linux-kernel - Re: [PATCH] cgroup: fix device deny of DEV

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Date:	Tue, 22 May 2012 10:14:24 +0800
From:	Amos Kong <akong@...hat.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
CC:	Li Zefan <lizefan@...wei.com>,
	Serge Hallyn <serge.hallyn@...onical.com>,
	containers@...ts.linux-foundation.org, mtosatti@...hat.com,
	linux-kernel@...r.kernel.org, tj@...nel.org,
	cgroups@...r.kernel.org
Subject: Re: [PATCH] cgroup: fix device deny of DEV_ALL

On 22/05/12 09:54, Serge E. Hallyn wrote:
> Quoting Li Zefan (lizefan@...wei.com):
>> Serge Hallyn wrote:
>>
>>> Quoting Amos Kong (akong@...hat.com):
>>>> @ mount -t cgroup -o devices none /cgroup
>>>> @ mkdir /cgroups/devices
>>>> @ ls -l /dev/dm-3
>>>>   brw-rw----. 1 root disk 253, 3 Oct 14 19:03 /dev/dm-3
>>>> @ echo 'b 253:3 rw'>  devices.deny
>>>> but I can still write it by 'dd if=/dev/zero of=/dev/dm-3'
>>>>
>>>> In devcgroup_create(), we create a new whitelist, and add first
>>>> entry which type is 'DEV_ALL'. Execute "# echo 'b 253:3 rw'>
>>>> devices.deny", dev_whitelist_rm() will update access of first
>>>> entry to 1(m), but type of first entry is still 'DEV_ALL'.
>>>
>>> Hi,
>>>
>>> thanks.  You raise a good point, but I think it needs some discussion.
>>>
>>> What happens right now is that if you have the 'a *:* rwm' entry and do
>>> echo 'b 253:3 rw'>  devices.deny, then when you next cat devices.list you
>>> will still see the 'a *:* rwm' entry.  So there should be no confusion
>>> over why the dd succeeds.

>>>   You didn't remove the entry, because there
>>> was no match echoed into devices.deny.

Hi serge,

My patch updated type,major,minor, it _equals to_ remove 'a *:* rwm' and 
add 'b *:* m'
It's a clear logic, why need to manually remove 'a *:* rwm'?


>> No, you'll see the entry has been changed to 'a *:* m', so I think we
>> should at least fix this.
>
> Yikes.  Agreed.  That's a bug.

which bug? should not update walk->access if wh->access is not 'rwm'?

diff --git a/security/device_cgroup.c b/security/device_cgroup.c
index c43a332..e619a34 100644
--- a/security/device_cgroup.c
+++ b/security/device_cgroup.c
@@ -145,7 +145,8 @@ static void dev_whitelist_rm(struct dev_cgroup 
*dev_cgroup,
                         continue;

  remove:
-               walk->access &= ~wh->access;
+               if (walk->type != DEV_ALL || wh->access == ACC_MASK)
+                       walk->access &= ~wh->access;
                 if (!walk->access) {
                         list_del_rcu(&walk->list);
                         kfree_rcu(walk, rcu);


-- 
			Amos.
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