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From:   Andy Shevchenko <andy.shevchenko@gmail.com>
Date:   Sun, 26 Jul 2020 11:55:48 +0300
Message-ID: <CAHp75Vf8h2kXnn-2xf3BFFDvBTSUH3w0z_S-FyKdB7MSJ_1j7g@mail.gmail.com>
Subject: Re: [PATCH] tty: fix pid refcount leak in tty_signal_session_leader
To:     Xin Xiong <xiongx18@fudan.edu.cn>
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jiri Slaby <jslaby@suse.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Xiyu Yang <xiyuyang19@fudan.edu.cn>,
        Xin Tan <tanxin.ctf@gmail.com>, yuanxzhang@fudan.edu.cn
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On Sun, Jul 26, 2020 at 8:30 AM Xin Xiong <xiongx18@fudan.edu.cn> wrote:
>
> In the loop, every time when p->signal->leader is true, the function
> tty_signal_session_leader() will invoke get_pid() and return a
> reference of tty->pgrp with increased refcount to the local variable
> tty_pgrp or return NULL if it fails. After finishing the loop, the
> function invokes put_pid() for only once, decreasing the refcount that
> tty_pgrp keeps.
>
> Refcount leaks may occur when the scenario that p->signal->leader is
> true happens more than once. In this assumption, if the above scenario
> happens n times in the loop, the function forgets to decrease the
> refcount for n-1 times, which causes refcount leaks.
>
> Fix the issue by decreasing the current refcount of the local variable
> tty_pgrp before assigning new objects to it.
>
> Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
> Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
> Signed-off-by: Xin Xiong <xiongx18@fudan.edu.cn>

This SoB chain is out of order. If you are the author, your SoB should
go first, if you are a commiter, the From line should correspond to
the first SoB (not yours), if it's a group of authors (funny for
one-/twoliner) then you consider to use Co-developed-by. Please, read
Submitting Patches document.

...

>                         put_pid(p->signal->tty_old_pgrp);  /* A noop */
>                         spin_lock(&tty->ctrl_lock);
> +                       if (tty_pgrp)
> +                               put_pid(tty_pgrp);
>                         tty_pgrp = get_pid(tty->pgrp);
>                         if (tty->pgrp)
>                                 p->signal->tty_old_pgrp = get_pid(tty->pgrp);

I guess this patch wasn't thought thru. You see the get_pid for it
happens twice in a row. Perhaps you have to get the logic behind all
these first?

P.S. ...on top of what Greg said.

-- 
With Best Regards,
Andy Shevchenko