tty: tty_jobctrl: fix pid memleak in disassociate_ctty()

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->ctrl.pgrp) //tty->ctrl.pgrp is not NULL
p->signal->tty_old_pgrp = get_pid(tty->ctrl.pgrp); //An extra get
spin_unlock_irq(&p->sighand->siglock);
...
tty_unlock(tty);
                               if (tty) {
                                   tty_lock(tty);
                                   ...
                                   put_pid(tty->ctrl.pgrp);
                                   tty->ctrl.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().

To fix this issue, we move "put_pid(current->signal->tty_old_pgrp)" after
"tty->ctrl.pgrp = NULL".

Fixes: c8bcd9c5be24 ("tty: Fix ->session locking")
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Co-developed-by: GUO Zihua <guozihua@huawei.com>
Signed-off-by: GUO Zihua <guozihua@huawei.com>
Link: https://lore.kernel.org/r/20230831023329.165737-1-yiyang13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index 0d04287da098441e29b5454e3174c100b2f305da..ef8741c3e662960cc69c7463bad4021d26be8bd0 100644 (file)
--- 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 +315,16 @@ void disassociate_ctty(int on_exit)
                tty_kref_put(tty);
        }
 
+       /* If tty->ctrl.pgrp is not NULL, it may be assigned to
+        * current->signal->tty_old_pgrp in a race condition, and
+        * cause pid memleak. Release current->signal->tty_old_pgrp
+        * after tty->ctrl.pgrp set to NULL.
+        */
+       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));