[PATCH] cgroups: fix a css_set not found bug in cgroup_attach_proc

[PATCH] cgroups: fix a css_set not found bug in cgroup_attach_proc

[Mandeep Singh Baines]

There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
is not called for the PF_EXITING case, find_existing_css_set() will return
NULL inside cgroup_task_migrate() causing a BUG.

This bug is easy to reproduce. Create a zombie and echo its pid to
cgroup.procs.

$ cat zombie.c
\#include <unistd.h>

int main()
{
  if (fork())
      pause();
  return 0;
}
$

We are hitting this bug pretty regularly on ChromeOS.

This bug is already fixed by Tejun Heo's cgroup patchset which is
targetted for the next merge window:

https://lkml.org/lkml/2011/11/1/356

I've create a smaller patch here which just fixes this bug so that a
fix can be merged into the current release and stable.

Signed-off-by: Mandeep Singh Baines <msb@chromium.org>
Downstream-Bug-Report: http://crosbug.com/23953
Cc: Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: containers@lists.linux-foundation.org
Cc: cgroups@vger.kernel.org
Cc: stable@kernel.org
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Paul Menage <paul@paulmenage.org>
Cc: Olof Johansson <olofj@chromium.org>
---
 kernel/cgroup.c |    5 -----
 1 files changed, 0 insertions(+), 5 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index d9d5648..a184470 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2098,11 +2098,6 @@ int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
 			continue;
 		/* get old css_set pointer */
 		task_lock(tsk);
-		if (tsk->flags & PF_EXITING) {
-			/* ignore this task if it's going away */
-			task_unlock(tsk);
-			continue;
-		}
 		oldcg = tsk->cgroups;
 		get_css_set(oldcg);
 		task_unlock(tsk);
-- 
1.7.3.1

Re: [PATCH] cgroups: fix a css_set not found bug in cgroup_attach_proc

[Tejun Heo]

On Thu, Dec 15, 2011 at 11:36:43AM -0800, Mandeep Singh Baines wrote:
> There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
> is not called for the PF_EXITING case, find_existing_css_set() will return
> NULL inside cgroup_task_migrate() causing a BUG.
> 
> This bug is easy to reproduce. Create a zombie and echo its pid to
> cgroup.procs.
> 
> $ cat zombie.c
> \#include <unistd.h>
> 
> int main()
> {
>   if (fork())
>       pause();
>   return 0;
> }
> $
> 
> We are hitting this bug pretty regularly on ChromeOS.
> 
> This bug is already fixed by Tejun Heo's cgroup patchset which is
> targetted for the next merge window:
> 
> https://lkml.org/lkml/2011/11/1/356
> 
> I've create a smaller patch here which just fixes this bug so that a
> fix can be merged into the current release and stable.

Looks safe to me.  Li?

Thanks.

-- 
tejun

Re: [PATCH] cgroups: fix a css_set not found bug in cgroup_attach_proc

[Li Zefan]

Tejun Heo wrote:
> On Thu, Dec 15, 2011 at 11:36:43AM -0800, Mandeep Singh Baines wrote:
>> There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
>> is not called for the PF_EXITING case, find_existing_css_set() will return
>> NULL inside cgroup_task_migrate() causing a BUG.
>>
>> This bug is easy to reproduce. Create a zombie and echo its pid to
>> cgroup.procs.
>>
>> $ cat zombie.c
>> \#include <unistd.h>
>>
>> int main()
>> {
>>   if (fork())
>>       pause();
>>   return 0;
>> }
>> $
>>
>> We are hitting this bug pretty regularly on ChromeOS.
>>
>> This bug is already fixed by Tejun Heo's cgroup patchset which is
>> targetted for the next merge window:
>>
>> https://lkml.org/lkml/2011/11/1/356
>>
>> I've create a smaller patch here which just fixes this bug so that a
>> fix can be merged into the current release and stable.
> 
> Looks safe to me.  Li?
> 

It should be fine.

Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>

Re: [PATCH] cgroups: fix a css_set not found bug in cgroup_attach_proc

[Tejun Heo]

On Fri, Dec 16, 2011 at 10:12:51AM +0800, Li Zefan wrote:
> Tejun Heo wrote:
> > On Thu, Dec 15, 2011 at 11:36:43AM -0800, Mandeep Singh Baines wrote:
> >> There is a BUG when migrating a PF_EXITING proc. Since css_set_prefetch()
> >> is not called for the PF_EXITING case, find_existing_css_set() will return
> >> NULL inside cgroup_task_migrate() causing a BUG.
> >>
> >> This bug is easy to reproduce. Create a zombie and echo its pid to
> >> cgroup.procs.
> >>
> >> $ cat zombie.c
> >> \#include <unistd.h>
> >>
> >> int main()
> >> {
> >>   if (fork())
> >>       pause();
> >>   return 0;
> >> }
> >> $
> >>
> >> We are hitting this bug pretty regularly on ChromeOS.
> >>
> >> This bug is already fixed by Tejun Heo's cgroup patchset which is
> >> targetted for the next merge window:
> >>
> >> https://lkml.org/lkml/2011/11/1/356
> >>
> >> I've create a smaller patch here which just fixes this bug so that a
> >> fix can be merged into the current release and stable.
> > 
> > Looks safe to me.  Li?
> > 
> 
> It should be fine.
> 
> Reviewed-by: Li Zefan <lizf@cn.fujitsu.com>

Applied to cgroup/for-fixes-3.2.  Will push to Linus in a couple of
days.  It's already rc6 and a bit late tho.

Thank you.

-- 
tejun