From: Stefan Berger <stefanb@linux.ibm.com>
To: Mimi Zohar <zohar@linux.ibm.com>,
Christian Brauner <christian.brauner@ubuntu.com>
Cc: linux-integrity@vger.kernel.org, serge@hallyn.com,
containers@lists.linux.dev, dmitry.kasatkin@gmail.com,
ebiederm@xmission.com, krzysztof.struczynski@huawei.com,
roberto.sassu@huawei.com, mpeters@redhat.com, lhinds@redhat.com,
lsturman@redhat.com, puiterwi@redhat.com, jejb@linux.ibm.com,
jamjoom@us.ibm.com, linux-kernel@vger.kernel.org,
paul@paul-moore.com, rgb@redhat.com,
linux-security-module@vger.kernel.org, jmorris@namei.org
Subject: Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses
Date: Wed, 15 Dec 2021 21:55:35 -0500 [thread overview]
Message-ID: <cd14c566-afaf-8295-5445-420164903336@linux.ibm.com> (raw)
In-Reply-To: <2cd00e8d0fa819b507cabd34e8f1760d5ff783c6.camel@linux.ibm.com>
On 12/15/21 18:04, Mimi Zohar wrote:
> On Wed, 2021-12-08 at 13:22 -0500, Stefan Berger wrote:
>> On 12/8/21 11:50, Stefan Berger wrote:
>>> On 12/8/21 07:23, Christian Brauner wrote:
>>>> On Wed, Dec 08, 2021 at 01:09:54PM +0100, Christian Brauner wrote:
>>>>> On Tue, Dec 07, 2021 at 03:21:21PM -0500, Stefan Berger wrote:
>>>>>> Implement hierarchical processing of file accesses in IMA
>>>>>> namespaces by
>>>>>> walking the list of IMA namespaces towards the init_ima_ns. This way
>>>>>> file accesses can be audited in an IMA namespace and also be evaluated
>>>>>> against the IMA policies of parent IMA namespaces.
>>>>>>
>>>>>> Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
>>>>>> ---
>>>>>> security/integrity/ima/ima_main.c | 29 +++++++++++++++++++++++++----
>>>>>> 1 file changed, 25 insertions(+), 4 deletions(-)
>>>>>>
>>>>>> diff --git a/security/integrity/ima/ima_main.c
>>>>>> b/security/integrity/ima/ima_main.c
>>>>>> index 2121a831f38a..e9fa46eedd27 100644
>>>>>> --- a/security/integrity/ima/ima_main.c
>>>>>> +++ b/security/integrity/ima/ima_main.c
>>>>>> @@ -200,10 +200,10 @@ void ima_file_free(struct file *file)
>>>>>> ima_check_last_writer(iint, inode, file);
>>>>>> }
>>>>>> -static int process_measurement(struct ima_namespace *ns,
>>>>>> - struct file *file, const struct cred *cred,
>>>>>> - u32 secid, char *buf, loff_t size, int mask,
>>>>>> - enum ima_hooks func)
>>>>>> +static int _process_measurement(struct ima_namespace *ns,
>>>>> Hm, it's much more common to use double underscores then single
>>>>> underscores to
>>>>>
>>>>> __process_measurement()
>>>>>
>>>>> reads a lot more natural to people perusing kernel code quite often.
>>>>>
>>>>>> + struct file *file, const struct cred *cred,
>>>>>> + u32 secid, char *buf, loff_t size, int mask,
>>>>>> + enum ima_hooks func)
>>>>>> {
>>>>>> struct inode *inode = file_inode(file);
>>>>>> struct integrity_iint_cache *iint = NULL;
>>>>>> @@ -405,6 +405,27 @@ static int process_measurement(struct
>>>>>> ima_namespace *ns,
>>>>>> return 0;
>>>>>> }
>>>>>> +static int process_measurement(struct ima_namespace *ns,
>>>>>> + struct file *file, const struct cred *cred,
>>>>>> + u32 secid, char *buf, loff_t size, int mask,
>>>>>> + enum ima_hooks func)
>>>>>> +{
>>>>>> + int ret = 0;
>>>>>> + struct user_namespace *user_ns;
>>>>>> +
>>>>>> + do {
>>>>>> + ret = _process_measurement(ns, file, cred, secid, buf,
>>>>>> size, mask, func);
>>>>>> + if (ret)
>>>>>> + break;
>>>>>> + user_ns = ns->user_ns->parent;
>>>>>> + if (!user_ns)
>>>>>> + break;
>>>>>> + ns = user_ns->ima_ns;
>>>>>> + } while (1);
>>>>> I'd rather write this as:
>>>>>
>>>>> struct user_namespace *user_ns = ns->user_ns;
>>>>>
>>>>> while (user_ns) {
>>>>> ns = user_ns->ima_ns;
>>>>>
>>>>> ret = __process_measurement(ns, file, cred, secid, buf,
>>>>> size, mask, func);
>>>>> if (ret)
>>>>> break;
>>>>> user_ns = user_ns->parent;
>>>>>
>>>>> }
>>>>>
>>>>> because the hierarchy is only an implicit property inherited by ima
>>>>> namespaces from the implementation of user namespaces. In other words,
>>>>> we're only indirectly walking a hierarchy of ima namespaces because
>>>>> we're walking a hierarchy of user namespaces. So the ima ns actually
>>>>> just gives us the entrypoint into the userns hierarchy which the double
>>>>> deref writing it with a while() makes obvious.
>>>> Which brings me to another point.
>>>>
>>>> Technically nothing seems to prevent an ima_ns to survive the
>>>> destruction of its associated userns in ima_ns->user_ns?
>>>>
>>>> One thread does get_ima_ns() and mucks around with it while another one
>>>> does put_user_ns().
>>>>
>>>> Assume it's the last reference to the userns which is now -
>>>> asynchronously - cleaned up from ->work. So at some point you're ending
>>>> with a dangling pointer in ima_ns->user_ns eventually causing a UAF.
>>>>
>>>> If I'm thinking correct than you need to fix this. I can think of two
>>>> ways right now where one of them I'm not sure how well that would work:
>>>> 1. ima_ns takes a reference count to userns at creation. Here you need
>>>> to make very sure that you're not ending up with reference counting
>>>> cycles where the two structs keep each other alive.
>>> Right. I am not sure what the trigger would be for ima_ns to release
>>> that one reference.
>>>
>>>
>>>> 2. rcu trickery. That's the one I'm not sure how well that would work
>>>> where you'd need rcu_read_lock()/rcu_read_unlock() with a
>>>> get_user_ns() in the middle whenever you're trying to get a ref to
>>>> the userns from an ima_ns and handle the case where the userns is
>>>> gone.
>>>>
>>>> Or maybe I'me missing something in the patch series that makes this all
>>>> a non-issue.
>>> I suppose one can always call current_user_ns() to get a pointer to
>>> the current user namespace that the process is accessing the file in
>>> that IMA now reacts to. With the hierarchical processing we are
>>> walking backwards towards init_user_ns. The problem should only exist
>>> if something else frees the current user namespace (or its parents) so
>>> that the hierarchy collapses. Assuming we are always in a process
>>> context then 'current' should protect us, no ?
>>>
>> All existing callers to process_measurements call it at least once with
>> current_cred().
>>
>> The only problem that I see where we are accessing the IMA namespace
>> outside a process context is in 4/16 'ima: Move delayed work queue and
>> variables into ima_namespace' where a delayed work queue is used. I
>> fixed this now by getting an additional reference to the user namesapce
>> before scheduling the delayed work and release it when it ran or when it
>> is canceled (cancel_delayed_work_sync()) but it didn't run.
>>
> From the "ima: Move delayed work queue and variables into
> ima_namespace" patch description:
> Since keys queued up for measurement currently are only relevant in
> the init_ima_ns, call ima_init_key_queue() only when the init_ima_ns
> is initialized.
>
> When IMA_QUEUE_EARLY_BOOT_KEYS is not enabled, ima_should_queue_key()
> simply returns false. Why do the keys workqueue need to be namespaced?
> Is this preparatory for some future IMA namespacing?
06 ima: Move policy related variables into ima_namespace
05 ima: Move IMA's keys queue related variables into ima_namespace
04 ima: Move delayed work queue and variables into ima_namespace
06 requires the ima_namespace parameter to be passed into
process_buffer_measurement(). The problem was ima_process_queued_keys()
that needs to pass the namespace but it's probably sufficient to use
&init_ima_ns there as the ima_namespace parameter, which would allow to
drop 05 and 04.
Stefan
>
> thanks,
>
> Mimi
>
next prev parent reply other threads:[~2021-12-16 2:55 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-07 20:21 [PATCH v4 00/16] ima: Namespace IMA with audit support in IMA-ns Stefan Berger
2021-12-07 20:21 ` [PATCH v4 01/16] ima: Add IMA namespace support Stefan Berger
2021-12-08 11:29 ` Christian Brauner
2021-12-08 11:54 ` Christian Brauner
2021-12-08 14:50 ` Stefan Berger
2021-12-07 20:21 ` [PATCH v4 02/16] ima: Define ns_status for storing namespaced iint data Stefan Berger
2021-12-07 20:21 ` [PATCH v4 03/16] ima: Namespace audit status flags Stefan Berger
2021-12-07 20:21 ` [PATCH v4 04/16] ima: Move delayed work queue and variables into ima_namespace Stefan Berger
2021-12-07 20:21 ` [PATCH v4 05/16] ima: Move IMA's keys queue related " Stefan Berger
2021-12-07 20:21 ` [PATCH v4 06/16] ima: Move policy " Stefan Berger
2021-12-07 20:21 ` [PATCH v4 07/16] ima: Move ima_htable " Stefan Berger
2021-12-07 20:21 ` [PATCH v4 08/16] ima: Move measurement list related variables " Stefan Berger
2021-12-07 20:21 ` [PATCH v4 09/16] ima: Only accept AUDIT rules for IMA non-init_ima_ns namespaces for now Stefan Berger
2021-12-07 20:21 ` [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses Stefan Berger
2021-12-08 12:09 ` Christian Brauner
2021-12-08 12:23 ` Christian Brauner
2021-12-08 16:50 ` Stefan Berger
2021-12-08 18:22 ` Stefan Berger
2021-12-15 23:04 ` Mimi Zohar
2021-12-16 2:55 ` Stefan Berger [this message]
2021-12-07 20:21 ` [PATCH v4 11/16] securityfs: Only use simple_pin_fs/simple_release_fs for init_user_ns Stefan Berger
2021-12-08 11:58 ` Christian Brauner
2021-12-08 14:03 ` Stefan Berger
2021-12-08 12:46 ` Christian Brauner
2021-12-11 14:16 ` Jarkko Sakkinen
2021-12-11 14:44 ` James Bottomley
2021-12-07 20:21 ` [PATCH v4 12/16] securityfs: Extend securityfs with namespacing support Stefan Berger
2021-12-07 20:21 ` [PATCH v4 13/16] ima: Move some IMA policy and filesystem related variables into ima_namespace Stefan Berger
2021-12-07 20:21 ` [PATCH v4 14/16] ima: Use mac_admin_ns_capable() to check corresponding capability Stefan Berger
2021-12-08 12:40 ` Christian Brauner
2021-12-07 20:21 ` [PATCH v4 15/16] ima: Move dentries into ima_namespace Stefan Berger
2021-12-07 20:21 ` [PATCH v4 16/16] ima: Setup securityfs for IMA namespace Stefan Berger
2021-12-08 12:58 ` Christian Brauner
2021-12-08 13:16 ` Christian Brauner
2021-12-08 14:11 ` James Bottomley
2021-12-08 14:46 ` Christian Brauner
2021-12-08 15:04 ` James Bottomley
2021-12-08 15:22 ` Christian Brauner
2021-12-08 15:39 ` Stefan Berger
2021-12-08 15:49 ` Christian Brauner
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