linux-kernel - Re: Kernel panic at Ubuntu: IMA + Apparmor

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Message-ID: <CACE9dm9CTU0-FFr3KveE8QV0dR-vxiDg91Km_2w3v37O5eTcYg@mail.gmail.com>
Date:	Sat, 26 Apr 2014 00:56:32 +0300
From:	Dmitry Kasatkin <dmitry.kasatkin@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	Dmitry Kasatkin <d.kasatkin@...sung.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	John Johansen <john.johansen@...onical.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	James Morris <jmorris@...ei.org>, viro@...iv.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kernel-team <kernel-team@...ts.ubuntu.com>
Subject: Re: Kernel panic at Ubuntu: IMA + Apparmor

On 26 April 2014 00:46, Dmitry Kasatkin <dmitry.kasatkin@...il.com> wrote:
> On 26 April 2014 00:27, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>> Dmitry Kasatkin <dmitry.kasatkin@...il.com> writes:
>>
>>> On 25 April 2014 23:45, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>>> Dmitry Kasatkin <dmitry.kasatkin@...il.com> writes:
>>>>
>>>>> On 25 April 2014 23:01, Oleg Nesterov <oleg@...hat.com> wrote:
>>>>>> On 04/25, Eric W. Biederman wrote:
>>>>>>>
>>>>>>> Oleg Nesterov <oleg@...hat.com> writes:
>>>>>>>
>>>>>>> > Well. I _think_ that __fput() and ima_file_free() in particular should not
>>>>>>> > depend on current and/or current->nsproxy. If nothing else, fput() can be
>>>>>>> > called by the unrelated task which looks into /proc/pid/.
>>>>>>> >
>>>>>>> > But again, task_work_add() has more and more users, and it seems that even
>>>>>>> > __fput() paths can do "everything", so perhaps it would be safer to allow
>>>>>>> > to use ->nsproxy in task_work_run.
>>>>>>>
>>>>>>> Like I said, give me a clear motivating case.
>>>>>>
>>>>>> I agree, we need a reason. Currently I do not see one.
>>>>>>
>>>>>>> Right now not allowing
>>>>>>> nsproxy is turning up bugs in __fput.  Which seems like a good thing.
>>>>>>
>>>>>> This is what I certainly agree with ;)
>>>>>>
>>>>>
>>>>> Hi,
>>>>>
>>>>> IMA uses kernel_read API which does not know anything about caller.
>>>>> And of course security frameworks are at guard as usual.
>>>>>
>>>>> Exactly after reading first Eric's respons, I thought why to scratch
>>>>> the head when task work queues are indeed designed for tasks...
>>>>
>>>> __fput has no guarantee of running in the task that close the file
>>>> descriptor.  If your code depends on that your code is broken.
>>>>
>>>>> And if you to dig for the history, IMA-appraisal was stuck due to
>>>>> lockdep reporting even though it was on non-everlaping cases.
>>>>> IIRC files vs. directories...
>>>>>
>>>>> After that IIRC Al Viro discussed about delayed fput and IIRC Oleg
>>>>> (sorry if I am wrong) introduced task work queues.
>>>>>
>>>>> So IMA-appraisal was able to be upstreamed... That was ~3.4 time frame, IIRC
>>>>>
>>>>> Name space also dated around ~3.4??
>>>>> Apparmor namespace change was also around that time.
>>>>>
>>>>> 3.10 introduces this name space order change and broke IMA-appraisal.
>>>>
>>>> IMA-appraisal is fundamentally broken because I can take a mandatory
>>>> file lock and prevent IMA-apprasial.
>>>>
>>>
>>> What file lock are you talking about?
>>> IMA-appraisal does not depends on file locks...
>>
>> It honors them.  Look at rw_verify_area, in vfs_read, in kernel_read.
>>
>> It sure looks like locks_mandatory_area can cause your kernel_read to
>> fail.
>>
>>>> Using kernel_read is what allows this.
>>>>
>>>>> Isn't it a clear motivating case???
>>>>
>>>> kernel_read is not appropriate for IMA use.  The rest of this is just
>>>> the messenger.
>>>>
>>>> IMA needs to use a cousin of kernel_read that operates at a lower level
>>>> than vfs_read.  A function that all of the permission checks and the
>>>> fsnotify work.
>>>>
>>>> I am sorry to be the bearer of bad news.  But kernel_read is totally
>>>> inappropriate for IMA.
>>>>
>>>
>>> So you break IMA-appraisal and declare that it cannot be used now?
>>
>> I didn't break it.  I read the code, and I read the back trace to see
>> where the bug was.
>>
>> I see IMA-appraisal trying to read file data as if it were a user space
>> application in such a way that it can get permission denied for a whole
>> host of reasons.
>>
>> My understanding of IMA-appraisal is that using a code path that can
>> give use permission denined when performing appraisal is a way for
>> clever people to attack and avoid IMA-appraisal without violating any
>> security policy.
>>
>
> Interesting thing is that file was already open before and LSM gave OK for this.
> Then re-reading the file on close in fact does not need any LSM
> permission checks.
> But as kernel_read API is still the same, it goes via the same checks...
>
> But on close with delayed fput nsproxy is missing ....
>
>> Am I wrong.  Is it ok for IMA-appraisal to get permission denied when it
>> wants to appraise a file?
>>
>
> IMA is called after may_open...
>
>
>> Eric
>
>

Is it really a show stopper to switch order of 2 functions as quick fix?
It was like that before 3.10 and seemed ok...


>
> --
> Thanks,
> Dmitry


-- 
Thanks,
Dmitry
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