Re: [PATCH V3 RESEND] tty: tty_jobctrl: fix pid memleak in disassociate_ctty()

[Greg KH <gregkh@linuxfoundation.org>]

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(&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>
> ---
>  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(&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));
> -- 
> 2.17.1
> 

Hi,

This is the friendly patch-bot of Greg Kroah-Hartman.  You have sent him
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