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@xxxxxxxxxxxxx> --- 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_namespace06 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
