Re: [PATCH ghak90 V9 02/13] audit: add container id

[Richard Guy Briggs]

On 2020-07-05 11:09, Paul Moore wrote:
> On Sat, Jun 27, 2020 at 9:22 AM Richard Guy Briggs <rgb@xxxxxxxxxx> wrote:
> >
> > Implement the proc fs write to set the audit container identifier of a
> > process, emitting an AUDIT_CONTAINER_OP record to document the event.
> >
> > This is a write from the container orchestrator task to a proc entry of
> > the form /proc/PID/audit_containerid where PID is the process ID of the
> > newly created task that is to become the first task in a container, or
> > an additional task added to a container.
> >
> > The write expects up to a u64 value (unset: 18446744073709551615).
> >
> > The writer must have capability CAP_AUDIT_CONTROL.
> >
> > This will produce a record such as this:
> >   type=CONTAINER_OP msg=audit(2018-06-06 12:39:29.636:26949) : op=set opid=2209 contid=123456 old-contid=18446744073709551615
> >
> > The "op" field indicates an initial set.  The "opid" field is the
> > object's PID, the process being "contained".  New and old audit
> > container identifier values are given in the "contid" fields.
> >
> > It is not permitted to unset the audit container identifier.
> > A child inherits its parent's audit container identifier.
> >
> > Store the audit container identifier in a refcounted kernel object that
> > is added to the master list of audit container identifiers.  This will
> > allow multiple container orchestrators/engines to work on the same
> > machine without danger of inadvertantly re-using an existing identifier.
> > It will also allow an orchestrator to inject a process into an existing
> > container by checking if the original container owner is the one
> > injecting the task.  A hash table list is used to optimize searches.
> >
> > Please see the github audit kernel issue for the main feature:
> >   https://github.com/linux-audit/audit-kernel/issues/90
> > Please see the github audit userspace issue for supporting additions:
> >   https://github.com/linux-audit/audit-userspace/issues/51
> > Please see the github audit testsuiite issue for the test case:
> >   https://github.com/linux-audit/audit-testsuite/issues/64
> > Please see the github audit wiki for the feature overview:
> >   https://github.com/linux-audit/audit-kernel/wiki/RFE-Audit-Container-ID
> >
> > Signed-off-by: Richard Guy Briggs <rgb@xxxxxxxxxx>
> > Acked-by: Serge Hallyn <serge@xxxxxxxxxx>
> > Acked-by: Steve Grubb <sgrubb@xxxxxxxxxx>
> > Acked-by: Neil Horman <nhorman@xxxxxxxxxxxxx>
> > Reviewed-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> > ---
> >  fs/proc/base.c             |  36 +++++++++++
> >  include/linux/audit.h      |  33 ++++++++++
> >  include/uapi/linux/audit.h |   2 +
> >  kernel/audit.c             | 148 +++++++++++++++++++++++++++++++++++++++++++++
> >  kernel/audit.h             |   8 +++
> >  5 files changed, 227 insertions(+)
> 
> ...
> 
> > diff --git a/include/linux/audit.h b/include/linux/audit.h
> > index c2150415f9df..2800d4f1a2a8 100644
> > --- a/include/linux/audit.h
> > +++ b/include/linux/audit.h
> > @@ -692,6 +715,16 @@ static inline bool audit_loginuid_set(struct task_struct *tsk)
> >         return uid_valid(audit_get_loginuid(tsk));
> >  }
> >
> > +static inline bool audit_contid_valid(u64 contid)
> > +{
> > +       return contid != AUDIT_CID_UNSET;
> > +}
> > +
> > +static inline bool audit_contid_set(struct task_struct *tsk)
> > +{
> > +       return audit_contid_valid(audit_get_contid(tsk));
> > +}
> 
> This is quasi-nitpicky, but it seems like audit_contid_valid() and
> audit_contid_set() should be moved to kernel/audit.h if possible
> (possibly even kernel/audit.c).  Maybe I'll see something later in the
> patchset, but right now I'm struggling to think of why anyone outside
> of audit would need to call these functions.

This was historical made moot by the conversion to contobj.  I moved
them to kernel/audit.c and then just went with an open coded test once
and even just looking at the existance of a contobj.

> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index 5d8147a29291..6d387793f702 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -138,6 +138,13 @@ struct auditd_connection {
> >
> >  /* Hash for inode-based rules */
> >  struct list_head audit_inode_hash[AUDIT_INODE_BUCKETS];
> > +/* Hash for contid object lists */
> > +struct list_head audit_contid_hash[AUDIT_CONTID_BUCKETS];
> > +/* Lock all additions and deletions to the contid hash lists, assignment
> > + * of container objects to tasks.  There should be no need for
> > + * interaction with tasklist_lock
> > + */
> > +static DEFINE_SPINLOCK(audit_contobj_list_lock);
> >
> >  static struct kmem_cache *audit_buffer_cache;
> >
> > @@ -212,6 +219,33 @@ void __init audit_task_init(void)
> >                                              0, SLAB_PANIC, NULL);
> >  }
> >
> > +/* rcu_read_lock must be held by caller unless new */
> > +static struct audit_contobj *_audit_contobj_hold(struct audit_contobj *cont)
> > +{
> > +       if (cont)
> > +               refcount_inc(&cont->refcount);
> > +       return cont;
> > +}
> > +
> > +static struct audit_contobj *_audit_contobj_get(struct task_struct *tsk)
> > +{
> > +       if (!tsk->audit)
> > +               return NULL;
> > +       return _audit_contobj_hold(tsk->audit->cont);
> > +}
> > +
> > +/* rcu_read_lock must be held by caller */
> > +static void _audit_contobj_put(struct audit_contobj *cont)
> > +{
> > +       if (!cont)
> > +               return;
> > +       if (refcount_dec_and_test(&cont->refcount)) {
> > +               put_task_struct(cont->owner);
> > +               list_del_rcu(&cont->list);
> 
> You should check your locking; I'm used to seeing exclusive locks
> (e.g. the spinlock) around list adds/removes, it just reads/traversals
> that can be done with just the RCU lock held.

Ok, I've redone the locking yet again.  I knew this on one level but
that didn't translate consistently to code...

> > +               kfree_rcu(cont, rcu);
> > +       }
> > +}
> 
> Another nitpick, but it might be nice to have similar arguments to the
> _get() and _put() functions, e.g. struct audit_contobj, but that is
> some serious bikeshedding (basically rename _hold() to _get() and
> rename _hold to audit_task_contid_hold() or similar).

I have some idea what you are trying to say, but I think you misspoke.
Did you mean rename _hold to _get, rename _get to
audit_task_contobj_hold()?

> >  /**
> >   * audit_alloc - allocate an audit info block for a task
> >   * @tsk: task
> > @@ -232,6 +266,9 @@ int audit_alloc(struct task_struct *tsk)
> >         }
> >         info->loginuid = audit_get_loginuid(current);
> >         info->sessionid = audit_get_sessionid(current);
> > +       rcu_read_lock();
> > +       info->cont = _audit_contobj_get(current);
> > +       rcu_read_unlock();
> 
> The RCU locks aren't strictly necessary here, are they?  In fact I
> suppose we could probably just replace the _get() call with a
> refcount_set(1) just as we do in audit_set_contid(), yes?

I don't understand what you are getting at here.  It needs a *contobj,
along with bumping up the refcount of the existing contobj.

> >         tsk->audit = info;
> >
> >         ret = audit_alloc_syscall(tsk);
> > @@ -246,6 +283,7 @@ int audit_alloc(struct task_struct *tsk)
> >  struct audit_task_info init_struct_audit = {
> >         .loginuid = INVALID_UID,
> >         .sessionid = AUDIT_SID_UNSET,
> > +       .cont = NULL,
> >  #ifdef CONFIG_AUDITSYSCALL
> >         .ctx = NULL,
> >  #endif
> > @@ -262,6 +300,9 @@ void audit_free(struct task_struct *tsk)
> >         struct audit_task_info *info = tsk->audit;
> >
> >         audit_free_syscall(tsk);
> > +       rcu_read_lock();
> > +       _audit_contobj_put(tsk->audit->cont);
> > +       rcu_read_unlock();
> >         /* Freeing the audit_task_info struct must be performed after
> >          * audit_log_exit() due to need for loginuid and sessionid.
> >          */
> > @@ -1709,6 +1750,9 @@ static int __init audit_init(void)
> >         for (i = 0; i < AUDIT_INODE_BUCKETS; i++)
> >                 INIT_LIST_HEAD(&audit_inode_hash[i]);
> >
> > +       for (i = 0; i < AUDIT_CONTID_BUCKETS; i++)
> > +               INIT_LIST_HEAD(&audit_contid_hash[i]);
> > +
> >         mutex_init(&audit_cmd_mutex.lock);
> >         audit_cmd_mutex.owner = NULL;
> >
> > @@ -2410,6 +2454,110 @@ int audit_signal_info(int sig, struct task_struct *t)
> >         return audit_signal_info_syscall(t);
> >  }
> >
> > +/*
> > + * audit_set_contid - set current task's audit contid
> > + * @task: target task
> > + * @contid: contid value
> > + *
> > + * Returns 0 on success, -EPERM on permission failure.
> > + *
> > + * If the original container owner goes away, no task injection is
> > + * possible to an existing container.
> > + *
> > + * Called (set) from fs/proc/base.c::proc_contid_write().
> > + */
> > +int audit_set_contid(struct task_struct *task, u64 contid)
> > +{
> > +       int rc = 0;
> > +       struct audit_buffer *ab;
> > +       struct audit_contobj *oldcont = NULL;
> > +
> > +       task_lock(task);
> > +       /* Can't set if audit disabled */
> > +       if (!task->audit) {
> > +               task_unlock(task);
> > +               return -ENOPROTOOPT;
> > +       }
> 
> See my question/comment in patch 1/13; this check may not be needed or
> it may need to be changed to something other than "!task->audit".
> 
> > +       read_lock(&tasklist_lock);
> > +       /* Don't allow the contid to be unset */
> > +       if (!audit_contid_valid(contid)) {
> > +               rc = -EINVAL;
> > +               goto unlock;
> > +       }
> > +       /* if we don't have caps, reject */
> > +       if (!capable(CAP_AUDIT_CONTROL)) {
> > +               rc = -EPERM;
> > +               goto unlock;
> > +       }
> > +       /* if task has children or is not single-threaded, deny */
> > +       if (!list_empty(&task->children) ||
> > +           !(thread_group_leader(task) && thread_group_empty(task))) {
> > +               rc = -EBUSY;
> > +               goto unlock;
> > +       }
> > +       /* if contid is already set, deny */
> > +       if (audit_contid_set(task))
> > +               rc = -EEXIST;
> > +unlock:
> 
> Can we move the "unlock" target to the end of the function where it
> just handles the unlocking and returns an error, including the
> AUDIT_CONTAINER_OP record if necessary?  From what I can see we only
> jump to "unlock" in case of error where we are not going to set the
> audit container ID, yet the "unlock" target is placed in a misleading
> location in the middle of the function.  It may be that everything
> works correctly, but I would argue this is a bad practice that
> increases the likelihood of buggy behavior in future code changes.
> 
> If you can't find way to arrange the code nicely, just duplicate the
> "tasklist_lock" unlock operation in the error handlers before jumping
> down to the end of the function.  It isn't perfect, but I believe it
> will be a lot less fragile than the current approach.

I think it makes most sense to convert it back to an else if ladder that
will simplify things a bit and make if flow a bit better.

> > +       read_unlock(&tasklist_lock);
> > +       rcu_read_lock();
> > +       oldcont = _audit_contobj_get(task);
> > +       if (!rc) {
> > +               struct audit_contobj *cont = NULL, *newcont = NULL;
> > +               int h = audit_hash_contid(contid);
> > +
> > +               spin_lock(&audit_contobj_list_lock);
> > +               list_for_each_entry_rcu(cont, &audit_contid_hash[h], list)
> > +                       if (cont->id == contid) {
> > +                               /* task injection to existing container */
> > +                               if (current == cont->owner) {
> > +                                       _audit_contobj_hold(cont);
> > +                                       newcont = cont;
> > +                               } else {
> > +                                       rc = -ENOTUNIQ;
> > +                                       spin_unlock(&audit_contobj_list_lock);
> > +                                       goto conterror;
> > +                               }
> > +                               break;
> > +                       }
> > +               if (!newcont) {
> > +                       newcont = kmalloc(sizeof(*newcont), GFP_ATOMIC);
> > +                       if (newcont) {
> > +                               INIT_LIST_HEAD(&newcont->list);
> > +                               newcont->id = contid;
> > +                               newcont->owner = get_task_struct(current);
> > +                               refcount_set(&newcont->refcount, 1);
> > +                               list_add_rcu(&newcont->list,
> > +                                            &audit_contid_hash[h]);
> > +                       } else {
> > +                               rc = -ENOMEM;
> > +                               spin_unlock(&audit_contobj_list_lock);
> > +                               goto conterror;
> > +                       }
> > +               }
> > +               spin_unlock(&audit_contobj_list_lock);
> > +               task->audit->cont = newcont;
> > +               _audit_contobj_put(oldcont);
> > +       }
> > +conterror:
> > +       task_unlock(task);
> > +
> > +       if (!audit_enabled)
> > +               return rc;
> > +
> > +       ab = audit_log_start(audit_context(), GFP_KERNEL, AUDIT_CONTAINER_OP);
> > +       if (!ab)
> > +               return rc;
> > +
> > +       audit_log_format(ab,
> > +                        "op=set opid=%d contid=%llu old-contid=%llu",
> > +                        task_tgid_nr(task), contid, oldcont ? oldcont->id : -1);
> > +       _audit_contobj_put(oldcont);
> > +       rcu_read_unlock();
> > +       audit_log_end(ab);
> > +       return rc;
> > +}
> 
> --
> paul moore
> www.paul-moore.com
> 

- RGB

--
Richard Guy Briggs <rgb@xxxxxxxxxx>
Sr. S/W Engineer, Kernel Security, Base Operating Systems
Remote, Ottawa, Red Hat Canada
IRC: rgb, SunRaycer
Voice: +1.647.777.2635, Internal: (81) 32635