On Wed, Aug 16, 2023 at 10:57:00AM +0800, Yi Yang wrote:
> 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(¤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);
> 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@xxxxxxxxxx>
> Co-developed-by: GUO Zihua <guozihua@xxxxxxxxxx>
> Signed-off-by: GUO Zihua <guozihua@xxxxxxxxxx>
> ---
> drivers/tty/tty_jobctrl.c | 17 +++++++++++------
> 1 file changed, 11 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
> index 0d04287da098..ef8741c3e662 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(¤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);
> -
> + 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(¤t->sighand->siglock);
> + put_pid(current->signal->tty_old_pgrp);
> + current->signal->tty_old_pgrp = NULL;
> + spin_unlock_irq(¤t->sighand->siglock);
> +
> /* Now clear signal->tty under the lock */
> read_lock(&tasklist_lock);
> session_clear_tty(task_session(current));
> --
> 2.17.1
>
Hi,
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