[PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

[PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

[Li Zefan]

Run these two scripts concurrently:

    for ((; ;))
    {
        mkdir /cgroup/sub
        rmdir /cgroup/sub
    }

    for ((; ;))
    {
        echo $$ > /cgroup/sub/cgroup.procs
        ech $$ > /cgce 6f2e0c38c2108a74 ]---
    }

A kernel bug will be triggered:

BUG: unable to handle kernel NULL pointer dereference at 00000038
IP: [<c10bbd69>] cgroup_put+0x9/0x80
...
Call Trace:
 [<c10bbe19>] cgroup_kn_unlock+0x39/0x50
 [<c10bbe91>] cgroup_kn_lock_live+0x61/0x70
 [<c10be3c1>] __cgroup_procs_write.isra.26+0x51/0x230
 [<c10be5b2>] cgroup_tasks_write+0x12/0x20
 [<c10bb7b0>] cgroup_file_write+0x40/0x130
 [<c11aee71>] kernfs_fop_write+0xd1/0x160
 [<c1148e58>] vfs_write+0x98/0x1e0
 [<c114934d>] SyS_write+0x4d/0xa0
 [<c16f656b>] sysenter_do_call+0x12/0x12

We clear cgrp->kn->priv in the end of cgroup_rmdir(), but another
concurrent thread can access kn->priv after the clearing.

We should move the clearing to css_release_work_fn(). At that time
no one is holding reference to the cgroup and no one can gain a new
reference to access it.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: Li Zefan <lizefan@huawei.com>
---

Toralf, Thanks for reporting the bug. I'm not able to repy to your email,
because I was kicked out of the cgroup mailing list so didn't receive
emails from mailing list for a week.

---
 kernel/cgroup.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 1c56924..e03fc62 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4185,6 +4185,15 @@ static void css_release_work_fn(struct work_struct *work)
 
 	mutex_unlock(&cgroup_mutex);
 
+	/*
+	 * There are two control paths which try to determine cgroup from
+	 * dentry without going through kernfs - cgroupstats_build() and
+	 * css_tryget_online_from_dir().  Those are supported by RCU
+	 * protecting clearing of cgrp->kn->priv backpointer.
+	 */
+	if (!ss && cgroup_parent(cgrp))
+		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
+
 	call_rcu(&css->rcu_head, css_free_rcu_fn);
 }
 
@@ -4601,16 +4610,6 @@ static int cgroup_rmdir(struct kernfs_node *kn)
 
 	cgroup_kn_unlock(kn);
 
-	/*
-	 * There are two control paths which try to determine cgroup from
-	 * dentry without going through kernfs - cgroupstats_build() and
-	 * css_tryget_online_from_dir().  Those are supported by RCU
-	 * protecting clearing of cgrp->kn->priv backpointer, which should
-	 * happen after all files under it have been removed.
-	 */
-	if (!ret)
-		RCU_INIT_POINTER(*(void __rcu __force **)&kn->priv, NULL);
-
 	cgroup_put(cgrp);
 	return ret;
 }
-- 
1.8.0.2

[PATCH 2/2] cgroup: check cgroup liveliness before unbreaking kernfs protection

[Li Zefan]

When cgroup_kn_lock_live() is called through some kernfs operation and
another thread is calling cgroup_rmdir(), we may trigger the warning in
cgroup_get().

------------[ cut here ]------------
WARNING: CPU: 1 PID: 1228 at kernel/cgroup.c:1034 cgroup_get+0x89/0xa0()
...
Call Trace:
 [<c16ee73d>] dump_stack+0x41/0x52
 [<c10468ef>] warn_slowpath_common+0x7f/0xa0
 [<c104692d>] warn_slowpath_null+0x1d/0x20
 [<c10bb999>] cgroup_get+0x89/0xa0
 [<c10bbe58>] cgroup_kn_lock_live+0x28/0x70
 [<c10be3c1>] __cgroup_procs_write.isra.26+0x51/0x230
 [<c10be5b2>] cgroup_tasks_write+0x12/0x20
 [<c10bb7b0>] cgroup_file_write+0x40/0x130
 [<c11aee71>] kernfs_fop_write+0xd1/0x160
 [<c1148e58>] vfs_write+0x98/0x1e0
 [<c114934d>] SyS_write+0x4d/0xa0
 [<c16f656b>] sysenter_do_call+0x12/0x12
---[ end trace 6f2e0c38c2108a74 ]---

Fix this by calling css_tryget() instead of cgroup_get().

Reported-by: Toralf Förster <toralf.foerster-Mmb7MZpHnFY@public.gmane.org>
Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
---
 kernel/cgroup.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index e03fc62..c8d07e5 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1025,6 +1025,11 @@ static umode_t cgroup_file_mode(const struct cftype *cft)
 	return mode;
 }
 
+static bool cgroup_tryget(struct cgroup *cgrp)
+{
+	return css_tryget(&cgrp->self);
+}
+
 static void cgroup_get(struct cgroup *cgrp)
 {
 	WARN_ON_ONCE(cgroup_is_dead(cgrp));
@@ -1091,7 +1096,8 @@ static struct cgroup *cgroup_kn_lock_live(struct kernfs_node *kn)
 	 * protection against removal.  Ensure @cgrp stays accessible and
 	 * break the active_ref protection.
 	 */
-	cgroup_get(cgrp);
+	if (!cgroup_tryget(cgrp))
+		return NULL;
 	kernfs_break_active_protection(kn);
 
 	mutex_lock(&cgroup_mutex);
-- 
1.8.0.2

Re: [PATCH 2/2] cgroup: check cgroup liveliness before unbreaking kernfs protection

[Tejun Heo]

On Tue, Sep 02, 2014 at 06:57:54PM +0800, Li Zefan wrote:
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index e03fc62..c8d07e5 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1025,6 +1025,11 @@ static umode_t cgroup_file_mode(const struct cftype *cft)
>  	return mode;
>  }
>  
> +static bool cgroup_tryget(struct cgroup *cgrp)
> +{
> +	return css_tryget(&cgrp->self);
> +}

Can you please move this right below cgroup_get() definition?

Thanks.

-- 
tejun

Re: [PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

[Tejun Heo]

Hello, Li.

On Tue, Sep 02, 2014 at 06:56:58PM +0800, Li Zefan wrote:
>     for ((; ;))
>     {
>         echo $$ > /cgroup/sub/cgroup.procs
>         ech $$ > /cgce 6f2e0c38c2108a74 ]---
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          copy & paste error?
...
> Reported-by: Toralf Förster <toralf.foerster-Mmb7MZpHnFY@public.gmane.org>
> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
> ---
> 
> Toralf, Thanks for reporting the bug. I'm not able to repy to your email,
> because I was kicked out of the cgroup mailing list so didn't receive
> emails from mailing list for a week.
> 
> ---
>  kernel/cgroup.c | 19 +++++++++----------
>  1 file changed, 9 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 1c56924..e03fc62 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -4185,6 +4185,15 @@ static void css_release_work_fn(struct work_struct *work)
>  
>  	mutex_unlock(&cgroup_mutex);
>  
> +	/*
> +	 * There are two control paths which try to determine cgroup from
> +	 * dentry without going through kernfs - cgroupstats_build() and
> +	 * css_tryget_online_from_dir().  Those are supported by RCU
> +	 * protecting clearing of cgrp->kn->priv backpointer.
> +	 */
> +	if (!ss && cgroup_parent(cgrp))
> +		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);

Can we move the above into the preceding else block?  I don't think
holding cgroup_mutex or not makes any difference here.  Also, why do
we need the cgroup_parent() check?  Do we deref root's kn->priv in the
destruction path?  If so, can you please note that in the comment?

Thanks.

-- 
tejun

Re: [PATCH 1/2] cgroup: Delay the clearing of cgrp->kn->priv

[Li Zefan]

于 2014/9/2 23:33, Tejun Heo 写道:
> Hello, Li.
> 
> On Tue, Sep 02, 2014 at 06:56:58PM +0800, Li Zefan wrote:
>>     for ((; ;))
>>     {
>>         echo $$ > /cgroup/sub/cgroup.procs
>>         ech $$ > /cgce 6f2e0c38c2108a74 ]---
>           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>           copy & paste error?
> ...

oops

>> Reported-by: Toralf Förster <toralf.foerster-Mmb7MZpHnFY@public.gmane.org>
>> Signed-off-by: Li Zefan <lizefan-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>> ---
>>
>> Toralf, Thanks for reporting the bug. I'm not able to repy to your email,
>> because I was kicked out of the cgroup mailing list so didn't receive
>> emails from mailing list for a week.
>>
>> ---
>>  kernel/cgroup.c | 19 +++++++++----------
>>  1 file changed, 9 insertions(+), 10 deletions(-)
>>
>> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
>> index 1c56924..e03fc62 100644
>> --- a/kernel/cgroup.c
>> +++ b/kernel/cgroup.c
>> @@ -4185,6 +4185,15 @@ static void css_release_work_fn(struct work_struct *work)
>>  
>>  	mutex_unlock(&cgroup_mutex);
>>  
>> +	/*
>> +	 * There are two control paths which try to determine cgroup from
>> +	 * dentry without going through kernfs - cgroupstats_build() and
>> +	 * css_tryget_online_from_dir().  Those are supported by RCU
>> +	 * protecting clearing of cgrp->kn->priv backpointer.
>> +	 */
>> +	if (!ss && cgroup_parent(cgrp))
>> +		RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);
> 
> Can we move the above into the preceding else block?  I don't think
> holding cgroup_mutex or not makes any difference here. 

> Also, why do
> we need the cgroup_parent() check?  Do we deref root's kn->priv in the
> destruction path?  If so, can you please note that in the comment?
> 

I think the check is not necessary. I was trying to make smaller difference
than the original code, and RCU_INIT_POINTER() is in cgroup_rmdir() which
won't be called on root cgroup.