36e23b8de28e761a ("Move
tty lookup/reopen to caller").

This was found by bisecting an fsfuzzer problem.  Admittedly I have no
idea how it managed to tickle this 100% reliably, but it is clearly a
regression and when hit leaves the box in a completely unusable state.
This patch lets the fsfuzzer test complete every time.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/char/tty_io.c b/drivers/char/tty_io.c
index d33e5ab06177..bc84e125c6bc 100644
--- a/drivers/char/tty_io.c
+++ b/drivers/char/tty_io.c
@@ -1817,8 +1817,10 @@ got_driver:
                 /* check whether we're reopening an existing tty */
                 tty = tty_driver_lookup_tty(driver, inode, index);
 
-                if (IS_ERR(tty))
+                if (IS_ERR(tty)) {
+                        mutex_unlock(&tty_mutex);
                         return PTR_ERR(tty);
+                }
         }
 
         if (tty) {