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

[Kees Cook <keescook@chromium.org>]

On Fri, Jan 17, 2020 at 06:16:23AM +0100, Christian Brauner wrote:
> 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()":
> [...]
> #define __task_cred(task)	\
> 	rcu_dereference((task)->real_cred)

Ah! Yes, thank you. cred vs real_cred. That's what I missed!

> > 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.

Thanks for the clarification. With that adjusted, please consider it:

Reviewed-by: Kees Cook <keescook@chromium.org>

(I wonder how hard it might be to build some self-tests for this to
catch future glitches...)

-- 
Kees Cook