[PATCH v2] tty: tty_jobctrl: fix pid memleak in disassociate_ctty()

[Yi Yang]

There is a pid leakage:
------------------------------
unreferenced object 0xffff88810c181940 (size 224):
  comm "sshd", pid 8191, jiffies 4294946950 (age 524.570s)
  hex dump (first 32 bytes):
    01 00 00 00 00 00 00 00 00 00 00 00 ad 4e ad de  .............N..
    ff ff ff ff 6b 6b 6b 6b ff ff ff ff ff ff ff ff  ....kkkk........
  backtrace:
    [<ffffffff814774e6>] kmem_cache_alloc+0x5c6/0x9b0
    [<ffffffff81177342>] alloc_pid+0x72/0x570
    [<ffffffff81140ac4>] copy_process+0x1374/0x2470
    [<ffffffff81141d77>] kernel_clone+0xb7/0x900
    [<ffffffff81142645>] __se_sys_clone+0x85/0xb0
    [<ffffffff8114269b>] __x64_sys_clone+0x2b/0x30
    [<ffffffff83965a72>] do_syscall_64+0x32/0x80
    [<ffffffff83a00085>] entry_SYSCALL_64_after_hwframe+0x61/0xc6

It turns out that there is a race condition between disassociate_ctty() and
tty_signal_session_leader(), which caused this leakage.

The pid memleak is triggered by the following race:
task[sshd]                     task[bash]
-----------------------        -----------------------
                               disassociate_ctty();
                               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);
tty_vhangup();
tty_lock(tty);
...
tty_signal_session_leader();
spin_lock_irq(&p->sighand->siglock);
...
if (tty->pgrp) //tty->pgrp is not NULL
p->signal->tty_old_pgrp = get_pid(tty->pgrp); //An extra get
spin_unlock_irq(&p->sighand->siglock);
...
tty_unlock(tty);
                               if (tty) {
                                   tty_lock(tty);
                                   ...
                                   put_pid(tty->pgrp);
                                   tty->pgrp = NULL; // It's too late
                                   ...
                                   tty_unlock(tty);
                               }

The issue is believed to be introduced by commit c8bcd9c5be24 ("tty:
Fix ->session locking") who moves the unlock of siglock in
disassociate_ctty() above "if (tty)", making a small window allowing
tty_signal_session_leader() to kick in. It can be easily reproduced by
adding a delay before "if (tty)" and at the entrance of
tty_signal_session_leader() "tty_signal_session_leader()".

To fix this issue, we move put_pid() after "if (tty)".

Fixes: c8bcd9c5be24 ("tty: Fix ->session locking")
Signed-off-by: Yi Yang <yiyang13@xxxxxxxxxx>
Co-developed-by: GUO Zihua <guozihua@xxxxxxxxxx>
Signed-off-by: GUO Zihua <guozihua@xxxxxxxxxx>
---
v2:Completely refactor the solution, avoid the use of PF_EXITING flag and
do put_pid() in disassociate_ctty() again instead.
---
 drivers/tty/tty_jobctrl.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 0d04287da098..17a6565f428b 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -300,12 +300,7 @@ void disassociate_ctty(int on_exit)
 		return;
 	}
 
-	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);
-
+	tty = get_current_tty();
 	if (tty) {
 		unsigned long flags;
 
@@ -320,6 +315,11 @@ void disassociate_ctty(int on_exit)
 		tty_kref_put(tty);
 	}
 
+	spin_lock_irq(&current->sighand->siglock);
+	put_pid(current->signal->tty_old_pgrp);
+	current->signal->tty_old_pgrp = NULL;
+	spin_unlock_irq(&current->sighand->siglock);
+
 	/* Now clear signal->tty under the lock */
 	read_lock(&tasklist_lock);
 	session_clear_tty(task_session(current));
-- 
2.17.1