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

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Date:	Fri, 25 Apr 2014 14:27:14 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Dmitry Kasatkin <dmitry.kasatkin@...il.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

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.

Am I wrong.  Is it ok for IMA-appraisal to get permission denied when it
wants to appraise a file?

Eric
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