Messages in this thread | | | Date | Wed, 15 Dec 2021 21:55:35 -0500 | Subject | Re: [PATCH v4 10/16] ima: Implement hierarchical processing of file accesses | From | Stefan Berger <> |
| |
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 >
| |