* [PATCH] tty: fix pid refcount leak in tty_signal_session_leader
@ 2020-07-26 5:28 Xin Xiong
2020-07-26 7:06 ` Greg Kroah-Hartman
2020-07-26 8:55 ` Andy Shevchenko
0 siblings, 2 replies; 3+ messages in thread
From: Xin Xiong @ 2020-07-26 5:28 UTC (permalink / raw)
To: Greg Kroah-Hartman, Jiri Slaby, linux-kernel
Cc: Xiyu Yang, Xin Tan, yuanxzhang, Xin Xiong
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>
---
drivers/tty/tty_jobctrl.c | 2 ++
1 file changed, 2 insertions(+)
diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
index f8ed50a16848..9e6bf693ade1 100644
--- a/drivers/tty/tty_jobctrl.c
+++ b/drivers/tty/tty_jobctrl.c
@@ -212,6 +212,8 @@ int tty_signal_session_leader(struct tty_struct *tty, int exit_session)
__group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
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);
--
2.25.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] tty: fix pid refcount leak in tty_signal_session_leader
2020-07-26 5:28 [PATCH] tty: fix pid refcount leak in tty_signal_session_leader Xin Xiong
@ 2020-07-26 7:06 ` Greg Kroah-Hartman
2020-07-26 8:55 ` Andy Shevchenko
1 sibling, 0 replies; 3+ messages in thread
From: Greg Kroah-Hartman @ 2020-07-26 7:06 UTC (permalink / raw)
To: Xin Xiong; +Cc: Jiri Slaby, linux-kernel, Xiyu Yang, Xin Tan, yuanxzhang
On Sun, Jul 26, 2020 at 01:28:04PM +0800, Xin Xiong 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>
> ---
> drivers/tty/tty_jobctrl.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/drivers/tty/tty_jobctrl.c b/drivers/tty/tty_jobctrl.c
> index f8ed50a16848..9e6bf693ade1 100644
> --- a/drivers/tty/tty_jobctrl.c
> +++ b/drivers/tty/tty_jobctrl.c
> @@ -212,6 +212,8 @@ int tty_signal_session_leader(struct tty_struct *tty, int exit_session)
> __group_send_sig_info(SIGCONT, SEND_SIG_PRIV, p);
> put_pid(p->signal->tty_old_pgrp); /* A noop */
> spin_lock(&tty->ctrl_lock);
> + if (tty_pgrp)
> + put_pid(tty_pgrp);
No need to check this before calling it.
But, the real question is why is this needed now? Nothing has changed
in this area of the kernel for a very long time, so how did things get
broken here?
How are you triggering this and what is the result when we have that
additional reference?
thanks,
greg k-h
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] tty: fix pid refcount leak in tty_signal_session_leader
2020-07-26 5:28 [PATCH] tty: fix pid refcount leak in tty_signal_session_leader Xin Xiong
2020-07-26 7:06 ` Greg Kroah-Hartman
@ 2020-07-26 8:55 ` Andy Shevchenko
1 sibling, 0 replies; 3+ messages in thread
From: Andy Shevchenko @ 2020-07-26 8:55 UTC (permalink / raw)
To: Xin Xiong
Cc: Greg Kroah-Hartman, Jiri Slaby, Linux Kernel Mailing List,
Xiyu Yang, Xin Tan, yuanxzhang
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
^ permalink raw reply [flat|nested] 3+ messages in thread
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2020-07-26 5:28 [PATCH] tty: fix pid refcount leak in tty_signal_session_leader Xin Xiong
2020-07-26 7:06 ` Greg Kroah-Hartman
2020-07-26 8:55 ` Andy Shevchenko
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