Re: [REVIEW PATCH v2] ptrace: reintroduce usage of subjective credentials in ptrace_has_cap()

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Jan 16, 2020 at 06:29:26PM -0800, Kees Cook wrote:
> On Thu, Jan 16, 2020 at 11:45:18PM +0100, Christian Brauner wrote:
> > Commit 69f594a38967 ("ptrace: do not audit capability check when outputing /proc/pid/stat")
> > introduced the ability to opt out of audit messages for accesses to
> > various proc files since they are not violations of policy.
> > While doing so it somehow switched the check from ns_capable() to
> > has_ns_capability{_noaudit}(). That means it switched from checking the
> > subjective credentials of the task to using the objective credentials. I
> > couldn't find the original lkml thread and so I don't know why this switch
> > was done. But it seems wrong since ptrace_has_cap() is currently only used
> > in ptrace_may_access(). And it's used to check whether the calling task
> > (subject) has the CAP_SYS_PTRACE capability in the provided user namespace
> > to operate on the target task (object). According to the cred.h comments
> > this would mean the subjective credentials of the calling task need to be
> > used.
> 
> I don't follow this description. As far as I can see, both the current
> code and your patch end up using current's cred, yes? I'm not following
> the subjective/objective change mentioned here.
> 
> Before:
> bool has_ns_capability(struct task_struct *t,
>                        struct user_namespace *ns, int cap)
> {
>         int ret;
> 
>         rcu_read_lock();
>         ret = security_capable(__task_cred(t), ns, cap, CAP_OPT_NONE);

If I'm not mistaken, you're looking at the cuplrit: "__task_cred()":

 /**
 * __task_cred - Access a task's objective credentials
 * @task: The task to query
 *
 * Access the objective credentials of a task.  The caller must hold the RCU
 * readlock.
 *
 * The result of this function should not be passed directly to get_cred();
 * rather get_task_cred() should be used instead.
 */
#define __task_cred(task)	\
	rcu_dereference((task)->real_cred)

>         rcu_read_unlock();
> 
>         return (ret == 0);
> }
> ...
> 		return has_ns_capability(current, ns, CAP_SYS_PTRACE)
> 
> After:
> 	const struct cred *cred = current_cred(), ...
> ...
> 		return security_capable(cred, ns, CAP_SYS_PTRACE, CAP_OPT_NOAUDIT);
> 
> The cred passed to security_capable() is the subject before and after.
> 
> > This switches it to use security_capable() because we only call
> > ptrace_has_cap() in ptrace_may_access() and in there we already have a
> > stable reference to the calling tasks creds under cred_guard_mutex so
> > there's no need to go through another series of dereferences and rcu
> > locking done in ns_capable{_noaudit}().
> 
> This makes sense to me -- now there's no possible race on the cred
> changing between the two ptrace_has_cap() checks, yes?
> 
> However, I'm still trying to see where cred_guard_mutex() comes into
> play for callers of ptrace_may_access(). I see it for the object
> ("task" arg in ptrace_may_access()), but if this is dealing with the cred
> on current, it's just the RCU read lock protecting it (which I think is
> fine here), but seems confusing in the commit log.

Ah, right. I'll drop that from the commit message and place in the rcu
lock.

Christian